Sound
• Sound beam is similar to x ray beam as both waves
transmit energy
DIAGNOSTIC
ULTRASOUND
X RAYS
Wave type
Longitudinal waves Electromagnetic
waves
transmission
Elastic medium
No medium
generation
Stressing the
medium
Accelerating
electric charges
velocity
Depends on the
medium
constant
Similar waves
Seismic, acoustic
Radio, light
Longitudinal Waves
• Ultrasound pulses are
transmitted, as longitudinal
waves, ie; the motion of
particles in the medium is
parallel to direction of wave
propagation.
• They produce bands of
compression and rarefaction
• Wave length is distance
between two bands of
compression or rarefaction
“Ultra”…….sound?
• Audible range is 20 to 20,000 cycles per
second
• Ultrasound has frequency greater than
20,000 cycles per second
Velocity of Sound
• It is independent of frequency ,
• but depends on material through which it
propagates
• Depends on
– Compressibility
– Density
Velocity of Sound
1. Compressibility
–
–
velocity is inversely proportional
liquids and solids propagate sound more rapidly than
gases (easily compressed)
2. Density
–
denser materials have greater inertia - so, decreased
velocity
ULTRASOUND CHARACTERISTICS
•
•
•
•
•
Frequency
Velocity
Wavelength
Amplitude
Intensity and Power
Frequency
•
The frequency of sound is
determined by the source.
(2-20MHz)
v=fλ
• In ultrasonic frequency range, the
velocity of sound is constant in
any particular medium.
f
1

• If the frequency increases then
the wave length must decrease as
they are inversely proportional to
each other.
Velocity
• The significance of
ultrasound velocity is
that it is used to
determine the depth
location of structures in
the body
• The period (T) is the
time required for one
vibration cycle. It is the
reciprocal of the
frequency
Amplitude
• The amplitude of an
ultrasound pulse is the
range of pressure
excursions , related to the
energy content.
• In diagnostic applications,
it is usually necessary to
know only the relative
amplitude of ultrasound
pulses.
• Units of decibels (dB).
Intensity
• Also called loudness
• Determined by amplitude of oscillation
• As amplitude is increased , intensity also
increases
Intensity
• Intensity is the rate at which ultrasound energy is
applied to a specific tissue location within the
patient's body.
• It is the quantity that must be considered with
respect to producing biological effects and safety
• The intensity of most diagnostic ultrasound
beams at the transducer surface is on the order
of a few milliwatts per square centimeter.
History
• First successful application – SONAR in world
war 2 (SOund Navigation And Ranging)
Successful medical application – 1940s
Uses of ultrasonic energy in the 1940s. Left, in gastric ulcers. Right, in arthritis
Ultrasonic
therapy
generator, the
"Medi-Sonar"
in the 1950s.
A British
ultrasonic
apparatus for
the treatment
of Meniere's
disease in the
late 1950s
History
Denier's Ultrasonoscopic apparatus with
ultrasound generator, emitter transducer
and oscilloscope. This can be adapted for
both therapeutic and diagnostic purposes
The first hand-held imaging instrument was
developed by John Wild and John Reid in the
early 1950's
The Present
Components
1.
2.
3.
4.
5.
6.
7.
Transmitter
Transducer
CPU
Display
Key board / cursor
Disc storage device.
Printer
Transducer
• Instrument which converts one form
of energy to other
• The conversion of electrical pulses to
mechanical vibrations and the
conversion of returned mechanical
vibrations back into electrical energy.
Electrical Energy Mechanical Energy
SELECTION OF TRANSDUCER
• Superficial vessels and organs within 1 to 3cms
depth and intra operative imaging –
• 7.5 to 15 Hz
• Deeper structures in abdomen and pelvis within
12 to 15cms –
• 2.25 to 3.5Hz
Transducer - Parts
• A simple single-element,
plane-piston source
transducer has major
components including the
–
–
–
–
–
–
–
Piezoelectric material,
Sensor electrodes,
Insulated layer,
Backing block,
Acoustic insulator
Insulating cover, and
Transducer housing.
Piezoelectric Element
• The active element is basically a piece of polarized
material - a piezoelectric ceramic sandwiched
between electrodes
• The piezoelectric element converts electrical signals
into mechanical vibrations (transmit mode) and
mechanical vibrations into electrical signals (receive
mode).
Piezoelectric materials
• Natural – Quartz
• Artificial
– most of USG materials
– ferroelectrics
• barium titanate
• PZT (lead zirconate titanate)
ADVANTAGE – they can be
formed into different
shapes
Ceramic  Piezoelectric Crystal?
• The piezoelectric attributes are attained after a
process of
- Molecular synthesis, (dipoles)
- Heating, (Curie Temperature)
- Orientation of internal dipole structures with an
applied external voltage, (Poling)
- Cooling to permanently maintain the dipole
orientation, and
- Cutting into a specific shape.
Ceramic  Piezoelectric Crystal?
• Once the material has cooled, the dipoles
retain their alignment.
• Heating the crystal above this temp reduces
its usefulness. So, transducers should not be
autoclaved.
Piezoelectric crystal
• At equilibrium, there is no
net charge on ceramic
surfaces.
• When compressed, an
imbalance of charge
produces a voltage between
the surfaces - piezoelectric
effect
• Similarly, when a voltage is
applied between electrodes
attached to both surfaces,
mechanical deformation
occurs - electrostriction
Piezoelectric crystal – how thick?
• The thickness of the
active element is
determined by the
desired frequency of
the transducer
• Piezoelectric crystals
are cut to a thickness
that is 1/2 the desired
radiated wavelength
Backing/Damping Block
• The rear face of the
piezoelectric crystal
material is usually
supported by a backing
material which is
tungsten loaded araldite,
so that the vibrations in
the piezoelectric material
are rapidly damped after
the initial excitation.
Backing/Damping Block
• This component also dampens the transducer vibration
to create an ultrasound pulse with a short spatial pulse
length, which is necessary to preserve detail along the
beam axis (axial resolution).
Couplant
• Material (usually liquid)
that facilitates the
transmission of
ultrasonic energy from
the transducer into the
test specimen.
• Necessary to overcome
the acoustic impedance
mismatch between air
and solids.
Modes of Vibration
2 TYPES:
1. thickness mode
•
•
most common
Used in medical
crystals
2. radial mode
Resonant Frequency
• Natural frequency to which the transducer is
sensitive
• Resonant frequency determined by thickness of
crystal
• Thick crystal – low frequency sound
• Natural frequency – one that produces internal
wavelengths that are twice the thickness of crystal
• Frequency corresponding to half the wavelength isfundamental resonant frequency
Transducers
Resonance Transducers
Non Resonance Transducers
Resonance Transducers
• They are manufactured to operate in a
“resonance” mode, whereby a voItage
(commonly 150 V) of very short duration (a
voltage spike of 1 msec) is applied, causing
the piezoelectric material to initially contract,
and subsequently vibrate at a natural
resonance frequency.
• The operating frequency is determined from
– the speed of sound in, and
– the thickness of, the piezoelectric material.
Resonance Transducers
• Higher frequencies are
achieved with thinner
elements, and lower
frequencies with thicker
elements.
Nonresonance (Broad-Bandwidth)
“Multifrequency” Transducers
• Modern transducer design coupled with digital signal
processing enables “multifrequency or “multihertz”
transducer operation, whereby the center frequency
can be adjusted in the transmit mode.
Unlike the resonance transducer design, the piezoelectric element is
intricately machined into a large number of small “rods,” and then filled
with an epoxy resin to create a smooth surface.
Nonresonance (Broad-Bandwidth)
“Multifrequency” Transducers
• Excitation of the multifrequency transducer is
accomplished with a short square wave burst
of 150 V with one to three cycles, unlike the
voltage spike used for resonance transducers.
• This allows the center frequency to be
selected within the limits of the transducer
bandwidth.
Nonresonance (Broad-Bandwidth)
“Multifrequency” Transducers
• Likewise, the broad bandwidth response
permits the reception of echoes within a wide
range of frequencies.
• For instance, ultrasound pulses can be
produced at a low frequency, and the echoes
received at higher frequency.
• “Harmonic imaging” is a recently introduced
technique that uses this ability.
• Lower frequency ultrasound is transmitted
into the patient, and the higher frequency
harmonics (e.g., two times the transmitted
center frequency) created from the interaction
with contrast agents and tissues, are received
as echoes.
Spatial Pulse Length
• The length of the sonic pulse.
• The number of waves multiplied by their wavelengths
• The backing block is incorporated to quench the
vibrations and to shorten the sonic pulse.
Transducer Q Factor (Q = Quality)
Refers to two characteristics of crystal
• purity of their sound and
• length of time the sound persists
Transducer Q Factor
• The “Q factor” describes the bandwidth of the sound
emanating from a transducer as:
fo
Q
Bandwidth
• where
fo
= center frequency
bandwidth = width of the frequency distribution.
• The interval between initiation of the wave
and complete cessation of vibrations is called
the “ring down time”.
• Dampening of the vibration lessens the purity
of the resonance frequency and introduces a
broadband frequency spectrum.
𝑓0
𝑄=
∝ 𝑆𝑝𝑎𝑡𝑖𝑎𝑙 𝑃𝑢𝑙𝑠𝑒 𝐿𝑒𝑛𝑔𝑡ℎ
𝐵𝑎𝑛𝑑𝑤𝑖𝑑𝑡ℎ
• A “high Q” transducer has a narrow bandwidth
(i.e., very little damping) and a corresponding
long spatial pulse length – organ imaging
• A “low Q” transducer has a wide bandwidth and
short spatial pulse length – doppler
Characteristics of Ultrasound Beam
• A single vibrating point sets out waves in all
directions
• Waves move away as concentric circles
When two sound waves interact , they
cancel each other or reinforce each other
Beam Properties
• The ultrasound beam propagates as a longitudinal
wave from the transducer surface into the
propagation medium, and exhibits two distinct
beam patterns:
– a slightly converging beam out to a distance specified
by the geometry and frequency of the transducer (the
near field), and
– a diverging beam beyond that point (the far field).
The Near Field
• The near field, also known as the Fresnel zone,
is adjacent to the transducer face and has a
converging beam profile.
• Beam convergence in the near field occurs
because of multiple constructive and
destructive interference patterns of the
ultrasound waves from the transducer surface.
• Huygen’s principle describes a large
transducer surface as an infinite number of
point sources of sound energy where each
point is characterized as a radial emitter.
• By analogy, a pebble dropped in a quiet pond
creates a radial wave pattern.
Near Field Length……….
• The near field length for an unfocused, single-element
transducer is dependent on the transducer frequency and
diameter:
𝒓𝟐
𝒓𝟐
Near Field Length =  = 𝒗 =
𝒇
𝒓𝟐 .𝒇
𝒗
• For multiple transducer element arrays, an
“effective” transducer diameter is determined
by the excitation of a group of’ transducer
elements.
• Because of the interactions of each of the
individual beams and the ability to focus and
steer the overall beam, the formulas for a
single-element, unfocused transducer are not
directly applicable.
Near Field Length =
𝑟 2 .𝑓
𝑣
• Lateral resolution (the ability of the system to
resolve objects in a direction perpendicular to
the beam direction) is dependent on the
beam diameter and is best at the end of the
near field for a single-element transducer.
• Lateral resolution is worst in areas close to
and far from the transducer surface.
• Pressure amplitude characteristics in the near
field are very complex, caused by the
constructive and destructive interference
wave patterns of the ultrasound beam.
• Peak ultrasound pressure occurs at the end of
the near field, corresponding to the minimum
beam diameter for a single-element
transducer.
• Pressures vary rapidly from peak compression
to peak rarefaction several times during
transit through the near field.
– Only when the far field is reached do the
ultrasound pressure variations decrease
continuously.
Far Field
• The far field is also known as the
Fraunhofer zone, and is where the beam
diverges.
• For a large-area single-element transducer,
the angle of ultrasound beam divergence, ,
for the far field is given by
sin   1.22

d
• where d is the effective diameter of the transducer and
 is the wavelength; both must have the same units of
distance.
𝑣

𝑣
𝑓
sin 𝜃 = 1.22 = 1.22
=
𝑑
𝑑
𝑑. 𝑓
• Less beam divergence occurs with:
– High - frequency transducers
– Large - diameter transducers
Near Field Length =
𝑟 2 .𝑓
𝑣
• High frequency beams –
– fresnel zone is longer
– depth resolution is superior
• Disadvantage :
– Tissue absorption is more, leading to deterioration of side
to side resolution
Solution: Focused Transducer
Focused Transducers
• Single-element transducers are
focused by using
– a curved piezoelectric element or
– a curved acoustic lens
to reduce the beam profile.
Focal Distance
• The focal distance, the
length from the
transducer to the
narrowest beam width,
is shorter than the focal
length of a non-focused
transducer and is fixed.
Focal Zone
• The focal zone is defined as the region over
which the width of the beam is less than two
times the width at the focal distance;
– Thus, the transducer frequency and dimensions
should be chosen to match the depth
requirements of the clinical situation.
Interactions B/N Ultrasound and Matter
1. Reflection
2. Refraction
3. Absorption
Reflection
• Diagnostic images are produced by reflected
portion of the beam.
• Percentage of beam reflected at tissue
interfaces depends on
– Tissue’s acoustic impedance
– Beam’s angle of incidence.
Acoustic Impedance (Z)
• The most important tissue property in imaging.
• This quantity is more properly called the specific
acoustic impedance of the medium
• A simplified definition is
Z=dv
d = density of the tissue (g/cm3)
v = velocity of sound (cm/sec)
• Z – Rayl (g/cm2 x 10-5)
• At most interfaces
within the body, only
a portion of the
ultrasound pulse is
reflected
• The pulse is divided
into two pulses one pulse, the echo,
is reflected back
toward the
transducer and the
other penetrates into
the other material.
• The brightness of a
structure in an
ultrasound image
depends on the
strength of the
reflection, or echo.
• This in turn
depends on the
difference in
acoustic
impedance of the
two materials.
Angle of incidence
• Angle b/n sound waves
and reflecting surfaces
• The more the angle ,
the less the reflection
• In medical USG,
reflected sound is not
detected when angle is
greater than 3o
Reflection
• Two distinct patterns of reflection give rise to
the echoes that make up an ultrasound image
– specular reflection and
– scattering
Specular reflection
• When an incident ultrasound pulse encounters a large,
smooth interface between two types of tissue with
different acoustic impedance values , the result is a
partially reflected echo that travels back toward the
transducer and a partially transmitted pulse that travels
deeper into the patient.
• Responsible for the bright appearance of fibrous structures
such as tendons and of boundaries between different
tissues.
Scattering / Diffuse Reflection
• If the ultrasound pulse encounters reflectors whose
dimensions (d) are smaller than the ultrasound
wavelength (ie, d << λ), scattering occurs.
• Scattering gives rise to the characteristic texture
(echo texture) of the image seen within soft tissue.
Refraction
• Bending of waves as they pass from
one medium to other
• The change in wavelength and
direction of propagation of sound
occurs, but frequency remains
constant
• Artifacts due to refraction are
– Loss of resolution of image
– Spatial distortion
Snell’s Law
𝒔𝒊𝒏 𝜽𝒊
𝒗𝟏
=
𝒔𝒊𝒏 𝜽𝒕
𝒗𝟐
Absorption / Attenuation
• Refers to conversion of
ultrasound energy to
heat energy
Absorption / Attenuation
• Depends On
– frequency of sound (increase absorption)
– viscosity of medium (increase absorption)
– relaxation time of medium
– temperature (varies with tissues)
• The transmission of sound waves without
much energy loss can be done by
– using mineral oil b/n transducer and patient skin
– mechanical impedance matching
• Any process that lessens the reflection. it is done by a
coupler who has intermediate density b/n transducer
and tissue
• The thickness of this matching layer must be equal to
one fourth the wavelength of sound in the matching
layer (Quarter-wave matching)
IMAGE DISPLAY
• Electronic representation of data generated
from returning signals and displayed on TV
monitor
– A MODE
– M MODE
– Real time B MODE
AMPLITUDE MODE
•
•
•
Echoes are displayed in
the form of spikes on
CRO traced along time
base
Amplitude of spike
measures the echo size
Information about the
depth of the structures
and the amplitude of
the returning echo
AMPLITUDE MODE
•
Used in
–
–
–
•
Ophthalmology
Echoencephalography
Echocardiography.
Disadv:
–
–
1D information,
takes lot of space in CRO
M-MODE & TM-MODE
•
•
•
•
•
Detects motion of
structures- cardiac valves
and of cardiac chambers ,
vessels
Echoes are displayed as
dots of varying intensity
M-Mode does not have
Time factor.
It also provides 1D
information
TM-Mode has Time factor
BRIGHTNESS MODE
• Echo signals as – Line of dots.
• Intensity of dot gives relative size of echo
• It provides depth of information and
variations in direction of beam
REAL TIME B MODE
• System is fast enough to allow movements to
be followed.
• At least 16 frames / sec
GRAY SCALE IMAGING
• Display variation of amplitudes of echoes
arising from tissues as varying shades of gray.
Controls
Controls
• Adjustment of image to obtain Optimal Gray
scale image done using several controls:
– TGC
– Coarse Gain
– Intensity
– Reject
– Delay
– Near Gain – Far Gain
-
governs the amplitude
diminish near echoes
enhance distant echoes
TGC
• Time Gain Compensator
• Compensates differences in echo
strength by adjusting the
variations in degree of
amplification
• The slope of the TGC adjusts the
degree of amplification
• The delay control regulates the
depth at which the TGC begins to
augment weaker signals
Pulse Rate
• The number of separate little packets of sound
that are sent out each second
• It determines the total number of echoes
returning to the transducer in a unit of time.
• High pulse rate is desirable.
• But then the receiving time decreases.
• So, the pulse rate must be set to
accommodate the thickest part that might be
examined.
Spatial Resolution
• In ultrasound, the major factor that limits the spatial resolution
and visibility of detail is the volume of the acoustic pulse.
• The axial, lateral, and elevational (slice thickness) dimensions
determine the minimal volume element.
• Each dimension has an effect on the resolvability of objects in
the image.
Axial Resolution
• Also known as linear, range, longitudinal, or
depth resolution
• Refers to the ability to discern two closely
spaced objects in the direction of the beam.
• Achieving good axial resolution requires that
the returning echoes be distinct without
overlap.
• The minimal required separation distance between two
reflectors is one-half of the spatial pulse length (SPL) to avoid
the overlap of returning echoes, as the distance traveled
between two reflectors is twice the separation distance.
Lateral Resolution
• Lateral resolution, also known as azimuthal
resolution
• Refers to the ability to discern as separate two
closely spaced objects perpendicular to the
beam direction.
• For both single
element transducers
and multielement
array transducers, the
beam diameter
determines the lateral
resolution.
• Since the beam
diameter varies with
the distance from the
transducer in the near
and far field, the
lateral resolution is
depth dependent.
• The best lateral
resolution occurs at
the near field—far
field face.
• At this depth, the
effective beam
diameter is
approximately equal to
half the transducer
diameter.
• In the far field, the
beam diverges and
substantially reduces
the lateral resolution.
• The typical lateral resolution for an unfocused
transducer is approximately 2 to 5 mm.
• A focused transducer uses an acoustic lens (a
curved acoustic material analogous to an
optical lens) to decrease the beam diameter at
a specified distance from the transducer.
Elevational Resolution
• The elevational or slice-thickness dimension of
the ultrasound beam is perpendicular to the
image plane.
• Slice thickness plays a significant part in image
resolution, particularly with respect to volume
averaging of acoustic details in the regions
close to the transducer and in the far field
beyond the focal zone.
• Elevational resolution
is dependent on the
transducer element
height in much the
same way that the
lateral resolution is
dependent on the
transducer element
width.
Scanning the Ultrasound Beam
Types:
• Mechanical Scanners
• Electronic Array Scanners
Mechanical scanners
• Early US systems relied on
the operator to manually
change the position and
orientation of the
transducer and scan the
ultrasound beam through a
plane in the patient to
obtain the echo data
necessary for each image.
Mechanical scanners
• 3 types:
– Oscillating Transducer (Unenclosed type)
– Oscillating Transducer (Enclosed type)
– Rotating Wheel Transducer
Mechanical scanners
• This was a time-consuming process but produced images that
covered large FOVs
• Image artefacts occurred if the patient moved during the acquisition
Electronic array scanning
• Most modern US imagers automatically scan
the ultrasound beam using transducers
consisting of arrays of many narrow
piezoelectric elements.
• The array may consist of as many as 128–196
elements
• Two modes of activation are used to produce a beam.
– These are the “linear” (sequential) and “phased”
activation/receive modes.
Linear
• The linear array scanners produce sound
waves parallel to each other and produces
a rectangular image.
• The width of the image and number of scan
lines are the same at all tissue levels.
• Often used with high frequencies ie 7MHz.
• Advantage -good near field resolution.
• Disadvantage is artifacts when applied to a
curved part of the body creating air gaps
between skin and transducer.
Sector/Vector
• Produces a fan like
image that is narrow
near the transducer and
increase in width with
deeper penetration.
• The disadvantge is poor
near field resolution.
Curved
• Often with frequencies
of 2 - 5 MHz (to allow
for a range of patients
from obese to slender).
• The array of elements is
arranged across a
convex arc (instead of a
straight line), which
rapidly scans a larger
FOV
Endoscopic Ultrasound
• Dedicated linear-array and radial
echo-endoscopes for structural
evaluation of the luminal wall
adjacent tissues.
• Flexible shaft with a central wire
that drives rotation of a
mechanical transducer at the tip.
• The transducer is surrounded by
oil that serves as an acoustic
interface with tissue,
providing 360-degree imaging
• With the introduction of
the curved linear array
echoendoscope , the
indications for EUS have
expanded.
• Allows the endoscopist to
perform a whole range of
interventional applications
ranging from fine needle
aspiration (FNA) of lesions
surrounding the
gastrointestinal tract to
celiac plexus block and
drainage of pancreatic
pseudocyst
• Ultrasound imaging of vascular and non-vascular lumina
employing miniaturized high-frequency (20-30
MHz) transducers inserted into catheters as small as 2 mm
in external diameter
Recent Innovations in B-mode US
•
•
•
•
•
•
•
Tissue Harmonic Imaging
Spatial Compound Imaging
Extended FOV Imaging
Coded Pulse Excitation
3D and 4D imaging
Elastography.
Ultrasound contrast media.
Tissue Harmonic Imaging
• A musical note has three characteristics.
– Pitch (or frequency)
– Loudness
– Quality (or tone).
• Quality is the audible difference heard between two
musical notes of the same pitch and loudness.
• Hence, a piano based C-note does not sound the same
as a C-note played on a guitar, due to the existence of
harmonic frequencies.
• The same note from various instruments has different
qualities because the sounds are not pure notes i.e. of
one frequency and these integral multiples of the
fundamental frequency (overtones) give an instrument
its characteristic sound.
Tissue Harmonic Imaging
• Harmonics are frequencies that occur at
multiples of the fundamental or transmitted
sonographic frequency.
• In conventional gray-scale sonography, the same
frequency spectrum that is transmitted into the
patient is subsequently received to produce the
sonographic image.
• In THI sonography, higher harmonic frequencies
generated by propagation of the ultrasound
beam through tissue are used to produce the
sonogram.
Why harmonics?
• The ultrasonic pulse gets altered
with time as it traverses the
tissues with non-linear motion.
• The peaks within the pulse
waveform move faster than the
troughs because the propagation
speed is higher in compressed
regions of tissue than in the areas
which are expanded by the
passing pressure wave.
• The degree of such acoustic
signal distortion in tissue depends
on the amplitude of the emitted Drawing of undistorted pressure wave (top) and of
pulse and the distance it has
pressure wave after undergoing nonlinear propagation
(bottom). Nonlinear effects cause high-pressure regions
travelled in the tissue.
of sound wave to travel faster than low-pressure
regions and result in progressive distortion of
transmitted wave with generation of sound at higher
harmonics of transmit frequency
• As much of the fundamental sonographic signal
as possible must be removed to make these
theoretical harmonic improvements a clinical
reality.
• This can be done by either frequency- based or
phase inversion methods.
• There are 2 basic harmonic imaging methods.
– conventional frequency-based second-harmonic
imaging
– pulse inversion or phase inversion harmonic imaging
Conventional Frequency-based
Second-harmonic Imaging
• A narrowband pulse is
emitted, and then highpass or narrowbandpass filtering is
applied to the received
echoes to filter out the
fundamental echo
components.
• This results in reduction
of both spatial and
contrast resolution.
Phase Inversion Harmonic Imaging
• In wideband harmonic imaging
(also called pulse inversion or
phase inversion harmonic
imaging)
• A train of 2 pulses is emitted,
with the phase of the second
inverted relative to the phase
of the first.
• When the echoes from the
transmitted pulses are added,
the linear components of the
echoes cancel each other,
whereas the nonlinear
components are amplified.
• This results in superior
contrast and spatial resolution.
Two phase-inverted but otherwise identical
sonographic pulses are transmitted. Summing the
returning echoes in a buffer cancels most of the
fundamental and odd harmonic echoes and effectively
amplifies the second harmonic.
ADVANTAGES OF THI
1. Improve lateral resolution
2. Reduce side lobe artifacts
3. Improve signal to noise ratio
4. Improved near field and far field image
quality
ADVANTAGES OF THI
5. Lesions are clearer & better defined.
6. Use of higher frequencies improves
resolution.
7. Helps differentiate cysts from hypoechoic
solid masses.
8. Better clarity of contents.
9. It is superior to conventional USG in
visualization of lesions containing highly
reflective tissues like fat, calcium & air.
ADVANTAGES OF THI
10.Harmonic imaging is generally considered to
be most useful for “technically difficult”
patients with thick and complicated body
wall
THI is useful in
•
•
•
•
•
•
•
•
Better defining the borders of pancreas
Better visualization of lower pole of left kidney
Better visualization of fatty livers
Small renal cysts can be easily seen
Presence or absence of sludge in GB
Metastatic lesions in liver easily seen
Minimal fluid present between liver and kidney
Provides better images of aortic wall, IVC, portal vein
and renal arteries
Spatial Compound Imaging
• Electronic steering of ultrasound beams from
an array transducer is used to image the same
tissue multiple times by using parallel beams
oriented along different directions.
• The echoes from these multiple acquisitions
(upto 9 sets) are then averaged together into a
single composite image.
Disadvantage
• More time is required for data acquisition
• The compound imaging frame rate is reduced
compared with that of conventional B-mode
imaging.
Advantage
• Spatial compound images often show reduced
levels of speckle, noise, clutter, and refractive
shadows and improved contrast and margin
definition
• Enhancement and shadowing artifacts may
also be reduced, which may be an advantage
or potential drawback, depending on the
imaging situation.
Clutter Suppression
Conventional image: acoustic clutter
produces spurious echoes within a simple
cyst (inset), which can be difficult to
eliminate regardless of incident angle or
adjustment of system parameters.
Compound image: compound Imaging
results in significant clutter suppression in
the cyst. Also note the good depth of field in
this single focus compound image,
comparable to the multi-focus conventional
image.
Extended FOV Imaging
• A benefit of early static B-mode scanners that was lost
with the introduction of mechanical and electronic
automatic scanning was large imaging FOVs.
• Extended FOV imaging, has sought to restore this
capability.
• The transducer is slowly translated laterally across the
large anatomic region of interest. During this motion,
multiple images are acquired from many transducer
positions.
• The registered image data are accumulated in a large
image buffer and then combined to form the complete
large FOV image
• Extended FOV images are not limited just to Bmode acquisitions.
• Restores the capability of visualizing large
anatomic regions in a single image and simplifies
measurements made over these large regions.
• It is useful in evaluating aneurysms, detecting
small ligament and tissue damage, and to image
and calibrate long sections of blood vessels, to
locate anatomical landmarks for location of
disease.
Coded Pulse Excitation
• A fundamental trade-off in US is that between
imaging depth and spatial resolution.
• Coded ultrasound pulses help overcome this
limitation, providing good penetration at the
higher frequencies necessary for high spatial
resolution.
• In this imaging approach, long ultrasound pulses
are used instead of the very short pulses. These
long pulses carry greater ultrasonic energy,
increasing the energy of echoes that return from
large depths in the patient.
The coded pulses are produced with a very specific, characteristic
shape, and the resulting echoes will have a similar shape.
The end result is an image with good echo signal and good spatial resolution at large depths.
Conventional B-mode (a) and coded pulse (b) US images of the liver show the benefits of coded pulse imaging. The spatial
resolution of the coded pulse image (b) is very comparable with that of the 13-MHz conventional image (a). However, the
useful imaging depth is about 7.5 cm for the coded pulse image (b) compared with only about 2.8 cm for the conventional
image (a).
ELASTOGRAPHY
• Dynamic technique that uses ultrasound to
provide information on tissue stiffness by
measuring the degree of distortion under the
application of an external force - viscoelastic
properties of tissue
• Surrogate for that obtained with manual
palpation.
• Principle: malignant tissues have more stiffness
and are harder to distort. On tissue compression
strain (displacement) within harder tissue is less
than in softer tissue.
Layers in Jello show soft and hard materials after light compression
Important Quantities
• Young’s modulus (E) describes longitudinal
deformation in terms of strain
• The shear modulus (G) relates to transverse strain to
transverse stress
• The bulk modulus (K) of elasticity describes the change
in volume
• Poisson ratio (v) which is a ratio of transverse
contraction per unit breadth divided by longitudinal
extension per unit length
Shear and Young’s moduli, are the most suitable elasticity
parameters to measure.
The tissue is insonified a) before and b) after a small uniform compression.
In the harder tissues (e.g. the circular lesion depicted) the echoes will be less distorted than in
the surrounding tissues, denoting thus smaller strain
Elastography - methods
• Three methods—
a) spatial correlation method,
b) the phase-shift tracking method, and
c) the combined autocorrelation method (CAM)—
• have been introduced for measuring tissue
strain at elastography.
Elastography - Uses
• In the liver it is a very useful method in
depicting liver hardness and prediction and
prognostication of liver cirrhosis.
• Breast imaging, cervix imaging,
musculoskeletal imaging (especially
supraspinatus and tendoachilles), thyroid
lesions assessment and assessment of cervical
nodes.
3D and 4D ultrasound
• 3D ultrasound - data set that contains a large number
of 2D planes (B-mode images).
• This volume data can be manipulated in different
planes by rotation.
• Dissected in any plane, to get multiplanar images
(similar to multislice CT).
• Special probes and software are necessary in order to
perform 3D and 4D imaging.
• 4D ultrasound is also known as "Real-time 3D
Ultrasound". The 3D datasets with their multiplanar
reformations and renderings in real time, give us
photographic quality images.
Techniques of 3D US
• Data acquisition
• Image reconstruction and
• Image display
Data Acquisition Techniques
a)
b)
c)
d)
Tracked freehand systems,
Untracked freehand systems,
Mechanical assemblies, and
2D arrays
• Regardless of which method is used, one must know
the relative position and angulation of each 2D image
and must acquire the images rapidly or with gating to
avoid motion artifacts. If these two criteria are not
met, the 3D images may be inaccurate.
(a) Tracked Freehand Systems
• The operator holds an assembly composed of
the transducer with an attachment and
manipulates it over the anatomic area being
evaluated
• Ensure that there are no significant imaging
gaps.
• Acoustically tracked 3D scanning
• Articulated-arm-tracked 3D scanning
• Magnetic field-tracked 3D scanning
(b) Untracked Freehand
• 2D images are digitized as the operator moves
the transducer with a smooth, steady motion
• Most convenient for the operator
• Image quality is variable and depends largely on
how smoothly and steadily the operator moves
the transducer
• Geometric measurements may be inaccurate
because there is no direct information regarding
the relative position of the digitized images.
• Mechanical movement of the transducer
across the skin.
• Tilted about a fixed point on the skin surface.
• Rotated about its own axis.
(c) Mechanical Assemblies
• The transducer is propelled or rotated
mechanically, and 2D images are digitized at
predetermined spatial or angular intervals .
• Cumbersome for the user
• Improve the geometric accuracy
• To date, their greatest utility has been in
intracavitary and intraluminal examinations, in
which the area of interest is relatively small
and motion artifact is less of a problem.
• Three different scanning techniques can be used:
– linear scanning,
– tilt scanning, and
– rotational scanning.
(d) Two-dimensional Arrays
• With the first three types of data acquisition
systems, mechanical motion is used to obtain 3D
images.
• An alternative is to keep the transducer stationary
and use electronic scanning with a 2D transducer
array generates pyramidal or conical US pulses to
generate 3D information in real time
Image Reconstruction Techniques
• Two types:
– 3D surface model
– Voxel-based volume model.
Three-dimensional Surface Model:
• Outline the boundaries of the areas of interest
on the 2D images manually or with a
computer algorithm
• Reduces the amount of 3D data needed
• Shorter 3D reconstruction times and greater
efficiency
• Identification of boundaries can be tedious
and time-consuming.
Voxel-based Volume Model
• The computer builds a 3D voxel-based volume
(3D grid)
• This process preserves the original
information
• Allows a variety of rendering techniques
• Generates very large data files, which slows
processing and requires large amounts of
computer memory.
Image Display Techniques
• Surface Rendering: operator identifies the
boundaries of pertinent structures either
manually or with an algorithm
• Multiplanar Reformatting: three orthogonal
planes or texture mapping – image rotated to
obtain the desired image orientation
• Combined Surface Rendering and
Multiplanar Reformatting.
• Volume Rendering
Artefacts
• Respiratory motion artifact
• Artifact caused by incorrect calibration
Advantages of 3D US
• Many benefits in obstetrical and gynaecological
scanning where it is important to analyze anatomy. It
has shown promising results in diagnosing cleft
lip/palate, spina bifida, polydactyly, club foot, facial
dysmorphism, low set ears ,fetal cardiac imaging.
• Excellent tool for demonstrating and accurately
diagnosing congenital uterine anomalies, pelvic floor
muscle and sphincters.
• Considerable interobserver variability in 2D; exact
relationship between anatomic structures is accurately
recorded in the 3D image
Advantages of 3D US
• Unrestricted access to an infinite number of
viewing planes.
• Suited for monitoring the effects of therapy
over a long period of time
• More accurate quantitative volume estimates
Limitations of 3D US
• More cumbersome , requiring more user input
• Data archiving and communication more challenging
• Waiting for the 3D image to appear can be frustrating
to users
• Slow the image interpretation process, especially if
inexperienced, as the right algorithm needs to be
chosen.
• The ability to obtain a good 3-D picture is very much
dependent on operator skill, the amount of liquor,
fetus position and the degree of maternal obesity.
Applications for 3D US
• Fetal Imaging
Applications for 3D US
• Gynecologic Imaging
volume data sets to be acquired with both
transvaginal and transabdominal probes
Applications for 3D US
• Three-dimensional Power Doppler Imaging
3D US angiography
• Prostate Imaging
Applications for 3D US
• Breast Imaging: demonstrate lesion margins
and topography
• Biopsy-related Imaging: needle localization
and guidance
ULTRASOUND CONTRAST MEDIA
• Microbubbles of air or other gases which act as echo
enhancers
• Microbubbles are less than 10 µm in diameter, and
• Contrary to most other contrast media which are
rapidly distributed to the extravascular, extracellular
space, most microbubbles are confined to the vascular
space.
• Microbubbles may produce up to 25 dB (more than
300-fold) increase in echo strength.
• Stability is increased by
– external bubble encapsulation (galactose,
phospholipids, denatured albumin or poly-butylcyanoacrylate) with or without surfactants
– using gases with a low diffusion coefficient
(perfluorocarbons) or
– a combination of both
• The gas components of the microbubbles are
normally eliminated via the lungs.
• Stabilizing components are eliminated via the
hepato-renal route.
Contrast Microbubbles
• The ideal diameter 2 µm to 8 µm
• Enhancement life-time
of the microbubble,
often several minutes
(8–10 min)
Microbubbles – Structure
• The coatings include
albumin, gelatin,
galactose microspheres,
polyglutaminic acid,
lipophilic monolayer
surfactants, and lipid
bilayers (liposomes).
• The gas inside the shell
may be either air or
various perfluorocarbons,
which are liquids at room
temperature but gas at
body temperature.
The ideal USCA
• Non-toxic,
• Injectable intravenously,
• Capable of crossing the pulmonary capillary
bed after a peripheral injection, and
• Stable enough to achieve enhancement for
the duration of the examination.
Microbubbles - Generations
• Several "generations" of gas microbubble contrast media
have evolved;
• The "1st generation" products do not pass the pulmonary
vascular bed, and are therefore limited to the venous
system and the right heart cavities after injection.
• The "2nd generation" contrast media are both sufficiently
small and stable to pass into the systemic circulation, and
these contrast media enhance the doppler signal in various
arteries after injection. They are short-lived, however, the
effect is over in a few minutes.
Microbubbles - Generations
• The "3rd generation" gas microbubble
contrast media are even more echogenic and
stable, and are able to enhance the
echogenicity of parenchyma on B-mode
images. They may thus show perfusion, even
in such a difficult region as the myocardium.
• The various gas microbubble contrast media
are generally safe with low toxicity in humans.
Ultrasound Contrast Agents - Types
• Different types of ultrasound contrast agents:
– tissue contrast agents;
– contrast agents for vascular enhancement;
– agents for targeted contrast imaging.
PRINCIPLES
• The main mechanisms for signal enhancement are
– backscattering,
– bubble resonance and
– bubble rupture.
• These mechanisms are highly dependent on the acoustic
power of the transmitted ultrasound, which is reflected by the
mechanical index (MI).
• very high reflectivity ( gas content makes them very
reflective)
Acoustic Excitation of Ultrasound
Contrast Agents
• The behavior of these agents under acoustic
excitation fall into three classes (Frinking et
al., 1999), depending on the structure of the
microbubble and the level of the insonifying
pressure amplitude and frequency:
– stable linear (low MI),
– stable nonlinear scattering (medium MI)
– transient nonlinear scattering (high MI).
Back Scattering
• At low acoustic power (MI < 0.1), gas microbubbles may be
regarded as point scatterers, and the mechanism of
ultrasound reflection is that of Rayleigh Tyndall scattering.
• The scattering strength of a point scatterer is proportional to
the sixth power of the particle radius and to the fourth power
of the ultrasound frequency; the echogenicity of such contrast
media is therefore highly dependent upon particle size and
transmit frequency.
• The backscattered intensity of a group of point scatterers is
furthermore directly proportional to the total number of
scatterers in the insonified volume; the concentration of the
contrast medium is therefore also of importance.
MI- 0.1-0.3
Reflect and scatter
Bubble Resonance and Harmonics
• At intermediate acoustic power (0.1 < MI < 0.5) gas
microbubbles may show strong oscillatory motion provided
the frequency of the incident ultrasound is close to the
resonant (fundamental) frequency of the microbubbles.
MICROBUBBLE AS LINEAR RESONATOR
• By virtue of their compressibility, microbubbles display unique
properties in an ultrasound beam, which sets them into
resonance when there is a match between their diameter and
the ultrasonic wavelength which for microbubbles 2-7microns
range at ultrasound frequency 2-10 Mhz.
MI- 0.5
Resonates
MICROBUBBLE AS NONLINEAR RESONATOR
• nonlinear behaviour (compression and expansion phases)
produces echoes that contain frequencies not present in
the transmitted pulses.
• An elegant way to extract these nonlinear signals is to send
a series of pulses down each line, varying their phase and
amplitude; the returning signals are combined to cancel the
linear signals from tissue and the remaining bubble-specific
signals are used to form an image that can be presented as
a colour overlay on the B-mode image or shown on a sideby-side display, all in real-time.
MI- 0.7-0.8
Oscillates in non harmonic pattern
Bubble rupture
• At high acoustic power (MI > 0.5), ultrasound at the
microbubble resonance frequency will cause the bubbles to
rupture.
• The result is a transient high-amplitude, broadband signal
containing all frequencies, not only the harmonics.
• It will create a transient, strong signal in B-mode, or a
short-lasting multicoloured, mosaic-like effect in colour
Doppler sonography.
• Several terms for the strong, transient signal have been
proposed: induced or stimulated acoustic emission, loss of
correlation imaging and sono-scintigraphy.
MI- =/>1
Disrupt and Burst
Applications
• Give angiographic capacity to ultrasound ie help in
demonstrating tissue perfusion, bleeding
points,vascularity of lesions etc.
• Widely used in imaging of solid organs, particularly the
liver
• Improvement in detection of colour Doppler signal
from large vessels
• Microbubble contrast has also found a niche outside
the vascular compartment in the setting of vesicoureteric reflux in children where a high sensitivity and
specificity compared with conventional micturating
cystourography (MCUG) has been demonstrated
Applications
• Gene therapy and targeted delivery of drugs
Conclusion
• Modern US equipment is based on many of the same
fundamental principles employed in the initial devices
used for human imaging over 50 years ago.
• US has the characteristics of being relatively
inexpensive, portable, safe, and real-time in nature, all
of which make it one of the most widely used imaging
modalities in medicine.
• In short, US science, technology, and applications are
expanding at a brisk pace and are far from mature.
• Even more exciting developments are on the horizon.