CH 1 The Basics:
Types of Relationships
Definition
Unrelated
Related, Proportional
Inversely Related, Inversely Proportional
Two items that are not associated
Two items that are associated, or affiliated.
Two items that are associated such that
when one item increases, the other
decreases.
Two items that are associated such that
when one item increases, the other increases
A special form of Inverse Relationship. When
two numbers with a reciprocal relationship
are multiplied together, the result is 1.
Directly Related, Directly Proportional
Reciprocal Relationship
Units
Definition
Length – Distance or Circumference
Area
Volume
Hz
“Increase by a factor”
“Decrease by a factor”
When converting from Large to Small Units
When converting from Small to Large Units
cm, ft
cm2, ft2
cm3, ft3
per second
Multiply by that number
Divide by that number
Multiply
Divide
Exponent
Prefix
Symbol
Meaning
Representation
109
giga
mega
kilo
hecto
deca
deci
centi
milli
micro
nano
G
M
k
h
da
d
c
m
u
n
billion
million
thousand
hundred
ten
tenth
hundredth
thousandth
millionth
billionth
1,000,000,000
1,000,000
1,000
100
10
0.1
0.01
0.001
0.000001
0.000000001
106
103
102
101
10-1
10-2
10-3
10-6
10-9
Pairs
Prefixes
Abbreviations
billions and billionths
millions and millionths
thousands and thousandths
hundreds and hundredths
tens and tenths
giga & nano
mega & micro
kilo & milli
hecto & centi
deca & deci
G&n
M&u
k&m
h&c
da & d
CH 2 Sound:
Acoustic Variable
Pressure
Density
Distance
Units
Definition
Formula
pascals (Pa)
kg/cm3
cm, mm
Concentration of force in an area
Concentration of mass in a volume
Measure of particle motion
= Force / Area
= Mass / Volume
Seven Acoustic Parameters
•
•
•
•
•
•
•
Period
Frequency
Amplitude
Power
Intensity
Wavelength
Propagation Speed
Term
Definition
Acoustic Propagation Properties
Biologic Effects
The effects of the medium upon the sound wave
The effects of the sound wave upon the biologic
tissue through which it passes (ex: heat, pressure)
particles move in a direction that is perpendicular to
the direction that the wave propagates
particles move in the same direction that the wave
propagates
when a pair of wave’s peaks occur at the same time
and location
when a pair of wave’s peaks occur at different times,
and so do their troughs
when two or more waves arrive at an identical
location at exactly the same time, lose their
individual characteristics at that moment and
combine to form a single wave.
the interference of a pair of in-phase waves results
in the formation of a single wave of greater
amplitude than either of its components
the interference of a pair of out-of-phase waves
results in the formation of a single wave of lesser
amplitude than at least one of its components; outof-phase waves of equal amplitude may cancel each
other out
Transverse Waves
Longitudinal Waves
In-phase
Out-of-phase/ Out-of-step
Interference
Constructive Interference
Destructive Interference
CH 3 Describing Sound Waves:
Parameter Adjustable
*period
*frequency
amplitude
power
intensity
*wavelength
speed
no
no
yes
yes
yes
no
no
Units
Determined by
Typical Value
seconds, us, time
per second, Hz
pascals, cm, g/cm3, dB
watts, dB
watts/cm2, dB
mm, distance
m/s
source
source
source
source
source
both
medium
0.06 – 0.5 us
2-15 MHz
1 - 3 MPa
4 – 90 mW
0.01 – 300 W/cm2
0.1 – 0.8 mm
1,500 – 1,600 m/s
*most important to know
Parameter
Formula
period
frequency
amplitude
power
intensity
wavelength
speed
= 1/frequency
= 1/period; = speed/wavelength
EEKEE.EE
inverse
period /higher frequency
lower
frequency
longer period /
shorter
(Amp)2
= Intensity/Area; directly related to
= Power/Area; directly related to (Amp)2
= 1.54/frequency
= frequency x wavelength; = distance/time
t-requencytwavelengthhig.hrwavelength
lower
frequency /shorter
frequency / longer
wave
lengt
£edᵈEeEE= directly
sound in slow medium /short
sound in a fast medium/
wavelength
long
wavelength
Parameter
Definition
period
time from the start of one cycle to the start of the next cycle. (duration of one peak +
one trough)
the number of cycles that occur in 1 second
the “bigness” of a wave; the difference between the maximum or minimum value and
the average value of the acoustic variable; peak-to-peak amplitude: difference
between the maximum and minimum values of an acoustic variable
the rate of energy transfer or the rate at which work is performed
the concentration of energy in a sound beam
the distance or length of one complete cycle
the rate at which a sound wave travels through a medium
frequency
amplitude
power
intensity
wavelength
propagation speed
Sound Wave
Frequency
Tissue Type
infrasound
audible sound
ultrasound
less than 20 Hz
20 Hz – 20,000 Hz (20 kHz)
greater than 20,000 Hz (20 kHz)
Lung
Fat
Soft Tissue (avg)
Liver
Blood
Muscle
Tendon
Bone
Speed (m/s)
500
1,450
1,540
1,560
1,560
1,600
1,700
3,500
Material Speed (m/s)
Air
Water
Metal
330
1,480
2,000 – 7,000
Definition
Stiffness the ability of an object to resist compression
Density the relative weight of a material
Speed
Increases Increases
Increases Decreases
CH 4 Describing Pulsed Waves:
Parameter
Definition
Formula
pulse duration (PD)
the actual time from the start of a
pulse to the end of that pulse; single
on-time/transmit
the time from the start of one pulse
to the start of the next pulse
the number of pulses that an u/s
system transmits into the body each
second
the distance that a pulse occupies in
space from the start to the end of a
pulse
the percentage or fraction of time
that the system transmits a pulse
= # cycles/freq; = # cycles x period
talking
time
pulse repetition period (PRP)
pulse repetition frequency (PRF)
spatial pulse length (SPL)
duty factor (DF)
Parameter
Adjustable
pulse duration (PD)*
pulse repetition period (PRP)
pulse repetition frequency (PRF)
spatial pulse length (SPL)*
duty factor (DF)*
*most important to know
no
yes; by DOV
yes; by DOV
no
yes; by DOV
Units
PRI
usec, time
msec, time
Hz, per sec
mm, distance
none
i
g
•
•
•
•
Less listening
Short PRP
Higher DF
Higher PRF (best image)
as
= # cycles x wavelength
= PD/PRP x 100; = (PD x PRF) (100)
source
source
source
both
source
0.5 – 3.0 us
0.1 – 1.0 ms
1 – 10 kHz
0.1 – 1.0 mm
0.2 – 0.5%
Deep Imaging
PRP increases ,
imaging
decreases
PRP
as
,
=
= 1/PRP
Determined by Typical Value
Shallow Imaging
•
•
•
•
= PD + off-time; = 1/PRF
depth
imaging
More listening
Long PRP
Lower DF
Lower PRF
increases
depth
decreases
CH 5 Intensities:
Intensity Measurement Methods
SPTP
SATP
SPTA *most relevant with respect to tissue heating
SATA
SPPA
SAPA
spatial peak, temporal peak
spatial average, temporal peak
spatial peak, temporal average
spatial average, temporal average
spatial peak, pulse average
spatial average, pulse average
Temporal Intensities from Largest to Smallest
Itp > Imax > Ipa > Ita
Beam Uniformity Coefficient
= SP/SA
*the closer to 1 the more homogenous (equal in intensity) across the beam
Rank of Intensities from Largest to Smallest
SPTP > Im > SPPA > SPTA > SATA
Intensity Considerations
Definition
Spatial Peak Intensity (Isp)
Spatial Average Intensity (Isa)
Temporal Peak Intensity (Itp)
Imax (Im)
Pulse Average Intensity (Ipa)
Temporal Average Intensity (Ita)
the beam’s intensity at the location where it is maximum
the average intensity across the beam’s entire cross-sectional area
the intensity of the beam at the instant in time of its maximal value
the average intensity during the most intense half-cycle
the average intensity during the pulse duration
the average intensity of the entire pulse repetition period
• The “Ten Commandments” of Intensity: see page 74 of textbook
CH 6 Interaction of Sound and Media:
Term
Definition
logarithm
log
decibel (dB)
decibel notation
a novel method of rating numbers
represents the number of 10s that are multiplied to create the original number
a common unit for measuring intensities
• a relative measurement
• a comparison
• a ratio
• logarithmic
decrease in intensity, power, and amplitude as sound travels
• total attenuation (dB) = AC x distance
increase in intensity, power, and amplitude as sound travels
when the boundary is smooth, the sound is reflected in only 1 direction in an
organized manner
when a wave reflects off an irregular surface, it radiates in more than one direction
attenuation
amplification
specular reflection
diffuse reflection/
backscatter
scattering
Rayleigh scattering
absorption
attenuation
coefficient (AC)
Half-value layer
thickness (HVL)
acoustic impedance
normal incidence
oblique incidence
incident intensity
reflected intensity
the random redirection of sound in many directions
a special form of scattering that occurs when the structure’s dimensions are much
smaller than the beam’s wavelength; redirects the sound wave equally in all directions
(ex: RBCs) *Rayleigh Scattering is directly related to frequency4
most sizeable component of attenuation; occurs when ultrasonic energy is converted
into another energy form, such as heat; directly related to frequency
the number of dBs of attenuation that occurs when sound travels 1cm; one half the
frequency
• = f/2
the distance sound travels in a tissue that reduces the intensity of sound to one-half
its original value
• 0.25 – 1.0 cm
aka: penetration depth, depth of penetration, half-boundary layer
the acoustic resistance to sound traveling in a medium
• = density x speed
• units: rayls
• typical value: 1.25 -1.75 Mrayls (MZ)
aka: characteristic impedance
the incident sound beam strikes the boundary at exactly 90 degrees
aka: perpendicular, orthogonal, right angle, 90 degrees
the incident sound beam strikes the boundary at any angle other than 90 degrees; the
physics of transmission and reflection are complex and may or may not occur
aka: non-perpendicular, not at right angles
• Acute: less than 90 degrees
• Obtuse: greater than 90 degrees
the sound wave’s intensity immediately before it strikes a boundary
• = reflected intensity + transmitted intensity
the intensity of the portion of the incident beam that, after striking a boundary,
returns back where it came from
transmitted
intensity
intensity reflection
coefficient (IRC)
intensity
transmission
coefficient (ITC)
conservation of
energy
angle of incident
refraction
Snell’s Law
Decibels
3 dB
10 dB
-3 dB
-10 dB
the intensity of the portion of the incident beam that, after striking a boundary,
continues forward in the same general direction that it was traveling
the percentage of the intensity that bounces back when a sound beam strikes the
boundary between 2 media
• = (reflected intensity/ incident intensity) x 100
the percentage of intensity that passes in the forward direction when the beam
strikes an interface between 2 media; most (99% or more) of a sound wave’s intensity
is transmitted at a boundary between 2 soft tissues
• = (transmitted intensity/ incident intensity) x 100
ITC + IRC = 100%
= angle of reflection
a change in direction of wave propagation when traveling from one medium to
another; occurs only if 2 conditions are satisfied:
• oblique incidence
• different propagation speeds of the 2 media
quantifies the physics of refraction
• sin (transmission angle) = speed of Medium 2
sin (incident angle)
speed of Medium 1
Meaning
double
ten times larger
half
one-tenth
More Attenuation
•
•
longer distances/ path length
higher frequencies
Less Attenuation
•
•
shorter distances/ path length
lower frequencies
Attenuation Factors
•
•
path length
frequency of sound
Sound Back to Txdcr
Sound in all Directions
Attenuation Processes
•
•
•
Organized
Disorganized
specular
Rayleigh scattering
diffuse or backscattering
scattering
Half Value Layer:
Thin Half Value
•
•
•
High frequency sound
media with high attenuation rate
ex: lung or bone
reflection
scattering
absorption
Thick Half Value
•
•
•
Low frequency sound
media with low attenuation rate
ex: fluids
Impedances b/t 2 Tissues
Reflection
identical
slightly different
substantially different
none
small
large
Attenuation of Ultrasound in Media:
Medium
Attenuation
water
blood, urine, biologic fluids
fat
soft tissue
muscle
bone and lung
air
extremely low
low
low
intermediate
higher
even higher
extremely high
Speeds
Angle of Transmission
Speed 2 = Speed 1
Speed 2 > Speed 1
Speed 2 < Speed 1
no refraction; transmission angle = incident angle
transmission angle > incident angle
transmission angle < incident angle
Event
Requirement
Reflection with Normal Incidence
Reflection with Oblique Incidence
Transmission
Refraction
different impedances required
we cannot predict, it’s too complex!
derived from reflection information; use law of conservation of energy
oblique incidence and different speeds required
CH 7 Range Equation:
Term
Definition
go-return time; time-of-flight
the elapsed time from pulse creation to pulse
reception; directly related to depth of reflector
(mm) = 1.54 mm/us x go-return time (us)
2
for every 13 usec of go-return time, the object
creating the reflection is 1 cm deep in the body
inversely
• (Hz) = 77, 000 cm/s
imaging depth (cm)
directly
• (us) = imaging depth (cm) x 13 us/cm
depth of reflector (equation)
13-microsecond Rule
PRF (as related to DOV)
PRP (as related to DOV)
Time of Flight
Reflector Depth
Total Distance Traveled
13 us
26 us
39 us
52 us
130 us
1 cm
2 cm
3 cm
4 cm
10 cm
2 cm
4 cm
6 cm
8 cm
20 cm
Deep DOV
Shallow DOV
low
long
high
short
PRF
PRP
CH 8 Transducers:
Transducer
Converts
from….
to…..
ultrasound transmit
ultrasound receive
electrical
acoustic
acoustic
electrical
Effect
Reverse Piezoelectric
Piezoelectric
Basic Ultrasound Transducer Construction:
Transducer Component Description
case
electrical shield
acoustic insulator
PZT or active element
wire
matching layer
backing layer or damping
element
Term
transmission
reception
Piezoelectric Effect
the outer most covering, constructed of metal or plastic, that protects the
internal components of the txdcr from damage; insulates the patient from
electrical shock
thin metallic barrier lining the inside of the case; prevents electrical noise
from contaminating the clinically important electrical signals used to create
diagnostic images
thin barrier of cork or rubber that isolates or “uncouples” the internal
components of the txdcr from the case. Prevents vibrations in the case from
inducing an electrical voltage in the PZT of the txdcr
the piezoelectric crystal itself. ½ wavelength thick; impedance is 20x greater
than that of the skin
provides an electrical connection between the PZT and the u/s system;
necessary for transmission and reception
positioned in front of the PZT at the face of the txdcr. ¼ wavelength thick
• increases the efficiency of sound energy transfer b/t the active
element and the body
• protects the active element
bonded to the back of the PZT to reduce “ringing” and create pulses short in
duration and length (improves Axial Resolution); commonly made of epoxy
resin impregnated with tungsten filaments
Definition
electrical energy from the system is converted into sound
the reflected sound pulse is converted into electricity
describes the property of certain materials to create a voltage when they are
mechanically deformed or when pressure is applied to them
Reverse Piezoelectric Effect piezoelectric materials change shape when a voltage is applied to them
Piezoelectric Materials
materials with convert sound into electricity (and vice versa), commonly
made of lead zirconate titanate (PZT)
aka: ceramic, active element, or crystal
decreased sensitivity
during reception, txdcrs with damping material are less able to convert lowlevel sound reflections into meaningful electrical signals
bandwidth
the range of frequencies in the pulse
• (Hz) = highest frequency – lowest frequency
Quality factor
a unitless number that is inversely related to bandwidth
• Q-factor = main frequency / bandwidth
Curie Temperature
the temperature at which PZT is polarized; 300 – 400 degrees Celsius
polarization
depolarization
sterilization
disinfection
the process by which piezoelectric materials are exposed to a strong electrical
field while being heated to a substantial temperature (the Curie
Temperature)
the loss of piezoelectric properties due to exposure of temperatures
exceeding the Curie point; thus, u/s txdcrs should only be disinfected with
Cidex or other cold germicides
the destruction of all microorganisms by exposure to extreme heat, chemical
agents, or radiation
the application of a chemical agent to reduce or eliminate infectious
organisms on an object; attempts to significantly reduce the microbial load
Component
Thickness
Active element (PZT)
Matching Layer
½ wavelength thick
¼ wavelength thick
Damping Material:
Characteristics
Consequences
•
•
Imaging Transducers (PW)
•
•
•
•
•
•
1) decreased sensitivity
2) wide bandwidth (broadband)
3) low quality factor (QF)
high degree of sound absorption
acoustic impedance similar to PZT
pulses with short duration and length (short
“click)
uses backing material to limit ringing
reduced sensitivity
wide bandwidth or broadband
lower Q-factor
improved axial resolution
Continuous Wave Transducer
electrical frequency = acoustic frequency
Non-Imaging Transducers (CW/Therapeutic)
•
•
•
•
•
•
pulses with long duration and length
(continuous wave)
no backing material
increased sensitivity
narrow bandwidth
higher Q-factor (due to no off-time)
cannot even create an image
Pulsed Wave Transducer
acoustic frequency determined by 2 characteristics:
1) speed of sound in the PZT (4 – 6mm/us)
2) thickness of the PZT (0.2 – 1 mm)
f (MHz) = Sound’s Speed in PZT (mm/us)
2 x thickness of PZT (mm)
Characteristics of a High Frequency PW
Imaging Transducer
• thinner PZT crystals
• PZT crystals with higher speeds
Characteristics of a Low Frequency PW
Imaging Transducer
• thicker PZT crystals
• PZT crystals with slower speeds
CH 9 Sound Beams:
Term
Definition
Focus
the location where the beam is the narrowest ( ½ its original diameter); the end of the
near zone; the beginning of the far zone; the middle of the focal zone
aka: focal point
the region from the transducer to the focus; in the beginning the beam diameter, or
width, is same as the transducer
aka: near field, Fresnel zone
the distance from the transducer to the focus
aka: focal depth, near zone length
the region that starts at the focus and extends deeper; where the beam diverges, or
spreads out (beam diameter returns to same width as the txdcr at 2 focal lengths)
aka: far field, Fraunhofer zone
a region around the focus where the beam is relatively narrow; where best images are
acquired; half the focal zone length is located in the near zone and the other half is in
the far field
narrowing of the sound beam
widening or spreading out of the sound beam
adjustable focus or multiple foci
V-shaped waves created by tiny pieces of PZT or Huygen’s sources
aka: diffraction patterns, Huygen’s wavelets, V-shaped waves
states that a large active element may be thought of as millions of tiny, distinct sound
sources that creates a Huygens’ wavelet with a V-shape; explains the shape of an
imaging transducer’s emitted sound beam based upon constructive and destructive
interference
Near Zone
Focal Length
Far Zone
Focal Zone
Convergence
Divergence
Phased Array
spherical waves
Huygens’
Principle
Location
Beam Diameter
at the transducer
at the focus
at 2 near zone lengths
deeper than 2 near zone lengths
beam diameter = transducer diameter
beam diameter = ½ transducer diameter
beam diameter = transducer diameter
beam diameter > transducer diameter
Shallow Focus
•
•
Factors that Affect Focal Depth (for a fixed focus txdcr)
Deep Focus
smaller diameter PZT
lower frequency (thick PZT)
*focal depth is inversely related to wavelength
Less Divergence
•
•
larger diameter
higher frequency (improves lateral resolution in
the far field)
*frequency is inversely related to beam divergence
•
•
larger diameter PZT
higher frequency (thin PZT)
More Divergence
•
•
smaller diameter
lower frequency
CH 10 Axial and Lateral Resolution:
Term
Definition
Resolution
Axial Resolution
accuracy in imaging
parallel to the sound beams main axis, the minimum distance that 2 structures,
positioned front-to-back can be apart and still produce 2 distinct echos; not
adjustable; 0.1 – 1.0 mm (lower the better; shorter pulses)
• (mm) = SPL/2
(mm) = wavelength x # cycles
2
aka: Longitudinal, Range, Radial, or Depth resolution (LARRD)
*improved with higher frequency txdcr because pulses are shorter
perpendicular to the sound beams main axis, the minimum distance that 2
structures, positioned side-by-side can be apart and still produce 2 distinct echos
• (mm) = beam width
aka: Angular, Transverse, Azimuthal resolution (LATA)
*improved in far field only with higher frequency txdcr because of less divergence
the focal depth and the extent of focusing are determined when the transducer is
made; cannot be changed
aka: conventional or mechanical focusing, includes both external and internal
techniques
Lateral Resolution
Fixed Focusing
Better Axial Resolution is associated with the following
•
•
•
•
•
shorter spatial pulse length
shorter pulse duration
higher frequencies (shorter wavelength)
fewer cycles per pulse (less ringing)
lower numerical values
Orientation
Mnemonic
Determined by
Best with:
Does it Change?
In Near Field, best with
In Far Field, best with
Axial
Lateral
front-to-back; parallel to beam
LARRD
spatial pulse length
shortest pulse
highest frequency and fewest cycles
No, same at all depths
shortest pulse
shortest pulse
side-by-side; perpendicular to beam
LATA
beam width
narrowest beam
changes with depth, best at focus
smallest diameter crystal
largest diameter & highest frequency
(least divergence)
Focusing Techniques:
Type
Method
Name
Lens
Curved active element
Electronic
external
internal
phased array
fixed, conventional, or mechanical
fixed, conventional, or mechanical
adjustable
4 Effects of Focusing
•
•
•
•
beam diameter in near field and focal length is reduced
focal depth is shallower
beam diameter in the far zone increases (more divergence)
focal zone is smaller
*only improves Lateral Resolution
Characteristics
Frequency – Continuous Wave
Frequency – Pulsed Wave
Focal Length
Beam Divergence
Lateral Resolution
Axial Resolution
Determinants of Sound Beams
Determined by
frequency of electrical signal from u/s system
thickness and speed of sound of ceramic (PZT)
diameter and frequency of sound of ceramic
diameter and frequency of sound of ceramic
beam width
SPL & PD
CH 11 Display Modes:
Mode
A-mode
(Amplitude)
Displayed as
a single line with a series of
upward spikes;* most
accurate in determining
depth of reflector
B-mode or Ba line of dots of varying
scan (Brightness) brightness; used to describe
any form of gray scale image
M-mode
a group of horizontal wavy
(Motion)
lines that move up and down,
indicating the reflector is
moving closer to or away
from the txdcr; straight
horizontal line indicates a
stationary reflector
*has best temporal
resolution; “sampling rate” is
very high and equal to the
system’s PRF
X-axis
Y-Axis
Z-Axis
depth
amplitude
-
depth
-
brightness/amplitude/strength
of reflector
time
depth
-
CH 12 Two-Dimensional Imaging:
Term
Definition
Elevational Resolution
aka: Slice Thickness, Volume Averaging
answers: are the reflections on the image created from structures directly in
the imaging plane or from structures that lie above or beneath the imaging
plane?
*Best with single, disc-shaped crystals found in Mechanical txdcrs because it
is equal to Lateral resolution
*Poorest with Linear arrays, due to rectangular shape of PZT
overcomes poor Elevational Resolution from rectangular shaped PZTs by
arranging multiple crystals in the up-and-down direction to focus the beam in
the thickness plane.*May comprise of 700 total elements, 100 wide by 7 high.
performed on a frozen image, called Post-processing; creates an image from
3D data acquired during the u/s exam; adding an element of realism (color,
texture, etc)
some of the sound beam energy that spills from the main axis in the Far field
that creates undesirable reflections on the image (artifact) and degrades
Lateral resolution; created by single element transducers.
similar to side lobes, but are created by Phased array transducers, and occur
all along the sound beam; reduce lateral resolution, degrade image quality,
and create artifacts
Most popular technique used to reduce side and grating lobes; stronger
electrical signals are used to excite the inner crystals, and progressively
weaker electrical spikes excite the outer crystals.
a crystal is divided into a group of smaller crystals called sub-elements. These
sub-elements are electrically joined, and act as if they are a single crystal,
which reduces grating lobes.
a technique used to make a sound beam narrow over a greater range of
depths and thus optimizing lateral resolution; when an Array transducer is
used, the u/s system may change the # of crystals used to transmit sound
beams and receive reflected echos. Aperature may be thought of as “listening
or transmit hole”.
aka: variable aperature
a machine that displays Doppler and 2D images
1 ½ D Array
Rendering
Side Lobes
Grating Lobes
Apodization
Subdicing
Dynamic Aperature
Duplex Scanner
Ch 12 cont..
Transducer
Image Shape Steering
Technique
Mechanical
Linear
Sequential/Switched
sector
rectangular
Linear Phased Array
sector
Focusing
Technique
mechanical
electronic;
fixed
electronic
PZTs fired in
small groups;
steering creates
parallelogram
shaped image
*phased and
dynamic receive
electronic
electronic
*phased and
dynamic receive
Annular Phased
sector
mechanical
electronic
Crystals: # and Shape
1, disc-shaped
120-250, rectangular,
1 wavelength wide; up to
10cm wide footprint
100-300, rectangular,
¼ - ½ wavelength wide;
compact footprint 1 x 1cm
4 +, ring-shaped
*phased delays
separated by
microseconds
Convex/Curvilinear/Curved
blunted sector
electronic
electronic
*phased and
dynamic receive
Vector/Virtual Sector
trapezoidal
electronic
electronic
*phased and
dynamic receive
120-250, rectangular,
1 wavelength wide; ; up to
10cm wide footprint
120-250, rectangular,
1 wavelength wide; small
footprint, only a few cms
*time delays in the firing patterns of phased, linear sequential, curved, and vector arrays are separated by 10 nanoseconds
Transducer Type
Mechanical
Linear and Convex Arrays
Linear Phased Array
Annular Phased Array
Active Element Malfunction
Effect on Image
Loss of entire image
Dropout of image information from the top to the bottom of the image.
The location of the line corresponds to the broken crystal.
Erratic steering and focusing. The extent to which the image is affected is
variable.
A horizontal or side-to-side band of dropout at a particular depth
Electronic Pattern (created by the Beam Former)
Sound Beam
Slope
Curvature
Straight Line
Slight Curvature
Higher Curvature
Left element is excited first, across to Right Last
Right element is excited first, across to Left Last
Steering
Transmit Focusing
Straight down/Unfocused
Deeper focus
Shallow focus
Beam steered to the Right
Beam steered to the Left
CH 13 Real-Time Imaging (Temporal Resolution):
Term
Definition
Frame Rate
the system’s ability to create numerous frames each second (Hz); Directly related to
Temporal Resolution and the system’s PRF. Inversely related to Tframe
Temporal
Resolution
Tframe
field of view
line density
Spatial Resolution
“sampling rate”
• PRF = # scan lines x frame rate
Accuracy in time, describes the ability to precisely position moving structures from
instant to instant. Excellent when a system produces many frames per second, and
substandard when it produces few frames per second (Hz).
Time it takes the system to create 1 frame.
• = # pulses x PRP
aka: sector size
the spacing between sound beams
• = # pulses / sector size
image detail; combination of Lateral and Axial resolutions
in M-mode, same as “frame rate”, is very high and equal to the system’s PRF
Factor’s Affecting Frame Rate
•
System Settings Affecting Frame Rate
Speed of Sound in the Medium (*fundamental
•
Imaging Depth
•
# of Pulses per frame
limitation)
•
Imaging Depth
*inversely related to both
Shallow Imaging
•
•
•
•
•
Deep Imaging
•
•
•
•
•
short go-return time
shorter Tframe
higher frame rate
Superior Temporal Resolution
higher frequency txdcr
long go-return time
longer Tframe
lower frame rate
Inferior Temporal Resolution
lower frequency txdcr
Factors Determining Number of Pulses per Frame
•
•
•
number of focal points
sector size
line density
Single Focus
•
•
•
•
•
•
one pulse per scan line
shorter Tframe
higher frame rate
better Temporal resolution
poorer Lateral resolution
good for moving structure imaging
Multi-Focus
•
•
•
•
•
•
many pulses per scan line
longer Tframe
lower frame rate
diminished Temporal resolution
improved Lateral resolution (*main advantage)
good for static structure imaging
Narrow Sector
•
•
•
•
fewer pulses per frame
shorter Tframe
higher frame rate
superior Temporal resolution
Low Line Density
•
•
•
•
•
•
widely spaced lines
fewer pulses per frame
shorter Tframe
higher frame rate
high Temporal resolution
poor Spatial resolution
Wide Sector
•
•
•
•
more pulses per frame
longer Tframe
lower frame rate
inferior Temporal resolution
High Line Density
•
•
•
•
•
•
tightly packed lines
more pulses per frame
longer Tframe
lower frame rate
low Temporal resolution
excellent Spatial (detail) resolution (*main
advantage)
Better Temporal Res- higher Frame Rate
•
•
•
•
shallower imaging (high freq)
single focus
narrow sector size
low line density
Worse Temporal Res- lower Frame Rate
•
•
•
•
deeper imaging (low freq)
multiple foci (improved lateral res)
wide sector size
high line density (improved spatial res)
CH 14 Pulsed Echo Instrumentation:
U/S System 2 Major Functions
1) Preparation and Transmission of electrical signals to the transducer, which creates a sound beam
2) Reception of electrical signals from the transducer, with subsequent processing into clinically
meaningful images and sounds
6 Major Components of U/S Systems
Transducer
Pulser and Beam Former
Receiver
Display
Storage
Master Synchronizer
Term
Pulser Voltage
Pulse Repetition Period
Noise
Signal-to-Noise ratio (S/N)
during transmission, it transforms electrical energy into
acoustic energy. during reception, it converts returning
acoustic energy into electrical energy
creates and controls the electrical signals sent to the
transducer that generate sound pulses.
• Pulser determines the amplitude (power output),
PRP, & PRF
• Beam Former determines the firing delay patterns
for phased arrays
transforms the electrical signals from the transducer into
a form suitable for display
presents processed data. May be a flat screen monitor, a
transparency, a spectral plot, or a variety of other formats
archives the ultrasound studies
maintains and organizes the proper timing and interaction
of the system’s components
Definition
• Effect on Image: the brightness of the entire
image changes
• Adjustable by user
• Concerns BioEffects: Thermal Index and
Mechanical Index.
• range: 0 – 100%
• directly related to S/N ratio
• determines PRP & PRF
aka: Output Gain, Output Power, Acoustic Power,
Transducer Output, Acoustic Output, Pulser Power,
Energy Output, Transmitter Output, Power, or Gain
Determined by Pulser voltage; the time between one
voltage spike and the next
Determines Imaging Depth
a random and persistent disturbance that obscures or
reduce’s a signal’s clarity; contaminates images with
low-level undesirable information
a comparision of the meaningful information (signal) in
an image, compared to the amount of contamination
(noise).
*most commonly improved by increasing Output Power
Beam Former
“transmit-receive” Switch
Channel
Receiver
Amplification
Preamplification
Compensation
Compression
Demodulation
Reject
A sophisticated electronic device that receive’s the
pulser’s single electrical spike and distributes it to the
numerous active elements of an array transducer.
* During reception, it establishes the correct time
delays used for dynamic receive focusing.
* Also adjusts electrical spike voltages to reduce lobe
artifacts via apodization
* in modern txdcrs, it is called a digital beam former
1) protects the delicate receiver components from
the from the powerful signals that are created
for pulse transmission
2) directs the electrical signals from the transducer
to the appropriate electronic and processing
components within the ultrasound system
made up of a single PZT element in the txdcr, the
electronics in the beam former/pulser, and the wire
that connects them
prepares the information contained in the miniscule
reflection signals for eventual display on the system’s
monitor
*see 5 Receiver Functions table*
Each electronic signal returning from the transducer is
made larger; (dB)
The process of improving the quality of a signal before
it is amplified; occurs as close to the active elements as
is practical
Corrects for attenuation with regards to path
length/depth to create an image of uniform brightness;
(dB)
Performed twice:
1) keeps electrical signal levels within the accuracy
range of the system’s electronics
2) keeps an image’s gray scale content within the
range of detection by the human eye (20
shades)
Only affects weak signals; User-controlled compression
modifies the gray scale mapping of ultrasound images;
(dB)
2 part process that changes the electrical signals within
the receiver into a form more suitable for display on a
monitor
1) Rectification: converts all negative voltages into
positive voltages
2) Smoothing/Enveloping: places a smooth line
around the “bumps” and evens them out
Allows the sonographer to control whether low-level
gray scale information within the data will appear on
the displayed image
Dynamic Frequency Tuning
U/S system takes higher frequency signals to create
shallow part of the image & lower frequency signals to
create deeper part of the image
As Low As Reasonably Achievable;
States that when modifications to either output power
or receiver gain can improve the image’s diagnostic
quality, the first and best choice is the one that will
minimize the patient’s ultrasound exposure.
ALARA
Shallow Imaging
•
•
•
shorter listening time
shorter PRP
higher PRF
Deep Imaging
•
•
•
longer listening time
longer PRP
lower PRF
3 Advantages of Digital Beam Formers
•
•
•
software programming
extremely stable
versatile
3 Brightness Controls
•
•
•
Power Output/ Acoustic Output
Time Gain Compensation
Receiver’s Gain
Anatomy of a TGC Curve
Near gain
Delay
Slope
Knee
Far Gain
First part of a TGC curve; At superficial depths, reflections undergo a small,
constant amount of compensation
Second part of a TGC curve; The depth at which variable compensation
begins
Third part of a TGC curve; compensation corrects for the effects of
increasing attenuation that result from increasing path length
Fourth part of a TGC curve; reflections are maximally compensated by the
system
Fifth part of a TGC curve; indicates the maximum amount of compensation
that the receiver can provide
Ch 14 cont..
Receiver Functions
Function (in order) Adjustable
Signals Processed
Effect on Image Synonyms
All signals treated
identically
Signals treated differently
based on reflector depth
Entire image gets
brighter or darker
Image will be
uniformly bright
from top to
bottom
Amplification
Yes
Compensation
Yes
Compression
Yes
Signals treated differently
depending on strength
Changes gray scale
mapping
Demodulation
No
None
Reject
Yes
Prepares electrical signals
to be suitable for display
via Rectification and
Enveloping/Smoothing
Only weak signals affected;
strong signals remain
unchanged
Output Power
•
•
•
•
•
Changes brightness of entire image
Alters signal-to-noise ratio
Alters patient exposure
Bioeffect concerns
Decrease this first if image is too bright
vs
Weak echoes
appear or are
eliminated from
image
Receiver gain
Time Gain
Compensation
(TGC), Depth
Gain
Compensation
(DGC), Swept
Gain
Log
compression,
dynamic range
Threshold,
suppression,
priority control
Receiver Gain
•
•
•
•
•
Changes brightness of entire image
Does not affect signal-to-noise ratio
Does not change patient exposure
No bioeffect concerns
Increase this first if image is too dark
CH 15 Displays and Image Processing:
Bistable
•
•
Grayscale
Images are composed of only 2
shades: black and white
High contrast
Term
Contrast
Brightness
Scan Converter
•
•
•
•
Present multiple levels of brightness: white, light gray,
medium gray, etc.
Assigns different gray shades to different echo amplitudes
Differentiates biologic tissues of different reflectivity
Low contrast
Definition
Determines the range of brilliancies (shades of gray) within the displayed image
Determines the brilliance of the displayed image
Translates the information from the spoke (pulse) format into the video format;
modern technology uses digital scan converters
• Storage of image data = “writing”
• image data is displayed = “reading”
Analog Numbers unlimited and continuous range of “real world” numbers that are found in everyday life
Digital Numbers
Associated with computer devices and have only discrete values
Electrons
Charged particles within an analog scan converter that contain the image information
and are shot out of the gun to the silicon wafer
Dielectric matrix Contained in the larger end of the tube of an analog scan converter which receive and
or Silicon wafer
store the electrons; may be thought of as picture divided into millions of tiny dots, each
containing an electron bucket which are read to retrieve the image information.
Spatial resolution Image detail
Pixel
Picture element, the smallest building block of a digital picture; a single shade of gray at
any instant in time
Pixel Density
The number of picture elements per inch
Bit
Binary digit, the smallest amount of computer memory; bistable, having a value of
either 0 or 1
Binary number
A group of bits and is simply a series of zeroes and ones
Byte
A group of eight bits of computer memory (i.e.: 10011111)
Word
Consists of 2 bytes, or 16 bits
Analog
Real World
Digital
Computer World
Analog-to-digital Converts electrical signals from the transducers during reception into digital form: a
(A-to-D)
string containing only 0s and 1s.
converter
Preprocessing
Any process of the reflected signals before storage; controlled by the sonographer;
cannot be reversed or undone
Postprocessing
Any process after storage in the digital scan converter; controlled by the sonographer;
can be reversed
Digital-to-analog Translates digital signals back into analog form to be displayed on analog video display
(D-to-A)
converter
Magnification
Zoom, enlargement of a portion of the image to fill the entire screen
ROI
Region of Interest, the selected part of the image
Read
Magnification
Write
Magnification
Coded Excitation
Spatial
Compounding
Frequency
compounding
Edge
Enhancement
Temporal
Compounding
Fill-in
Interpolation
Elastography
PACS
DICOM
Occurs after the image data is stored (postprocessing); system scans the anatomy,
image data is converted from A-to-D and stored in the scan converter, sonographer
identifies the ROI and the system reads and displays only the original data that pertains
to the ROI; nothing is rescanned, # of pixels remains the same, but are enlarged
Applied during data acquisition, but before storage (preprocessing); system scans the
anatomy, image data is converted from A-to-D and stored in the scan converter, ROI is
identified and the system discards existing info in the scan converter, ROI is rescanned
and new data is written into the scan converter; # of pixels is increased, improving
spatial resolution, pixels are the same size.
A sophisticated method, developed within the context of Bioeffects, of improving image
quality. Creates very long sound pulses containing a wide range of frequencies, keeping
the peak intensity below the FDA’s limit. Special mathematical techniques shortens the
long reflected pulses improving resolution and providing high imaging quality. *Occurs
in the pulser.
Image is obtained by acquiring multiple frames from different angles and then
combined to create a single real-time image; electronic beam steering is required, thus
this is only available with phased array txdcrs; the more frames acquired in the
compound sequence, the better the image quality; reduces speckle and shadowing
artifacts; limitations: reduced frame rate and temporal resolution
Image is created using the entire reflection with all reflected frequencies from a single
pulse down a scan line; reduces speckle artifact and reduces noise.
An image processing method that makes pictures look sharper; increases the image
contrast in the area immediately around the edge; creates subtle bright and dark
highlights on either side of these boundaries to make them appear more defined.
Aka: Persistence, temporal averaging. Image processing technique that continues to
display the same view with a “history” from earlier frames creating a smoother image
with reduced noise, higher S/N ratio, and improved image quality. Limitation reduction
in frame rate, reducing temporal resolution.
With sector shaped images, the distance between scan lines increases as depth
increases; this method of image processing (preprocessing) fills in the gaps of missing
data by predicting the gray scale levels from known levels of neighboring pixels;
increases line density, improving spatial resolution; *improves the ability to precisely
visualize the borders of round structures. Reduces temporal resolution
Emerging technique that relates images to the mechanical properties of tissue; tissues
will deform differently following the application of a force, estimates of tissue
stiffness/elasticity are obtained and combined with ultrasound reflections to create an
elastogram; potential as a complimentary tool for the diagnosis of cancer as it can be
applied to differentiate malignant from benign lesions
Picture Archiving and Communications System; physical computer network/laboratory
in which images and additional medical information are digitized and stored on.
3 Advantages:
1) Instant access
2) No degradation of data
3) Ability to electronically transmit images an reports to remote sites
Digital Imaging and Computers in Medicine; a set of rules that allows imaging systems
on the network to communicate
Analog Numbers
•
•
•
Digital Numbers
•
•
•
•
Real world, 0-9
Unlimited # of choices
Continuous values
Computer world, 0 & 1
Limited choices
Discrete values
Binary numbers
Analog Scan Converter
•
•
•
•
Digital Scan Converter
1st
type of scan converter, making gray scale
imaging possible
Funnel-shaped vacuum tube with an electron
gun located in its smaller end and a dielectric
matrix in the larger end
Excellent Spatial Resolution because of the
large number of storage elements within the
matrix
Obsolete due to the following limitations:
1) Image Fade- stored charges on the silicon
wafer dissipate over time
2) Image Flicker- caused by switching
between read and write modes
3) Instability- picture quality depends on
many factors including length of use,
room temperature, and humidity
4) Deterioration- image degrades as device
ages
Low Pixel Density
•
•
•
•
# of
bits
1
2
3
4
5
10
n
Used in modern ultrasound machines
•
Uses a process called digitizing by computer
technology to convert images into numbers;
2 important elements are: pixel and bit
Image is stored as series of ones and zeros
and then processed and re-translated into an
image displayed on a monitor
Advantages include:
1) Uniformity- consistent gray scale quality
throughout the image
2) Stability- does not fade or drift
3) Durability- not affected by age or heavy
use
4) Speed- nearly instant processing
5) Accuracy- error-free
•
•
High Pixel Density
•
•
•
•
Few pixels per inch
Larger pixels
Less detailed image
Lower spatial resolution
Bits and Shades of Gray
Exponent (Max) Number of
Shades
21
22
23
24
25
210
2n
•
2
4
8
16
32
1024
Many pixels per inch
Smaller pixels
More detailed image
Higher spatial resolution
Digital Images
Pixels
•
•
•
Image element
Image detail
Spatial Res
Bits
•
•
•
Computer memory
Gray shades
Contrast Res
Fewer Bits per Pixel
•
•
Fewer shades of gray
Degraded contrast resolution
More Bits per Pixel
•
•
More shades of gray
Improved contrast resolution
Translating Analog and Digital Information
1) Voltage is always analog and must be converted to digital form (a string of 0s and 1s) by the analogto-digital (A-to-D) converter.
2) Digital information is stored, or “written”, in the scan converter’s computer memory. *Any
processing before storage in the scan converter is preprocessing
3) Image information, in digital form, continues to be processed by the u/s system’s computer. *Any
processing after storage in the digital scan converter is postprocessing
4) If the display unit is analog (TV monitor), then the image information must be translated from digital
form back to analog by a digital-to-analog (D-to-A) converter. If the display is a digital unit (flat
screen) then there is no need for a D-to-A converter.
5) The signal is presented on the display for interpretation.
Preprocessing
•
•
•
•
•
•
•
Time gain compensation (TGC)
Log compression/ Dynamic Range
Write magnification
Persistence/ Temporal compounding
Spatial compounding
Edge enhancement
Fill-in interpretation (phased array txdcrs only)
Read Magnification
Postprocessing
•
•
•
•
•
Write Magnification
•
•
•
3 steps
Uses old data
Postprocessing
•
•
•
•
•
•
•
Larger pixel size
Same # of pixels as in the original ROI
Unchanged spatial resolution
Unchanged temporal resolution
•
•
•
•
Coded Excitation provides…
•
•
•
•
•
Higher S/N ratio
Improved axial resolution
Improved spatial resolution
Improved contrast resolution
Deeper penetration
Any change after freeze frame
Black/white inversion
Read magnification
Contrast variation
3-D rendering
4 steps
Acquires new data
Preprocessing (cannot be used on a frozen
image)
Identical pixel size
More pixels than the original ROI
Improved spatial resolution
May improve temporal resolution, if the
bottom of the ROI is shallower than the
Original image’s DOV
Types of Data Storage:
Type of Medium
Paper Media
Examples
•
Charts from pen
writers
Advantages
•
•
Portability
Does not require a
device to read
Disadvantages
•
•
•
Magnetic Media
•
•
•
•
Chemically Mediated
Photographs
Optical Media
•
•
•
•
•
“Floppy”
Computer Discs
Computer
memory
Magnetic Tape
Video Tape
Photographs
Flat films
Multiformat
camera film
Laser discs
Compact discs
(CDs)
•
•
•
•
•
•
•
•
•
Able to store large
amounts of
information
efficiently
Can store and play
dynamic (moving)
images
Can record color
High resolution
Accepted in the
medical community
Can produce color
images
•
Store huge
amounts of data
Inexpensive
Not erased by
exposure to
magnetic fields
•
•
•
•
•
Bulky, hard to store
Difficult to make
copies
Cannot display
dynamic images
Can be erased by
strong magnetic
fields
Bulky, difficult to
store and retrieve
Requires chemical
processing
Artifacts can arise
from dirt or
chemical
contamination
Requires a display
system
No standardized
format for image
display and storage
CH 16 Dynamic Range:
Term
Definition
Dynamic Range
# of choices; a method of reporting the extent to
which signals vary and can still produce accurate
images
- Units: dB
- A comparison, a relative measure, or a ratio,
between the largest and smallest signals that
are measured accurately
Smallest signals that are accurately measured; min.
intensity which should be exceeded to get a certain
change
- Signals below the threshold are too weak and
will not be recognized by the system and is
tossed out
Largest signals (max. intensity) that are accurately
measured
- Signals above the saturation point are too
strong and are read as the max. intensity
value (ex: if a signal is 7dB and the sat. point
is 6.5dB then the signal is read as 6.5dB)
= (threshold) to (saturation point)
Ability to distinguish the different/ adjacent reflector
and still produce similar echoes
Limited choices; bistable (black and white)
- High contrast
- Low contrast resolution
Threshold
Saturation Point
Range of accurate measurement
Contrast Resolution
Narrow Dynamic Range
Wide Dynamic Range
Many choices; gray scale imaging (possible due to
digital scan converter)
Compression
Low contrast
High contrast resolution
Performed by the u/s machine so it can display the
image; signal ranking remains the same, but the
range is changed: smallest signal remains smallest
value and largest signal remains largest value
-
Adjustable by Sonographer via Gray Scale
Mapping
*General Rule: the dynamic range of information decreases the more it is processed*
U/S System Component
Dynamic Range
Transducer
Receiver
Amplifier
Scan Converter
Display
Archive
Human Eye
120 dB
100 - 120 dB
60 – 100 dB
40 – 50 dB
20 - 30 dB
10 – 30 dB
10 – 20 dB * always less than display
Mathematics of Compression: decibels add or subtract
• Uncompressed signal – amount of compression = dynamic range of compressed signal
• Compressed signal + amount of compression = dynamic range of original signal
Images and their Dynamic Ranges
Fewer Shades
•
•
•
•
•
Few choices
Black and white (bistable)
Narrow dynamic range
High contrast
Low contrast resolution
More Shades
•
•
•
•
•
Many choices
Gray scale
Wide dynamic range
Low contrast
High contrast resolution
CH 17 Harmonics and Contrast Agents:
Term
Definition
Fundamental Frequency
Frequency of sound transmitted into the body by the
transducer; main frequency (or) fixed frequency of
the transducer
- 1st Harmonic
Twice the fundamental frequency; resonant
frequency
- 2nd Harmonic
- Sound waves arise from nonlinear behavior
Image created by main frequency
- Creates superficial image
- More distortion, lobe artifacts
The creation of an image from sound reflections at
twice the frequency of the transmitted sound.
- Improves the image quality because
harmonic frequencies undergo less distortion
(do not create lobe artifacts) than
fundamental frequency
- 2 Forms: Tissue & Contrast
Means irregular or disproportionate, asymmetrical;
Difference of speed between compression (fast) and
rarefaction (slow); creates a “spilling” of energy, or
harmonics
Means proportional or symmetrical; responds in an
even manner
Resonance of tissue: Created by the conversion of a
miniscule amount of energy from the fundamental
frequency to the harmonic frequency of a sound
wave as it travels through the body during
transmission (nonlinear behavior)
- Do not develop in very superficial layers
- Develop in deeper/intermediate layers
- Disappear in deepest layers
- Increases S/N ratio
- Distortion free
- No grating lobes
- Arise from main axis, strongest part of the
sound beam
Most commonly used technique that separates
fundamental freq. from harmonic freq.: 2
inverted/opposite consecutive pulses are
transmitted down each scan line.
1) Fundamental signals exhibit linear behavior,
thus destructively interfere and completely
cancel each other out, leaving only the
harmonic portion of the reflections
Harmonic Frequency
Fundamental Image
Harmonic imaging
Nonlinear behavior
Linear behavior
Tissue Harmonics
Pulse Inversion Harmonics
Power Modulation
Contrast Agents
2) Harmonic signals exhibit nonlinear behavior,
they interfere constructively and produce
echoes of higher quality
Disadvantage:
- Temporal Resolution is reduced
Another technique used to augment harmonic
reflections, while eliminating distorted fundamental
reflections: 2 consecutive pulses are sent down each
scan line, the second pulse is 2x the strength of the
first one. The first, weaker pulse contains no
harmonics.
- During reception, reflections from the first
pulse (fundamental only) are amplified to
DOUBLE which then cancels out the
fundamental frequency of the second pulse’s
reflections, leaving only the harmonic
frequency portion of the reflection to create
an echo of higher quality
Disadvantage:
- Temporal Resolution is reduced
Microbubbles; gas bubbles encapsulated in a shell,
that are either ingested or injected in the circulation,
designed to create strong reflections that actually
“light up” blood chambers, vessels, or other
anatomic regions
• 5 requirements:
1) safe
2) metabolically inert (inactive)
3) long lasting
4) strong reflector of u/s
5) small enough to pass through capillaries
- Microbubbles are strong scatterers of sound
because they are the same size as RBCs and
resonate when exposed to freq. of 2-4MHz
• 2 Characteristics:
1) The nature of the outer shell should be
flexible
2) The gas molecule that fills the
microbubble; Larger gas molecules find
the shell less permeable and remain
trapped within the bubble
Contrast Harmonics
Mechanical Index
Much stronger than tissue harmonics; Created
during reflection as energy is converted from the
fundamental freq. to the harmonic freq. Nonlinear
changes in size of microbubbles when struck by
sound waves, or resonance: bubbles expand to a
greater extent than they shrink.
- Peak rarefaction pressure, which expands
the bubble, is most important with regard to
contrast harmonics
Advantages:
- Increased spatial resolution
- Increased contrast resolution
- Reduces artifact
A number that estimates the amount of contrast
harmonics produced; depends on the frequency
(inversely related) of the transmitted sound and
peak rarefaction pressure (directly related) of the
sound wave.
MI = peak rarefaction pressure
√frequency
Other Non-linear Behavior of Tissue Harmonics that minimizes distortion:
Beam Strength
Amount of Tissue Harmonics created
Weak
Intermediate
Strong
none
Tiny amount
Significant amount
Summary of Tissue Harmonics
•
•
•
•
•
Not present as sound leaves the transducer
Created deeper in the tissues
Created in the tissues during transmission
Created by nonlinear behavior in the speed of sound: Sound in compressions travels faster than
sound in rarefactions
Primarily created along the beam’s main axis; beams that are more likely to create harmonics are
least likely to create artifacts
Lower MI
•
•
•
Small pressure variation
Higher frequency
Less Bioeffects
Higher MI
•
•
•
Large pressure variation
Lower frequency
More bioeffects
Low MI: < 0.1
•
•
•
•
•
•
No harmonics
Backscatter
Linear behavior
Higher frequency sound
Low beam strength
Bubble expands very
little
Higher MI: 0.1 to 1.0
•
•
•
•
•
•
Some harmonics
Resonance
Nonlinear behavior
Lower frequency sound
Higher beam strength
Bubble expands moderately
Tissue Harmonics
•
•
•
•
•
•
Strongest harmonics
Bubble disruption
Extreme nonlinear behavior
Lowest frequency sound
Highest beam strength
Bubble expands greatly
Contrast Harmonics
•
•
Created during transmission in tissue
Occurs as sound propagates in tissue
•
•
•
Results from nonlinear behavior of
transmitted sound beam
Weaker harmonic signal
•
•
Highest MI: > 1.0
•
•
Created during reflection off of microbubble
Occurs only when contrast agents are
present and with MIs > 0.1
Results from nonlinear behavior of
microbubble
Stronger harmonic signal
Affected by the microbubble’s shell and the
gas within it
CH 18 Hemodynamics:
Term
Definition
Hemodynamics
Flow
Study of blood moving/circulating through the circulatory system
Aka: Volume flow rate; indicates the volume of blood moving during a
particular time. Flow measurements answer the question “how much?”
- Units: L/min (volume/time)
- 3 Types: Pulsatile, Phasic, Steady
Indicates the speed or swiftness of a fluid moving from one location to
another. Velocity answers the question “how fast?”
- Units: cm/s (distance/time)
Occurs when blood moves with a variable velocity. Blood accelerates and
decelerates as a result of cardiac contraction; therefore, pulsatile flow
commonly appears in the arterial circulation.
Aka: Spontaneous flow; Also occurs when blood moves with a variable
velocity. Blood accelerates and decelerates as a result of respiration;
therefore, phasic flow often appears in the venous circulation
Occurs when a fluid moves at a constant speed or velocity. Present in the
venous circulation when individuals stop breathing for a brief moment.
Normal physiological flow; flow streamlines are aligned and parallel;
characterized by layers of blood that travel at individual speeds. Silent flow
- 2 Types: Plug flow, Parabolic flow
- Reynolds number of < 1,500
All layers and blood cells travel at the same velocity
Bullet shaped profile; velocity is highest in the middle (center of the
lumen), and then gradually decreases to its minimum at the vessel wall.
Predicts whether flow is laminar or turbulent; ratio between viscosity and
inertia
Abnormal flow characterized by chaotic flow patterns in many different
directions and at many speeds; streamlines are often obliterated;
associated with cardiovascular pathology and elevated blood velocities
seen downstream from a significant stenosis in a vessel. Converts flow
energy into other forms such as sound and vibration.
- Eddy current/swirling: small, hurricane-like, swirling, rotational
patterns appear in turbulent flow
- Reynolds number of > 2,000
Sound associated with turbulent flow
Tissue vibration associated with turbulent flow; aka: palpable murmur,
which is one you can feel with your fingertips
Difference of energy; blood moves from regions of high energy to low
energy by contraction of the heart during systole.
Associated with all moving objects; determined by:
1) an object’s mass
2) the speed at which it moves
KE = ½ MV2
Velocity
Pulsatile Flow
Phasic Flow
Steady Flow
Laminar Flow
Plug Flow
Parabolic Flow
Reynolds Number
Turbulent Flow
Murmur, Bruit
Thrill
Energy Gradient
Kinetic Energy
Pressure/ Potential Energy
Gravitational Energy
Viscosity
Viscous Energy Loss
Hematocrit
Frictional Energy Loss
Inertial Energy Loss
Inertia
Stenosis
Post-stenotic turbulence
Bernoulli’s Principle
Ohm’s fluid Law
Veins
Hydrostatic Pressure
A form of stored energy; has the ability to perform work; a major form of
energy for circulating blood and creates flow by overcoming resistance
PE = Mass x Gravity x Height
A form of stored or potential energy associated with any elevated object
Describes the thickness of a fluid
- Units: Poise
More energy is lost with movement of high viscosity fluids; viscous loss is
associated with blood overcoming its internal stickiness; determined by
Hematocrit
The percentage of blood made up of RBCs; normal value = 45%
Occurs when flow energy is converted to heat as one object rubs across
another, i.e: blood rubbing/sliding against vessel walls
Energy is lost when the speed of a fluid changes, regardless of whether the
fluid speeds up or slows down; occurs during 3 events:
1) Pulsatile flow (arterial)
2) Phasic flow (venous)
3) Velocity changes at a stenosis
Relates to the tendency of a fluid to resist changes in its velocity
A narrowing in the lumen of a vessel
Turbulent flow downstream from the stenosis
A modified law of conservation of energy; describes the relationship
between velocity (kinetic energy) and pressure in a moving fluid (blood)
- When velocity is high, pressure is low
- When velocity is low, pressure is high
Pressure gradient = flow x resistance; Resistance to flow increases
whenever there is a stenosis; in the circulatory system, the resistance
vessels are called arterioles
thin-walled, collapsible, low-resistance vessels; allows for large volume
increases with very small pressure increases by changing shape from an
hourglass, to oval, to round as inflow increases.
Pressure related to the weight of blood pressing on a vessel measured at a
height above or below heart level; units: mmHG
- Hydrostatic pressure is 0mmHG everywhere in a supine patient, or
at heart level in an upright patient
- Hydrostatic pressure is negative at locations above the heart in an
upright patient, creating erroneously low measured pressures
- Hydrostatic pressure is positive at locations below the heart in an
upright patient, creating erroneously high measured pressures:
Measured pressure = circulatory press. + hydrostatic pressure
Effects of a Stenosis
•
•
•
•
•
Change in flow direction
Increased velocity as vessel narrows
Turbulence downstream from the stenosis
Pressure gradient across the stenosis
Loss of pulsatility
A- “Upstream”, flow reversal seen during
diastole; pressure reduces, velocity increases
B- Area of stenosis, velocity is maximum, pressure
is lowest
C- “Downstream”, Pressure is higher than B, but
less than A; flow reversal seen during systole;
post-stenotic turbulence
Ohm’s Law – Fluid
•
•
•
Ohm’s Law- Electricity
•
•
•
Pressure
Flow
Resistance
Voltage
Current
Resistance
Measurement
Site
Blood Pressure
(example)
Level
Hydrostatic
Pressure
Measured
Pressure
Ankle
knee
Waist
Mid chest
Top of head
140 mmHg
140 mmHg
140 mmHg
140 mmHg
140 mmHg
Far below heart level
Somewhat below heart
Slightly below heart
Heart level
Above heart
100 mmHg
75 mmHg
50 mmHg
0 mmHg
-30 mmHg
240 mmHg
215 mmHg
190 mmHg
140 mmHg
110 mmHg
Breathing and Venous Flow
Inspiration
Expiration
•
•
•
•
•
Diaphragm moves downward toward the
abdomen
Thoracic pressure decreases
Abdominal pressure increases
Venous return to the heart increases
Venous flow in legs decreases
•
Diaphragm moves upward into thorax
•
•
•
•
Thoracic pressure increases
Abdominal pressure decreases
Venous return to the heart decreases
Venous flow in legs increases
CH 19 Doppler:
Term
Definition
Doppler principle
Physical principle used to measure the velocity of blood in the
circulation; (velocities sampled every second)
Produced by relative motion of sound source (transducer) and receiver
(RBCs); relative motion of RBCs towards or away from the txdcr that
causes a change in the main frequency is called Doppler frequency;
The Doppler shift is a low frequency that “rides” on top of the much
higher txdcr frequency
Doppler shift (Hz) = reflected freq. – transmitted freq.
The process of extracting the low Doppler frequency from the txdcr’s
carrier frequency
When blood cells move toward the txdcr, the reflected frequency is
higher than the transmitted frequency;
When blood cells move away from the transducer, the reflected
frequency is lower than the transmitted frequency
Purely a magnitude, indicates the distance that a RBC moves in 1 sec
Defined by a magnitude and a direction
Doppler shift = 2 x velocity of blood x txdcr freq. x cosine
Propagation speed
Cosine: angle between sound and vessels
2: represents the fact that there are actually 2 doppler shifts
1st shift = reception of sound wave by moving RBC
2nd shift = transducer’s reception of the sound wave from the
moving RBC
Doppler shift/ Doppler frequency
Demodulation
Positive Doppler Shifts
Negative Doppler Shifts
Speed
Velocity
The Doppler Equation
Measured velocity
Bidirectional Doppler
Phase quadrature/ Quadrature
detection
Continuous Wave (CW) Doppler
Range ambiguity
Duplex imaging/scanning
= true velocity x cosine
Distinguishes the direction of flow toward or away from the transducer
with spectral tracing, and/or audio: spectrum below the baseline
indicates a negative shift, or flow away from the txdcr; spectrum above
the baseline indicates a positive shift, or flow toward the txdcr
A commonly used signal processing technique for bidirectional Doppler
Requires 2 crystals in the txdcr, one always transmitting, the other
always receiving.
(+) ability to accurately measure very high velocities
(+) very small doppler shifts can be detected (highly sensitive)
(+) artifact free
(-) cannot determine exact location of the moving blood cells (range
ambiguity) due to overlap of transmit and receive beams
(-) lack of TGC
(-) anatomic imaging cannot be performed with CW txdcr
A disadvantage of CW doppler; Depth of reflections cannot be
determined because reflections are being measured from entire
overlap area of transmit and receive beams
Simultaneous anatomic imaging and Doppler
Fusion: 2D US + MRI/CT
Pulsed Wave (PW) Doppler
Range resolution
Aliasing
Nyquist limit
Gray Shades of a Spectrum
Color Flow Doppler
Color Map
Velocity Mode
1 PZT alternates between transmit and receive; velocities are sampled
many times per second; sampling rate = system’s PRF
(+) range resolution
(-) inaccurate measurement of high velocity signals, aliasing
(-) cannot pick up very low doppler shifts
Greatest advantage of PW Doppler; the ability to select the exact
location where velocities are measured by placing a small marker,
called the gate, or sample volume, on a 2D image. The US system then
calculates the ToF for a sound pulse traveling to and from the gate.
(aka: range specificity, freedom from range ambiguity artifact)
Most common error associated with Doppler ultrasound: High velocity
flow in one direction is incorrectly displayed as traveling in the
opposite direction; a false identity
• Never occurs with CW Doppler
• Occurs when scanning deeper and Doppler sampling rate (PRF)
is too low in comparison to the measured blood velocities
The highest Doppler frequency or velocity that can be measured
without the appearance of aliasing; The very top of the spectral graph
display when blood flow is toward the txdcr, and the very bottom of
the display when flow is away from the txcdr
Nyquist limit (Hz) = PRF
2
Related to amplitude of the reflected signal (# of blood cells creating
the reflection)
A form of 2D Doppler, where velocity information is coded into colors
and superimposed on a 2D gray scale, anatomic image. Reports
average or mean velocities.
- Is a pulsed u/s technique
- Has range resolution (or range specificity)
- Is subject to aliasing
- Is considered semi-quantitive
- Provides info regarding direction of flow
- Uses a Color map
- Knowledge of angle between directions of sound and flow is
less important than compared to CW or PW Doppler
A “dictionary” or lookup table, used by color flow Doppler to convert
measured velocities into colors that appear on the image. Displayed as
a vertical bar of various colors, with a black bar in the center which
indicates “no Doppler shift”.
2 most common dictionaries:
- Velocity mode
- Variance mode
The color provides information on flow direction and velocity. Colors
above the black stripe indicate flow towards the txdcr, or positive
Doppler shifts. Colors below the black stripe indicate flow away from
the txdcr, or negative Doppler shifts.
- Velocity maps with multiple colors above and below the black
stripe indicate slower velocities closer to the black, and faster
Variance Mode
Doppler Packets
Power Doppler
Spectral Analysis
Fast Fourier Transform (FFT)
Spectral Window
Spectral Broadening
Autocorrelation
velocities farthest from the black, either towards or away from
the txdcr (above or below the black stripe)
- Color change is always up and down, never side to side
Same information provided as velocity mode, but also distinguishes
laminar flow from turbulent flow
- Color change is side-to-side, as well as up and down:
• L:L - Left is Laminar
• R:T – Right is Turbulent
With color Doppler, multiple u/s pulses per scan line are used to
accurately determine blood velocities (aka: ensemble); with larger
packets or longer ensemble lengths are composed of larger number of
pulses per scan line:
(+) More accurate velocity measurement
(+) Increased sensitivity to low flow
(-) more time needed to acquire data
(-) reduced frame rate
(-) decreased temporal resolution
(-) subject to aliasing (mixing of colors)
* Packet size should be selected to balance 2 competing interests:
accurate velocity measurements and adequate temporal resolution
(aka: energy mode or color angio) Non-directional color Doppler; no
velocity or direction information, only detects flow: the amplitude of
the reflection is directly related to the number of moving blood cells.
(+) not angle dependent
(+) no aliasing
(+) sensitive to low flow or velocity (such as venous flow)
(-) no measurement of velocity or direction
(-) lower frame rates than conv. color flow (reduced temp. reso.)
(-) prone to “flash artifact” with slight motion of txdcr, patient, or
soft tissues
A tool that breaks the complex signals into its basic “building blocks”
and identifies velocities that make up the reflected Doppler signal
2 Methods:
- Fast Fourier Transform (FFT)
- Autocorrelation
A digital technique used to process both PW and CW Doppler signals
(+) exceedingly accurate
(+) displays all individual velocity components that make up the
complex reflected signal; distinguishes laminar from turbulent flow
The region of a (FFT) spectral trace between the baseline and the
spectrum; when clear = laminar flow, spectral broadening = turbulent
flow
With turbulent flow, the pulsed Doppler spectral window is filled in; a
wider range of velocities and Doppler shifts within the sample volume
(aka: correlation function) the digital technique used to analyze color
flow Doppler because of the enormous amount of data that is
processed.
(+) substantially faster than FFT
(-) less accurate than FFT
Doppler Shift measured in Hertz
•
•
•
Directly related to velocity
Directly related to transducer frequency
Directly related to the cosine of the
angle between the direction of the flow
and the direction of sound
Angle (degrees)
Cosine
Percent of True Velocity
0
1.0
30
0.87
60
0.5
90
0
120
-0.5
150
-0.87
180
-1.0
*at angles other than 0 and 180, only a portion of the true velocity is
measured
Factors that affect Aliasing
Less Aliasing
•
•
•
Slower blood velocity
Lower frequency transducer
Shallow gate (high PRF)
100%
87%
50%
0%
50%
87%
100%
More Aliasing
•
•
•
Faster blood velocity
Higher frequency transducer
Deep gate (low PRF)
5 Techniques that Eliminate Aliasing (use in order):
Method
Strategy/ (-) disadvantage
1) Adjust the scale (PRF) to its
maximum
2) New, Shallower view (sample
volume)
3) Lower transducer frequency
Increases Nyquist limit
(-) less sensitive to very small Doppler shifts/low velocities
Increases Nyquist limit (PRF)
(-) None
Decreases Doppler shift/ reduces height of doppler spectrum
(-) resolution degrades/ lower quality anatomic image
4) Use baseline shift (down/zero)
Aliasing remains, but display is more appealing
(-) ineffective if Doppler shift wraps around itself completely
5) Use CW Doppler
Never aliases, but (-) range ambiguity
*Eliminating aliasing “improves the ability to measure the maximum velocity with Doppler”
PW Doppler
•
•
•
•
Range resolution
Sample volume
Limited maximum velocity – Nyquist
Aliasing
CW Doppler
•
•
•
•
Range ambiguity
Region of overlap
Unlimited maximum velocity
No aliasing
Pulsed Doppler Transducer
•
•
•
•
•
CW Doppler Transducer
At least one crystal
Dampened PZT (backing material)
Low Q-factor
Wide bandwidth
Lower sensitivity
•
•
•
•
•
At least 2 crystals
Undampened PZT (no backing material)
High Q-factor
Narrow bandwidth
Higher sensitivity
Imaging
Doppler
•
•
•
•
•
•
•
•
Normal incidence – 90 deg
Higher frequency – improves resolution
Pulsed wave only
Minimum of 1 crystal
0 or 180 deg
Lower frequency – avoids aliasing
Pulsed or CW
Min. of 1 (pulsed) or 2 (CW) crystals
Doppler Artifact
Doppler modality/ Appearance
How to eliminate
Ghosting
Clutter
CF- spilling of color outside vessel
PW – low doppler shifts appear below spectral
baseline
CF- “mirror image” appears as second vessel
color flow below anatomic ROI
PW- “mirror image” appears as identical
Doppler spectrum both above and below
baseline
Increase Wall Filter (reject)
Increase Wall Filter (aka: high pass
filters)
Decrease Gain
Check angle between txdcr and
vessel
Crosstalk
Summary of Doppler Modalities
Continuous Wave
Pulsed Wave
•
Identifies highest
velocity jets
anywhere along
the length of the
u/s beam
(-) range ambiguity
• Most sensitive
•
Accurately
identifies the
location of flow
(+) range resolution
• Moderately
sensitive
(+) Very good temporal
resolution
(+) Very good temporal
resolution
(+) no aliasing
(-) subject to aliasing
•
Peak velocity
measurements
•
Peak velocity
measurements
Color Flow
•
Provides 2D flow
information
directly on
anatomic image
(+) range resolution
• Moderately
sensitive
• Size of color jet is
most affected by
Doppler gain
settings
(+/-) Reduced temporal
resolution due to
multiple packets
(-) based on pulsed u/s,
subject to aliasing
• Mean velocity
measurements
Power Mode
•
Used with low
velocity or small
volume blood
flow
(+) range resolution
• Greater
sensitivity than
color flow
(-) lowest temporal
resolution
(-) subject to flash
artifact, not to aliasing
• NO velocity
measurements
CH 20 Optimizing Doppler Imaging:
System Setting
Importance
How it affects Doppler
image
Steering of Color Box
-Normal incidence, or 90deg angle,
between the direction of flow and the
direction of the sound beam will result in
NO Doppler shift. The cosine of 90 is 0.
-NO color appears on image with
Normal Incidence.
-The Color Box should always be steered at
any angle other than 90
-Too high of color gain can cause color to
appear throughout the color box
-Color is created in the lumen of
the blood vessels
-Color confetti appears
throughout color box
-Too low of color gain
-No color is present
Color Doppler Gain
Spectral Doppler Gain
*Be sure to check angle of color box to
vessel before adjusting gain
-Too high pulsed Doppler gain
-Too low pulsed Doppler gain
Wall Filter
-increasing Wall Filter rejects low Doppler
shifts
-does not affect appearance of higher
velocity flows
Velocity Scale (PRF)
-Increasing the PRF, or velocity scale, will
make the transducer less sensitive to low
Doppler shifts
• Refer to CH 20 in the text for image comparisons
-gray scale noise appears
throughout the spectrum
-ALL gray scale will disappear in
the image: noise and meaningful
Doppler spectrum
-Ghosting of color doppler is
eliminated
-with pulsed Doppler, eliminates
low velocity flows near the
baseline
-Aliasing still appears
-Aliasing will be eliminated; low
velocity flow vessels will be absent
color with color doppler
CH 21 Artifacts:
Term
Definition
Artifact
Hyperechoic
an error in imaging
Portions of an image that are brighter than
surrounding tissues, or tissues that appear brighter
than normal
Portions of an image that are not as bright as
surrounding tissues, or tissues that appear less
bright than normal.
An extreme form of hypoechoic, meaning entirely
without echoes (echo-free).
Describes structures with equal echo brightness
A portion of tissue or an image that has similar echo
characteristics throughout
A portion of tissue or an image that has differing
echo characteristics throughout
Six assumptions are incorporated into the design of
every ultrasound machine (see table below).
Artifacts arise when these assumptions are not true.
Reduces an image’s noise content. Increases the
signal-to-noise ratio (2x the fundamental freq)
Hypoechoic
Anechoic
Isoechoic
Homogenous
Heterogenous
Assumptions
Harmonic imaging
Types of Artifacts
•
•
•
•
Not real
Not seen on the image
Incorrect shape or size
Incorrect brightness
Causes of Artifacts
•
•
•
•
Violation of assumptions
Equipment malfunction or poor design
The physics of ultrasound
Operator error
Six Assumptions of Imaging Systems
1)
2)
3)
4)
5)
6)
Sound travels in a straight line
Sound travels directly to a reflector and back.
Sound travels in soft tissue at exactly 1,540 m/s
Reflections arise only from structures positioned in the beam’s main axis
The imaging plane is very thin
The strength of a reflection is related to the characteristics of the tissue creating the reflection
Appearance
Possible Cause
Anatomic reflectors absent on image/ less # of
reflectors
Anatomic reflector appears multiple times on
image. Artifact positioned deeper than the true
anatomy/ vertical misregistration
Anatomic reflector appears multiple times on
image. Artifact displaced to the side of the true
anatomy/ horizontal misregistration
Anatomic reflectors appear with abnormal
brightness
Anatomic structures appear at incorrect depth
Anatomic structures appear in the incorrect
imaging planes
Anatomic structures do not correspond to echoes
on the image
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Shadowing
Shadowing by refraction/ Edge Shadowing
Lateral Resolution
Axial Resolution
Comet tail
Ring down
Reverberation
Mirror image
Refraction
Side lobe
Grating lobe
Enhancement (hyperechoic)
Focal Banding (hyperechoic)
Shadowing (hypoechoic)
Shadowing by refraction/ Edge Shadow
(hypoechoic)
Speed errors
Range ambiguity artifact
Slice or section thickness/ Elevational
Resolution artifact
Acoustic speckle
Multipath
** See “Artifacts Table” Document**
CH 22 Quality Assurance:
Term
Definition
Quality Assurance
the routine, periodic evaluation of an ultrasound system to
guarantee optimal image quality. Medical and legal necessity for
every laboratory. The sonographer is responsible for implementing
a QA action plan that is based on objective standards.
Completely unbiased. Is factual, repeatable, and able to be
counted. Is the same, even when obtained from different people.
Not affected by an individual’s previous experience, preference, or
taste.
One that is influenced by an individual’s experience or beliefs.
Often, it cannot be verified using concrete facts and figures. Is
affected by opinion, belief, or assumption and frequently varies
from person to person.
Composed of TMM (tissue mimicking material) embedded with
nylon strings to produce reflections at strategic locations,
structures that mimic hollow cysts, and solid masses. Are similar to
soft tissue in the following ways:
- Speed of sound (1540 m/s)
- Attenuation
- Scattering characteristics
- Echogenicity
Evaluates:
- Gray scale, TGC, A.O., Foci, Calipers, Axial and Lateral
Resolutions
Evaluate all modalities of Doppler. 3 Types:
- Flow phantoms (most common)
- Vibrating string
- Moving belt
Determines beam profile/ elevational resolution. Contains a diffuse
scattering plane that is at an angle to the incident sound beam
(linear phantom). When u/s beam is overly thick, cystic structures
may appear filled in.
Refers to the ability of a system to display low-level echoes. Is
assessed when the sonographer adjusts the system controls to
change echo brightness from barely visible to full brightness
(saturation). Evaluated in 2 ways: Normal and Maximum
Normal sensitivity settings should not vary from one routine
evaluation to the next. All pins, solid masses, and cystic structures
in the test phantom are accurately displayed. Output Power, TGC,
and amplification (gain) are adjusted to establish normal
sensitivity.
Evaluated with Amplification (gain) and Output Power of the
system set to maximum practical levels. Depth of tissue-like texture
on the display is measured. Maximum visualization depth is used to
assess sensitivity and should not differ from one routine evaluation
to the next.
Objective Standard/Statement
Subjective Standard/Statement
Tissue Equivalent Phantom
Doppler Phantom
Slice Thickness Phantom
Sensitivity
Normal Sensitivity
Maximum Sensitivity
Dead Zone
Registration Accuracy
Range Accuracy/Vertical depth
calibration
Depth calibration
Horizontal Calibration
Digital Calipers
Axial Resolution
Lateral Resolution
Uniformity/ Compensation
Operation
Display, Hardcopy Output (printout),
and Gray Scale Dynamic Range
The region close to the txcdr where images are inaccurate. It
extends from the txdcr to the shallowest depth from which
meaningful reflections appear. Results from txdcr ringing and the
time it takes the system to switch from transmit to receive mode.
- Higher frequency = thinner dead zone
- Lower frequency = thicker dead zone
• An acoustic standoff, or gel pad, or 50cc IV bag positioned
b/t the txdcr and the patient allows accurate imaging of
important superficial structures.
• An increasingly deeper dead zone = cracked PZT, detached
backing material, or a longer PD.
The ability of the system to place reflections in proper positions
while imaging from different orientations
The system’s accuracy in placing reflectors at correct depths
located parallel to the sound beam. If differences appear, error
may be caused by:
- System malfunction
- Speed of sound in tissue phantom is different than 1540m/s
The accuracy of reflector depth positioning in A-mode, M-mode, Bmode, and 2D imaging
The system’s ability to place echoes in their correct position when
the reflectors are perpendicular to the sound beam
Used to measure distances of structures during exams. Should be
evaluated in both vertical and horizontal directions.
Evaluated by scanning sets of successively closer spaced pins within
the phantom
Evaluated by measuring the width of reflections on the display that
are created by point targets in the phantom.
The system’s ability to display similar reflectors in the phantom
with echoes of equal brightness. With proper TGC, identical
reflectors should have the same appearance on the monitor,
regardless of their depth.
Adjusting the system’s output power and amplification of the
system should alter the appearance of the image on the system’s
display and all output devices. Adjusting brightness or contrast of a
monitor should only alter the image on the display.
QA Requirements
QA Goals
Assessment of system
components
Guarantee proper
operation of the system
Repairs
Detect gradual changes
Preventative
maintenance
Minimize downtime
Record keeping
Reduce the number of
non-diagnostic exams
Reduce number of
repeat scans
QA Devices (TMM)
1) Tissue equivalent
phantom
Measures:
- Gray scale
- TGC (normal and
Max)
- A.O.
- Foci
- Calipers
- Axial and Lateral
Res.
2) Doppler phantom
(measures all
doppler
modalities)
Types:
- Vibrating string
- Moving belt
- Flow phantom
3) Beam profile/
slice thickness
phantom
(measures
Elevational Res.)
QA Methods
test under known,
defined conditions
Use constant instrument
settings
Use a phantom with
measurable
characteristics
Image in an identical
environment
CH 23: Sonographers in the Clinical Setting
Term
Definition
Major Principles of Medical Ethics
1) Respect for autonomy
2) Nonmaleficence
3) Beneficence
4) Justice
The patient has the capacity to act intentionally,
with understanding, and with free will. The basis of
informed consent. In making voluntary,
knowledgeable decisions, the patient is assumed to
be of sound mind.
Avoid needless harm or injury to the patient,
whether by action or inaction. Requires medical
competence.
The actions of health care providers should benefit
the patient.
Fairness. People who are equals should qualify for
equal treatment, regardless of age, gender,
educational background, and other factors.
The process by which patients are educated about
the essentials of a medical procedure which
includes:
- The nature of the procedure
- Reasonable alternatives
- The risks, benefits, and uncertainties related
to each alternative
- Assessment of the patient’s understanding
- The patient’s acceptance
When to question a patient’s ability to participate in
decision making:
- Underage (<18)
- Patient is incapacitated or incapable
- Patient does not speak English. A translator is
required, but only the patient can provide
consent.
Revocation of consent by the patient can occur at
any time. The sonographer must end the exam as
soon as it is safe to do so.
Upon meeting a patient:
- Treat the patient respectfully. Introduce
yourself, and describe your role and the
procedure.
- Identify the patient before starting an exam
NEVER provide a clinical interpretation of the exam
to the patient!
Respect for autonomy
Nonmaleficence
Beneficence
Justice
Informed Consent
Patient-Sonographer Interaction
Interpretation of the Exam
Dignity
Ergonomics
Causes of Injury
Standard Precautions
SOPs (Standard Operating Procedures)
the quality of being worthy, honored, or esteemed;
includes a patient’s perception of being in control
and having self-worth. Every patient has the right to
be treated in a dignified manner, one that is
respectful and ethical.
Factors of loss of dignity:
- Advanced age
- Infirmity
- Lack of privacy
Studies the interaction between the sonographer,
patient, and equipment in order to optimize the
well-being of sonographers in their professional
environment. More than 3 out of 4 sonos experience
WRMSIs and nearly 20% end their careers as a result
of these injuries.
Repetitive motions, forceful or awkward
movements, poor posture, improper positioning,
strain, and pressure on joints for extended periods.
A set of guidelines to minimize the exposure and risk
of health care workers when in contact with a
patient. Based on the idea that all patients should be
treated as potentially infectious.
A detailed work practice program that helps prevent
WRIs.
Sonographer should know the location of SOPs.
Supervisor is responsible to make sure all employees
are following SOPs.
CH 24: Bioeffects
Term
Definition
Bioeffects
Hydrophone/Microprobe
Effects of sound on living tissue created by intense sound beam
Similar to a small hypodermic needle with a tiny piece of PZT
attached to its end; connected by a wire to an oscilloscope.
Measures the characteristics of a sound beam at specific
locations. A voltage from the hydrophone relates to the pressure
and is displayed on the oscilloscope. Also measures: Period, PRP,
PRF, PD, Amplitude, and DF.
Provides known relationship b/t the acoustic pressure signal and
the voltage created by the PZT. Intensities and other output
measures can be derived from the signals of a calibrated
hydrophone.
- Output Power is highest = Pulsed Doppler
- Output power is lowest= gray scale imaging
- Output power is intermediate= CF Doppler and M-mode
Constructed from a very thin membrane of PZT plastic. A very
small area, located at the center of the membrane, is pressure
sensitive: Only detects intensity
Force exerted on tissue by the sound beam. If the target is a
balance or a float, the measured force relates to the power in
the beam. Sheer stresses and streaming of fluids can distort or
disturb biologic structures.
Based on the interaction of sound and light.
A shadowing system that allows us to visualize the shape of a
sound beam in a medium
Process of conversion of energy to heat
1) Calorimeter
2) Thermocouple
3) Liquid crystal
Measures total power/ entire intensity in a sound beam through
absorption
A tiny electronic thermometer. Can measure intensity at a
particular location
Change color based on their temperature
A primary mandate regarding clinical ultrasound is that the
benefits to the patient must outweigh the risks of the exam.
- Diagnostic ultrasound has no known harmful bioeffects
- Under controlled circumstances, bioeffects are beneficial.
For example: therapeutic u/s is widely used to treat
musculoskeletal injuries
The science of identifying and measuring the characteristics of
an u/s beam that are relevant to its potential for producing
biological effects.
Within the living body
Calibrated hydrophone
Membrane hydrophone
Radiation Force/ Feedback
Microbalance
Acousto-Optics
Schlieren
3 Devices that measure Output of txdcr
by Absorption:
Calorimeter
Thermocouple
Liquid Crystal
Risk-Benefit Relationship
Dosimetry
In vivo
In vitro
“in glass”; outside the living body
- Research indicates that very high intensities can cause
genetic damage and cell death
- Important in the research of bioeffects
- Bioeffects are real even though they may not apply to
clinical setting
- Bioeffect research that claims direct clinical significance
(w/o in vivo validation) should be viewed with caution
Mechanistic Approach
1 of 2 techniques used to study bioeffects. Searches for a
relationship b/t cause & effect
Empirical Approach
1 of 2 techniques used to study bioeffects. Searches for a
relationship b/t exposure & response
• The strongest conclusions are made when the mechanistic and empirical conclusions are in
agreement.
Mechanistic
Empirical
• Broad exposure range can be evaluated (+)
• No need to understand mechanism (+ and -)
• Uncertainty about assumptions (-)
• Biological significance is obvious (+)
• Are other mechanisms involved? (-)
• Species differences may alter results (-)
• Is the bioeffect clinically significant? (-)
Thermal Mechanism/ Index (TI)
A useful predictor of maximum temperature increase under
most clinically relevant conditions. Any rise in temperature from
37֯C, by 2-4C,֯ or exposure time is more than 50 hours, harmful
bioeffects will occur. Temp. of 41֯C + during a testicular exam can
cause infertility. Fetal tissues are less tolerant than adult tissue,
but no harmful bioeffects have been observed below 39֯C
- FDA has set SPTA regulatory limit to 720 mW/cm²
- TIS: soft tissue
- TIB: bone
- TIC: cranial bone
Nonthermal Mechanism
Consist of cavitation (implosion of gaseous nuclei, bubbles, or
contrast agents) and radiation force (microstreaming of fluids).
Cavitation
Interaction of sound waves with microscopic, stabilized, gas
bubbles (gaseous nuclei) in the tissues. Describes the creation of
gaseous nuclei from dissolved gases in a fluid.
Stable cavitation
Occurs at lower MI levels. Gaseous nuclei expand and contract,
or oscillate, but do not burst
Transient/Normal/Inertial cavitation
Occurs at higher MI levels. Bubble-bursting! Produces highly
localized, violent effects such as colossal temps & shock waves
(enormous pressures).
• The pressure threshold for transient cavitation is only
10% higher than that required for stable
Stable
•
•
•
Oscillating bubbles
microstreaming and shear stresses
lower MI
Transient
•
•
•
•
also called normal or inertial
bursting bubbles
higher MI
Shock waves and very high temperatures
Mechanical Index (MI)
Lower MI
•
•
•
Less cavitation
Less pressure
Higher frequency
Epidemiology
Limitations of Epidemiologic Studies
A calculated number related to the likelihood of harmful
bioeffects from cavitation. Related to 2 sound wave
characteristics:
1) Peak rarefaction pressure
2) Lower frequency
MI = Peak Rarefaction Pressure
√frequency
Higher MI
•
•
•
More cavitation
More pressure
Lower frequency
A branch of medicine associated with population
studies and the prevalence of disease. It is Empirical
(exposure & response) retrograde – uses clinical
surveys. Most deal with in utero fetal exposures to
u/s because:
- Large percentage of pregnant women are
scanned
- u/s is routinely used during normal
pregnancies
- harmful effects, if present, have the potential
to affect the fetus for life
Evaluates:
- fetal weight
- Abd. Circumference
- Head circumference
- Femur length
- Congenital abnormality
- APGAR scores
- Hearing
- Infection
Data indicates that u/s exposure is not associated
with adverse fetal outcome.
1) Studies are often retrograde. *Antegrade
studies are always better.
2) Ambiguities may exist in the data (ie:
justification for the exam, gestational age,
number of scans, technique, and exposure
time)
3) Risk factors other than exposure to u/s may
precipitate a bad outcome in the fetus.
Including: environmental factors, poor
nutrition, smoking, or alcohol and drug
abuse.
Best Epidemiologic Studies are
Clinical Safety and Prudent Use Conclusions of the
AIUM:
Training and Research Conclusions of the AIUM:
Electrical Safety
Overall Safety Considerations
Prospective and Randomized
1) The advantage of a prospective study is that
a complete and accurate compilation of
meaningful information is obtained
2) Randomized studies create 2 groups of
patients. The advantage of randomized study
is that other risk factors that could negatively
affect fetal outcome are present in both
groups and can be accounted for.
• No confirmed harmful bioeffects from
exposure to diagnostic u/s have ever been
reported
• It is possible that bioeffects may be identified
in the future
• The benefits to the patient outweighs the
risks
• It is appropriate to diagnostic u/s prudently
to provide benefit to the patient
• It is inappropriate to use diagnostic u/s in a
non-medical setting for entertainment
• No confirmed bioeffects on patients or sonos
have been found with the use of diag u/s
• Experience with diag u/s may differ from
research and training, due in part to longer
research exams and greater exposure
• When used w/o direct medical benefit to the
patient, the subject should be informed of
how the research study differs from standard
diag procedures.
The greatest risk arises form electrical shock from a
cracked transducer housing. In addition, image
quality may be compromised when using damaged
transducers
• Only perform studies with valid medical
justification
• ALARA: do not prolong studies
• Minimize patient exposure
BE:
• Prudent
• Careful
• Judicious