usg physics.pptusg physics.pptusg physics.ppt

usg physics.pptusg physics.pptusg physics.pptusg physics.pptusg physics.ppt

1. By
Mr. Dinesh sekar MSc Radiology
Asst Professor
Department of Radiology
 
Ultrasound physicsUltrasound physics

2. SoundSound
•It is the
 physical phenomenon that
stimulates the sense of hearing.
•A sound beam is a wave transmitting
energy
• In human beings, hearing takes
place whenever there are vibrations
of frequencies between about 15 and
20,000 hertz

3. Sound waveSound wave
•In
 general, waves can be propagated transversely or
longitudinally. In both cases, only the energy of wave
motion is propagated through the medium; no portion of
the medium itself actually moves very far.
•A sound wave, is a longitudinal wave. As the energy of
wave motion is propagated outward from the centre of
disturbance, the individual air molecules that carry the
sound move back and forth, parallel to the direction of
wave motion.
• Thus, a sound wave is a series of alternate compressions
and rarefactions of the air. Each individual molecule passes
the energy on to neighboring molecules but, after the
sound wave has passed, each molecule remains in about
the same location

6. FrequencyFrequency

•We perceive frequency as “higher” or “lower”
sounds. The frequency of a sound is the number
of cycles, or oscillations, a sound wave completes
in a given time. Frequency is measured in hertz,
or cycles per second. Because the amplitude
(height) of the waves above remains constant,
we are able to hear the same note at different
frequencies
•Waves propagate at both higher and lower
frequencies, but humans are unable to hear them
outside a relatively narrow range.

9. Amplitude and VolumeAmplitude and Volume

•Amplitude is the characteristic of sound waves
that we perceive as volume.
•The maximum distance a wave travels from the
normal, or zero, position is the amplitude; this
distance corresponds to the degree of motion in
the air molecules of a wave. As the degree of
motion in the molecules is increased, they strike
the ear drum with progressively greater force.
This causes the ear to perceive a louder sound.
•A comparison of samples at low, medium, and
high amplitudes having the same frequency
should sound the same except for a perceptible
volume difference.

11. Sound IntensitiesSound Intensities
•Intensity is determined by the length of
oscillation of the particle conducting the waves
•Sound intensities are measured in decibels (dB).
For example, the intensity at the threshold of
hearing is 0 dB, the intensity of whispering is
typically about 10 dB, and the intensity of
rustling leaves reaches almost 20 dB. Sound
intensities are arranged on a logarithmic scale,
which means that an increase of 10 dB
corresponds to an increase in intensity by a factor
of 10. Thus, rustling leaves are about 10 times
louder than whispering.

13. Wavelength Wavelength
•For longitudinal waves, it is the distance
from compression to compression or
rarefaction to rarefaction.
• The frequency of the wave is the number
of vibrations per second.
•The velocity of the wave, which is the
speed at which it advances, is equal to the
wavelength times the frequency.
•The maximum displacement involved in
the vibration of a mechanical wave is
called the amplitude of the wave.

15. •The wavelength is inversely
proportional to frequency and
directly proportional to velocity. In
mathematical terms, this relationship
is expressed by the equation V = λ f,
where V is velocity, f is frequency,
and λ is wavelength.

18. Ultrasound Ultrasound
•Sounds of frequencies higher than about 20,000 hertz are called
ultrasonic.
•The science of ultrasonics has many applications in various fields
of physics, chemistry, technology, and medicine. Ultrasonic waves
have long been used for detection and communication devices
called sonar (sound navigation & ranging) of great importance in
present-day navigation, and especially in submarine warfare.
•Applications of ultrasonics in physics include the determination of
such properties of matter as compressibility, specific heat ratios,
and elasticity.
•Ultrasonics is employed in producing emulsions, such as
homogenized milk and photographic film, and for detecting flaws
in industrial materials. Ultrasound in the gigahertz range can be
used to produce an acoustic “microscope”, able to visualize detail
down to 1 micrometre (40 millionths of an inch). Surface acoustic
waves of ultrasonic frequency form an important component of
electronic control devices.

19. Robert BallardRobert Ballard
The oceanographer and explorer Robert Ballard (on the right) and the photographer The oceanographer and explorer Robert Ballard (on the right) and the photographer
Emory Kristof stand next to the French sonar search instrument SAR in this 1985 view. Emory Kristof stand next to the French sonar search instrument SAR in this 1985 view.
SAR was used in the initial reconnaissance of the seabed that led to the discovery of the SAR was used in the initial reconnaissance of the seabed that led to the discovery of the
wreck of the wreck of the TitanicTitanic. Ballard has used similar methods to pinpoint the location of many . Ballard has used similar methods to pinpoint the location of many
other famous wrecks in the years since.other famous wrecks in the years since.

20. •In medicine, ultrasonics is used as a diagnostic tool, to
destroy diseased tissue, and to repair damaged tissue.
• Ultrasonic waves have been employed to treat bursitis,
various types of rheumatoid arthritis, gout, and muscular
injuries and to destroy kidney stones. As a diagnostic tool,
ultrasonic waves are often more revealing than X-rays,
which do not prove as useful in detecting the subtle density
differences found in certain forms of cancer.
•When ultrasonic waves are passed through a tissue the
waves are reflected in varying degrees, depending on the
density and elasticity of the tissue.
• Using an ultrasonic “scalpel”, a surgeon can make an finer
incision than with a conventional surgical knife. Such
techniques have been used in delicate surgery on the brain
and the ear. Diathermic devices in which ultrasonic waves
are used to produce heat internally as a result of tissue
resistance have been used successfully in physical therapy.

21. Diagnostic ultrasoundDiagnostic ultrasound
•All diagnostic applications are based
on the detection & display of acoustic
energy reflected from interfaces
within the body
•These interactions provide the info.
Needed to generate high resolution
grey scale images of the body as well
display info. related to the blood flow

22. Propagation of sound in the Propagation of sound in the
body body
•Ultrasound uses brief bursts or
pulses of energy which is transmitted
along the direction of the particle
movement (longitudinal waves)
•The speed at which the pressure
wave moves through tissues varies
greatly & is affected by the physical
properties of the tissue like density,
stiffness or elasticity

24. Propagation velocity Propagation velocity
artifactsartifacts
•When a sound passes through a
lesion containing fat in the liver,echo
return is delayed as fat has a
propagation velocity of 1450 & liver
has 1540m/sec
•So the final image shows a
misregistration artifact in which the
diaphragm shows a deeper position
than expected

26. Acoustic impedance Acoustic impedance
•Current diagnostic ultrasound
scanners depend on the detection &
display of reflected sound or echoes
•To produce an echo a reflecting
interface must be present
•Sound passing through a totally
homogenous medium encounters no
interfaces to reflect sound & the
medium appears anechoic

27. •The amount of reflection or
backscatter is determined by the
difference in the acoustic impedences
of the material forming the interface
•Acoustic impedence Z = рc where p
is the density of the medium & c is
the velocity of the sound in that
medium

28. Reflection Reflection
•Reflection, phenomenon of light and other
wave motions in which the light or other
wave motion is returned after impinging
on a surface, or the boundary between
two media
•If the interface is large & relatively smooth
it can reflect sound as a mirror can reflect
light
•Such interfaces are called specular
reflectors eg. Diaphragm ,wall of the urine
filled bladder ,endomerial stripe

29. •Echoes which do not arise from
specular reflectors ,but come from
much smaller interfaces the echoes
are scattered in all the directions
•These are called diffuse reflectors
•The constructive & destructive
interference of sound scattered these
lead to the formation of ultrasound
speckle

33. Refraction Refraction
•When sound passes from a tissue
with one acoustic propagation
velocity to another of either higher
or lower sound velocity ,there is a
change in the direction of sound
wave
•This is one of the causes of
misregistration of a structure in a
ultrasound image

38. Attenuation Attenuation
•As the acoustic energy moves through a
uniform medium ,work is performed &
energy is transferred to the transmitting
medium as heat
•So as sound passes through the tissue it
looses energy & pressure waves decrease
in amplitudeas they travel further from the
source
•Attenuation is therefore a result of the
combined effects of absorption ,scattering
& reflection

40. •High frequencies are attenuated
more rapidly than low frequencies

41. Piezoelectricity Piezoelectricity
•Piezoelectric Effect is the appearance of an
electric potential across certain faces of a
crystal when it is subjected to mechanical
pressure.
• Conversely, when an electric field is
applied on certain faces of the crystal, the
crystal undergoes mechanical distortion.
•Pierre Curie and his brother Jacques
discovered the phenomenon in quartz and
Rochelle salt in 1880 and named the effect
piezoelectricity (from Greek piezein,”to
press”).

42. •The piezoelectric effect occurs in several crystalline
substances, such as barium titanate and tourmaline.
•The effect is explained by the displacement of ions in
crystals that have a nonsymmetrical unit cell, the simplest
polyhedron that makes up the crystal structure.
•When the crystal is compressed, the ions in each unit cell
are displaced, causing the electric polarization of the unit
cell.
•Due to the regularity of crystalline structure, these effects
accumulate, producing an electric potential difference
between certain faces of the crystal.
•When an external electric field is applied to the crystal, the
ions in each unit cell are displaced by electrostatic forces,
resulting in the mechanical deformation of the whole
crystal.

43. •Due to their capacity to convert mechanical
deformation into electric voltages, and electric
voltages into mechanical motion, piezoelectric
crystals are used in such devices as the
transducer, record-playing pickup elements, and
the microphone.
•Piezoelectric crystals are also used as resonators
in electronic oscillators and high-frequency
amplifiers, because the mechanical resonance
frequency of adequately cut crystals is stable and
well defined.

44. •Nowadays the piezoelectric crystals
used are lead zirconate titanate
commonly called as PZT
•Several types are available with
slight variation in chemical
additions& thermal treatment
producing different properties

45. Transducers Transducers
•It is a device that converts one form
of energy into another
•Usg transducers convert an electric
signal into ultrasonic energy that can
be transmitted to tissues & then to
convert the returning ultrasonic
energy to an electric signal

46. Construction of a single Construction of a single
piezoelectric crystal transducer piezoelectric crystal transducer
•The most important component of a
transducer is the thin piezoelectric
crystal located near the face of the
transducer
•The front & back faces of the crystal
are coated with a thin conducting
film to ensure good contact with the
two electrodes that will supply the
electric field for straining the crystal

47. •The outside electrode is grounded to
protect the pt. from electric shock & its
outside surface is coated with a watertight
electrical insulator’
•The inside electrode abuts against a thick
backing block that absorbs sound waves
transmitted back into the transducer
•The housing is usually made of plastic an
acoustic insulator of rubber os cork
prevents the sound from passing into the
housing

49. Evolution of transducersEvolution of transducers
•Early ultrasound scanners used
transducers consisting of a single
peizoelectric element
•To generate a real time images with these
transducers required mechanical devices
to move the transducer in a linear &
circular motion
•These mechanical sector scanners do not
allow variable focusing
•This problem was overcome by using a
annular array mechanical transducer with
electronically steered focusing

52. Arrays Arrays
•Current technology uses a transducer
composed of multiple elements usually
produced by the precise slicing of a piece
of piezoelectric material into numerous
small units each with its own electrodes
•They contain no mechanical moving parts
•Most commonly used arrays are
linear ,curved & phased
•High density 2D arrays are also developed
for 3D & 4D images

53. Linear arrayLinear array
•Commonly used for small parts vascular &
obg applications
•In these transducers individual elements
are arranged in a linear fashion
•By firing the transducer in sequence either
individually or in groups a series of parallel
pulses is generated each forming a line of
sight perpendicular to the transducer face
•These individual line of sights combine to
give the image field of view

57. Curved arrayCurved array
•Transducers have been shaped into
convex curves which produce an image
that combines a relatively large surface
field of view with a sector display format
•The larger ones are used for abdominal &
pelvic scans
•The smaller ones are used for trans
vaginal ,trans rectal & for paed. imaging

59. Phased arrays Phased arrays
•In this a sector field of view is produced
by multiple transducer elements fired in
precise sequence
•By controlling the time & sequence in
which the individual transducer elements
are fired the ultrasound waves can be
steered in different directions as well
focused at different depths
•These transducers are useful for
intercostal scanning

60. 2D arrays2D arrays
•Arrays can be formed by slicing a rectangular
piece of transducer material perpendicular to its
long axis to produce a no. of small rectangular
element or by creating a series of concentric
elements nested within one another
•The advantage of this is that the beam can be
precisely focused both in elevation & in lateral
plane
•These arrays offer improvements in spatial
resolution & contrast as well as reduction in
clutter & are well suited for use in 3D processing
& display

62. Ultrasound instrumentationUltrasound instrumentation
•Ultrasound scanners are one of the most
complex & sophisticated imaging devices
currently in use
•All scanners consist of a transmitter or
pulsar to energize the transducer, the usg
transducer ,a receiver & processor to
detect & amplify the backscattered energy
& a display that presents the usg image or
data in a form, suitable for analysis &
interpretation & a method to record &
store the images

63. Transmitter Transmitter
•Most clinical applications use pulsed
ultrasound in which brief bursts of
acoustic energy are transmitted into
the body
•The ultrasound transducer, which is
the source of these pulses is
energized by application of precisely
timed, high amplitude voltage

64. Pulse repetition frequencyPulse repetition frequency
•The transmitter also controls the rate of
pulses emitted by the transducer or the
PRF
•It determines the time interval between
ultrasound pulses & is important in
determining the depth from which the
data can be obtained
•The ultrasound pulses must be spaced
with enough time between the pulses to
permit the sound to travel to the depth of
interest & return before the next pulse is
sent

65. •For imaging PRF from 1 to 10 kHz
are used ,resulting in an interval of
0.1 to 1 ms between the pulses

66. Transducer Transducer
•The ultrasound machine transducer
converts electric energy provided by
the transmitter to the acoustic pulses
directed into the pt.
•The transducer also serves as a
receiver of reflected echoes
converting the weak pressure
changes into electric signals for
processing

67. •Changing the polarity of the voltage
applied to the transducer changes
the thickness of the transducer, so it
expands & contracts as the polarity
changes
•This results in the generation of
mechanical pressure waves that can
be transmitted into the body

68. •The peizoelectric effect also results in
generation of small potentials across the
transducer when the returning echoes hit
it
•Positive pressures during compressions
causes a small polarity to develop across
the transducer
•Negative pressure during rarefaction
produces the opposite polarity across the
transducer

70. •These tiny polarity changes & the voltage
associated with them are the source of all
the information processed to generate an
ultrasound image
•In the pulsed operating mode used for
most clinical ultrasounds ,the pulses
contain additional frequencies both higher
& lower than the preferential frequency
•The range of frequencies produced by the
given transducer is called the bandwidth

71. •Modern systems employ broad bandwidth
technology
•This helps to reduce the speckle artifact by
a process of frequency compounding
•This is possible due to speckle patterns at
different frequencies are independent of
one another & combining data from
multiple frequency bands results in a
reduction of speckle in the final image

72. •The ultrasound pulses produced by a transducer
result in a series of wavefronts that form a 3D
beam of ultrasound
•Interferance of the pressure waves produces a
area near the transducer in which the pressure
amplitude varies greatly called the near field or
Fresnel zone
At a distance determined by the radius of the
transducer & the frequency at which the sound
wave begins to diverge & the pressure amplitude
decreases at a steady rate with increasing
distance from the transducer is called the
Frauenhofer zone

76. •In multielement transducer arrays
precise timing of the firing of
elements allows correction of this
divergence by focusing at selected
depths

77. Receiver Receiver
•When returning echoes strike the
transducer face minute voltages are
produced ,the receiver detects & amplifies
these signals
•The receiver also provides a means for
compensating for differences in echo
strength which result from attenuation by
different tissues by the control of time
gain compensation

78. •The TGC control can permit the user
to selectively amplify the signal from
deeper structure & suppress the
signal from superficial structures
•Another important function of the
receiver is the compression of the
wide range of amplitudes returning
to the transducer into the range that
can be displayed to the user

80. Dynamic rangeDynamic range
•The ratio of the highest to the lowest
amplification that can be displayed
may be expressed in decibels is
called as the dynamic range
•Although the amplifiers used in usg
machines can handle a dynamic
range upto120 dB ,grey scale
displays are limited to display signal
intensity of 35 to 40 dB

82. Image displayImage display
•Over the years imaging has evolved from
simple A mode & bistable display to high
resolution real time grey scale imaging
•The A mode devices displayed the voltage
produced across the transducer by the
backscatter echo as a vertical deflection
on the face of the oscilloscope & the
horizontal sweep was calibrated to indicate
the distance from the transducer to the
reflecting surface
•With this only the position & strength of
the reflecting structure are recorded

83. M modeM mode
•Another simple form is the M mode
•This displays the echo amplitude & shows
the position of moving reflectors
•This mode uses brightness of the display
to indicate the intensity of the reflected
signal
•M mode is interpreted by assessing the
motion patterns of specific reflectors &
determining the anatomic relationship
from characteristic patterns of motion

85. •The major application of M mode is
in the evaluation of rapid motion of
cardiac valves & of the cardiac
chamber & vessel walls

86. Real time, gray scale, B Real time, gray scale, B
mode displaymode display
•To generate a 2D image multiple
ultrasound pulses are sent down a series
of successive scan lines building a 2D
representation of echoes arising from the
object being scanned
•When an ultrasound image is displayed on
a black background signals of greatest
intensity appears white ,absence of signal
is shown as black & signals of
intermediate intensity appear as shades of
grey

87. •Realtime ultrasound produces the
impression of motion by generating a
series of individual 2D images at the
rate of 15 to 60 frames per sec.
•So transducers used for real time
imaging should be able to steer the
beam as 30 to 60 complete images
should be generated per sec.

89. Transducer selectionTransducer selection
•The highest ultrasound frequency
permitting penetration to a depth of
interest should be selected
•For superficial structures 7.5 to 15
MHz are used ,for deeper structures
like the abdomen 2.5 to 3.5 MHz
may be used

90. Special imaging modesSpecial imaging modes
•Harmonic imaging:
•Variation of the propagation velocity of sound in
fat & other tissues near the transducer results in
phase aberration that distorts the usg field
producing noise & clutter in the usg image
•Nonlinear propagation of ultrasound through
tissue is associated with more rapid propagation
of high pressure component ,this results in
increasing distortion of the acoustic pulse &
causes generation of multiple harmonics of the
transmitted frequency

91. •Tissue harmonic imaging takes
advantage of the generation of these
harmonic waves deep in the tissue
•As most imaging artifacts appear in
the superficial plane near the
transducer skin interface ,harmonic
imaging is beneficial as these
artifacts are eliminated using filters

94. Spatial compoundingSpatial compounding
•An important source of image degradation
is ultrasound speckle
•They result from the constructive &
destructive interaction of the acoustic
fields generated by the scattering of
ultrasound
•By summing images from different
scanning angles ,significant improvement
in the contrast to noise ratio can be
achieved

98. Image qualityImage quality
•Spatial resolution is the ability to
differentiate two closely situated objects
as distinct structures
It must be considered in three planes:
Axial resolution-
It is the maximum resolution along the
beam axis & is determined by the pulse
length i.e. the product of the wavelength
& the no. of cycles in the pulse

99. So higher the transducer frequency
higher is the image resolution

101. Lateral resolution :
•Refers to the resolution in the plane
perpendicular to the beam & parallel to
the transducer
•It is determined by the width of the beam
•It can be controlled by focusing the beam
by electronic phasing to alter the beam
width at a selected depth of interest

103. Azimuth or elevation resolution:
•It refers to the slice thickness in a
plane perpendicular to the beam & to
the transducer
•It is mainly determined by the
construction of the transducer &
generally cannot be controlled by the
user

104. Artifacts Artifacts
•Many artifacts suggest the presence
of structures not actually present eg.
Reverberation ,refraction & side
lobes
•Some artifacts remove real echoes
from the display or obscure info. &
important pathology may be missed
eg. Shadowing ,multipath artifact

105. Propagation velocity Propagation velocity
artifactsartifacts
•When a sound passes through a
lesion containing fat in the liver,echo
return is delayed as fat has a
propagation velocity of 1450 & liver
has 1540m/sec
•So the final image shows a
misregistration artifact in which the
diaphragm shows a deeper position
than expected

107. Refraction artifactRefraction artifact
•Refraction causes bending of the sound
wave so that targets not along the axis of
the transducer are insonated
•Their reflections are then detected &
displayed in the image
•This may cause structures to appear in the
image that actually lie outside the volume
the investigator assumes is being
examined

109. Side lobes artifactsSide lobes artifacts
•Though most of the energy generated by
the transducer is emitted in a beam along
he central axis ,some energy is
transmitted from the sides
•These are called side lobes & are lower
intensity than a primary beam
•These may interact with strong reflectors
that lie outside of the scan plane &
produce artifacts that are displayed in the
usg image

112. Reverberation artifactReverberation artifact
•They arise when the usg signal
reflects repeatedly between highly
reflective surfaces that are usually
near the transducer
•This results in delayed echo return to
the transducer
•Appears as a series of regularly
spaced echoes at increasing depth

114. Shadowing Shadowing
•When there is marked reduction in
the intensity of ultrasound deep to a
strong reflector or attenuator there
is a partial or complete loss of info.
•Another common cause is improper
setting of TGC control ,poor scanning
angles, inadequate penetration, poor
resolution

115. Multipath artifactMultipath artifact
•Echoes reflected from highly
reflective surfaces like the diaphragm
or the bladder create complex echo
paths that delay return of echoes to
the transducer
•This results in display of these
echoes at a greater depth than they
should normally appear

117. Thank You!