Ultrasound
rev 2016-02-25
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Ultrasound Imaging
D. J. McMahon 150221
rev cewood 2016-02-25
Key Points
Ultrasound:
Know the physics of sound waves
- average speed of ultrasound in human tissue
- typical frequencies used in ultrasound at deep vs shallow depths
Know the basics of an ultrasound transducer
Recognize B-mode and M-mode images
Know the three types of transducer
Know what color Doppler shows
What is a ultrasound phantom, and when would it be used ?
excellent (long) writeup:
http://folk.ntnu.no/stoylen/strainrate/Ultrasound
Ultrasound History
• 1st use in medical diagnosis was in the mid 1940’s by
Dr. Karl Dussik using transmission ultrasound.
• Medical Ultrasound post WWII from SONAR
(SOund Navigation And Ranging)
John J. Wild, MD
“The Father of Ultrasound”
using reflected echography.
"Use of high-frequency ultrasonic
waves for detecting changes of
texture in living tissues"
Lancet March 1951
Ultrasound History
Physical Properties
• About Sound Waves
• Definitions
• Types of Echos
• Formation of Ultrasound Waves
1985
1990
1995
About Sound Waves
• Sound waves are mechanical
• Moving disturbance
• Disturbance advances, not the medium
• Carries energy
• Require a transmission medium
• Sound wave propagate through liquids (e.g. human body) as
longitudinal waves by displacing molecules within the medium.
About Sound Waves
Ultrasound (U/S) : sound with frequency > 20kHz.
(actual frequencies in medical use: 2 - 40 MHz)
• Advantages:
1.U/S can direct as a beam.
2.It obeys the laws of reflection and refraction.
3.It is reflected by objects of small size.
• Disadvantages:
1.It propagates poorly through a gaseous medium.
2.The amount of U/S reflected depends on the acoustic mismatch
(which define the boundaries of the tissue).
Definitions
• Acoustic Impedance - dependent on the density of the material in
which sound is propagated through. The more dense the material
the greater the impedance.
• Reflection - the portion of a sound that is returned from the
boundary of a medium.
• Refraction - the change of sound direction on passing from one
medium to another.
• Acoustic Mismatch - the boundary between two different media
where reflection and refraction occurs.
• Attenuation - the decrease in amplitude and intensity as a sound
wave travels through a medium.
Reflection & Propagation
• Propagation poorly
through gas
• Reflects through
dense zones –
• Structures below
dense zones are
poorly imaged.
• Examples of dense
materials - bone,
calcium, metal.
Diagnostic Frequencies
• Determined by PZT Thickness
• Typically 2x the thickness = the wavelength
• Frequency selected for operating depth
• Lower frequency for greater depth
• Higher frequency for better resolution
Application
Frequency
Depth
Cardiac
2 – 7 MHz
2 – 18 cm
Abdomen
3 – 5 MHz
2 – 16 cm
Vascular
5 – 10 MHz
1 – 5 cm
OB / GYN
3 – 7 MHz
3 – 12 cm
Ultrasound in Tissue
When Ultrasound Travels Through the Tissues, Four Events Occur:
1)
Propagation – the transfer of acoustic energy
• Intensity & Velocity
2)
Absorption – the conversion of acoustic energy into heat
• Attenuation & Penetration
3)
Reflection – the process of sending or bending back acoustic energy
incident on a tissue interface
4)
Refraction – the process of bending a traveling wave from its linear
propagation path at an interface that has a change in the propagation
velocity
Velocity of Ultrasound
Propagation Velocity is directly proportional to density
• Averages: 1540 m/sec in tissue
• 1 cm / 13 msec
Ultrasound Velocities in various materials
dry air
gelatine (10%)
(340 m/s)
tooth
brass
natural rubber
steel
glass
bone
lung
0
1000
gall stone
2000
3000
4000
5000
6000
speed of sound (m/s)
skin
muscle
brain
saline
fat
1400
water
1500
tendon
eye lens
blood
1600
speed of sound (m/s)
1700
Attenuation Rate
Tissue
Propagation Velocity m/sec
Attenuation dB /cm /MHz
Blood
1,549 – 1565
0.18
Fat
1,476
.63
Liver (normal)
1,585
.94
Liver (diseased)
1,570
.97
Kidney
1,558 – 1,572
1.0
Spleen
1,570
0.7
Heart Muscle
1,568 – 1,580
1.8
Bone
3,406 – 4,030
5.0
Skull
3,360
4.2
Air
330
12
Water
1,480
0.002
Average attenuation is 1dB / MHz / cm
Sound Wave Formation
receiver
• Piezoelectric Transducer
• most common material:
lead zirconate titanate (PZT)
• Frequency based on thickness
• Acts as receiver & transmitter
transmitter
Zones:
Sound Profile
Near - the region of a sound beam
in which the beam diameter
decreases as the distance from the
transducer increases. This zone is
called the Fresnel (Fra-nel, the s is
silent) zone.
Focal - the region where the beam
diameter is most concentrated
giving the greatest degree of focus.
Far - the region where the beam
diameter increases as the distance
from the transducer increases. This
zone is called the Fraunhoffer zone
Types of Echoes
• Specular - echoes
originating from relatively
large, regularly shaped
objects with smooth
surfaces. These echoes are
relatively intense and angle
dependent. (i.e. IVS, valves)
• Scattered - echoes
originating from relatively
small, weakly reflective,
irregularly shaped objects
are less angle dependant
and less intense. (ie. blood
cells)
Specular Reflection
Insonification angle : 0°
Ultrasound is perpendicular to the
three layers of different materials.
(insonification = holding the probe)
(specular = related to a point)
Insonification angle : 45°
The three layers are rotated with an angle of
45° to the ultrasound incident direction.
Depending on the material the ultrasound is
reflected diffuse or directed. The left and
right materials are diffuse reflectors, while
the material in the middle is a strong
reflector because the ultrasound is not
reflected back to its origin at this angle.
Beam Refraction
As the Air –water
interface refracts the
light beam, so do
changes in tissue
density refract an
ultrasound beam.
Scattering Reflector
Scattering (Non-Specular or Diffuse) Reflector.
e.g. Red Blood Cells
Acoustic Scattering occurs when an
object is encountered that is less than
the size of the wavelength
A-Mode in ultrasound:
Essentially an oscilloscope display of location vs depth.
Not used as an ultrasound image.
The “B” is for Brightness
The “A” is for Amplitude
B-Mode in ultrasound:
Presents the spatial changes in echoes in real time. Typically a
“pie-shaped” display in which the sensor is at the top.
The “B” is for Brightness
M-Mode in ultrasound:
A presentation of the temporal changes in echoes in which the
depth of echo-producing interfaces is displayed along one axis
and time is displayed along the second axis.
The “M” is for Motion
The reflected signals can be displayed in three different modes.
A-mode (Amplitude) shows the depth and the reflected energy from each scatterer.
B-mode (Brightness) shows the energy or signal amplitude as the brightness (in this case
the higher energy is shown darker, against a light background) of the point.
The bottom scatterer is moving. If the depth is shown in a time plot, the motion is seen as a
curve, (and horizontal lines for the non moving scatterers) in a M-mode plot (Motion).
C-Mode in ultrasound:
Uses both A and B modes, starting by using A-Mode to create
a specific region which is then scanned with B-Mode
ultrasound to create an image of that region.
Color Doppler Mode Images
Typical Block Diagram (legacy)
SCANNER
XDCR
1
TRANSMITTERS
RECEIVERS
64 - 512 CH
XDCR
SELECT
TIMING/CONTROL
RF
TO
IF
PROBE
ID
PULSERS
TGC’S
XDCR
2
TO
SCAN
CONV
KEYBD
KNOBS
FOOT
SW
DISK
DRIVES
ROM
RAM
SCAN CONVERTOR
A/D
PREPROCESSING
X,Y TO TV RASTER
MEMORY
CLOCKS
POWER SUPPLIES
110/220
50/60H
Z AC
5 VOLT
DIGITAL
FROM
SCANNER
CPU
POSTPROCESSING
VIDEO AND
GRAPHICS
I/O
MONITORS, CAMERAS, VCRS
PRINTERS
ANALOG
VOLTAGES
PROGRA
MABLE
HIGH
VOLTAGE
PULSER
Ultrasound System Parts
Display
Keyboard
CPU
Power
Supplier
Recorders
Probe Connector
Disk Storage
Average Size 26 W x 35 D x 54 H
Average weight: 450 lbs
System Parts (modern)
Basic Transducer
Converts energy forms
• Electrical signal into pressure
wave (speaker)
• Pressure wave into electrical
signal (microphone)
Coaxial cable
Transducer housing
Acoustic absorber
Backing block
Electrodes
Piezoelectric crystal
Acoustic Lens
Transducer Parts
Bend Relief
Handle
Nose Piece
Cable
Acoustic Lens
Probe Nomenclature
Transducer Types
Type
Geometry
Linear Array
Phased Array
Curved Array
Application
Vascular, Small Parts,
Musculoskeletal, OB
Cardiac, Upper
Abdomen, Pelvic
Abdominal, OB,
Renal, Urological
Beam Steering
Beam Steering
- wave summation
- probe frequency
- inter-element pitch
Focus / Beam Steering
Focus
- Delay lines
analog
digital
- Resolution
axial
lateral
Multiple element array
38 - 42 ga. coax
Case
Backing
Piezoelectric
Layer
Element
Matching
Layer
Acoustic Lens
RF Shield
X-ray Image
… What’s inside?
Acoustic Lens
Acoustic Array
Flex Circuit
… What breaks ?
Backing Delamination
• Increased Pulse Length
• Reduced Axial Resolution
Lens Delamination
• Loss of signal
• Fluid infiltration
• Cross contamination
Broken Cables
• Loss of signal
• Loss of focus
• Increased noise
Cracked / Depolorized Element
• Loss of signal
• Loss of focus
• Reduced lateral resolution
• Increased noise
38 - 42 ga. coax
RF Shield
• RFI susceptibility
• Noise immunity
Cracked Case
•Fluid infiltration
•Cross contamination
•Lens delamination
•Acoustic stack delamination
2D Array
2D Array
Resolution Measurements
Determine Image Quality:
• Axial
• Lateral
• Detail
• Contrast
• Temporal
Axial Resolution
Contrast Resolution
Lateral Resolution
Axial Resolution
Axial resolution is the ability to resolve targets that lie along the length of
the beam.
Axial resolution is directly proportional to the transducer frequency.
The higher the frequency the higher the axial resolution.
This state results from the shorter wavelength.
Nc
PD =
f(MHz) ms
Pulse Duration
2
1
3
T
PD
N= # of cycles
Lateral Resolution
Ability to separate targets side to side
Key Factors:
• Beamformer accuracy
• Element pitch
• Aperture size
• Transducer frequency
• Element height
Detail Resolution
• Ability to differentiate
between adjacent
targets
• Combination of axial
and lateral resolution
Contrast Resolution
• Ability to differentiate
between small differences
in tissue densities
• Also known as Dynamic
Range
• Improves with Harmonic
Imaging
Harmonic Imaging
Splenic mass
• Transmit at fundamental
• Receive and process 2nd harmonic
• Reduces haze and clutter
• Improved Contrast Resolution
Temporal Resolution
Temporal resolution
• A measure of the time needed to create an
image. For an image to be a real-time
application, it must generate images at a rate
of at least 30 per second. At this rate, it is
possible to produce clear image of the
beating heart.
• Also known as frame rate
• Will be slower in color flow
• Will be slower with increased depth settings
Doppler
Doppler Effect
1992 Austria stamp honors Johann
Christian Doppler who in the mid-1800s
proposed the theory for what is today
known as Doppler ultrasound. First
investigated by Japanese scientists in
the 1950s, Doppler color ultrasound is
an important tool for today's
sonographer and physician.
Doppler
How Doppler Works
Flow Velocity Calculation
A measure of frequency shift
expressed as velocity on ultrasound
systems
Transmitted frequency
Reflector velocity
f =
2fo * v * cos 
Angle of incidence
c
Constant velocity of
sound in soft tissue
Frequency shift
Spectral Doppler
Doppler Processing
• Pulsed
•Continuous Wave
• High PRF
(pulse repetition freq)
Angle Correction
cos is the cosine of the angle
Between the transmitted beam
and the reflector path
1.0
Cosine
0.5
0.0
-0.5
-1.0
45
90
135
Angle (°)
180
Doppler Trade-offs
Disadvantages Advantages
Continuous
Wave
Accurately
measures high
velocity flows
Lacks range
resolution
Pulsed wave
Aliasing of velocities
above the Nyquist
limit (inability to
measure high
velocities
accurately)
Ability to measure
velocities at a
specific location
(range resolution)
Color Flow Doppler
Phase Shift

v = 4f
c
1
PRF
Color Flow Doppler
New Advancements
Extended FOV
3D / 4D Ultrasound
Clinical Uses
Musculoskeletal
Cardiac
Surgical
OB / GYN
Abdominal
OB Ultrasound
Sonogram
OB Ultrasound
Sonogram
Cardiac Ultrasound
Echocardiography
Trans-Esophageal Echo (TEE) Cardiogram
https://www.youtube.com/watch?v=8cjK8a-kK7Q
The ‘BladderScan’
by Verathon
Compact & Portable Models
GE’s Pocket Scan
GE’s V-Scan
Ultrasound test equipment
(not for typical field repair)
Electrical Safety
checks of
TEE probes:
Probes are “applied parts”.
Probe is submerged in a bath and
tested with a device that interfaces
with the bath and the probe
connector.
Ultrasound testing phantoms --
Tissue Phantoms
Tissue Phantoms
1.0 mm
What they do and do not
Measure
• System performance more
than probe performance
2.0 mm
• System controls remove the
objectivity
• Tissue phantoms need to be
calibrated
1.0 mm
• Useful for measurement and
geometry checking
0.5 mm
0.25 mm
Major Players in
Ultrasound Systems:
> Philips
> Hewlett-Packard
> Siemens
> Sonosite (Fuji)
> GE
> Diagnostic Ultrasound
Popular Ultrasound Systems
Major Players in Ultrasound
Probes and Service: