ULTRASOUND PHYSICS MADE EASY

1. ULTRASOUND PHYSICS AND
IMAGE OPTIMIZATION
DR PRAJWITH K J RAI

2. Overview
 Basic principles
 Instrumentation
 Image optimization
 Bioeffects of Ultrasound

3. What is ultrasound?
 Ultrasound or ultrasonography is a medical imaging
technique that uses high frequency sound waves to
obtain cross sectional images of the body.
 Also known as ‘pulse echo’ technique

4. Audible sound 15 – 20 000 Hz
< 15 infrasound > 20 000 ultrasound
Medical USG
1-20 MHz

6. Sound
 Sound is a mechanical energy that travels through
matter, as a result of vibration of the particles of the
medium through which the sound wave is moving.
 Vibration of particles  Transport the energy through
medium  pressure wave
 Change in pressure  sinusoidal waveform

8. compression compression compression compression
rarefaction
rarefaction rarefaction rarefaction
compression
rarefaction

9. Waves
Transverse waves Longitudinal waves
waves parallel to the
direction of energy
transfer
Waves perpendicular to
the direction of energy
transfer

11. Sound waves
longitudinal waves (air)
Compression
Space where density and pressure is
elevated (positive)
Rarefaction
Space where density and pressure is
depressed (negative)

12.  Unit of acoustic frequency(number of cycles in a unit of time) -
Hertz (Hz)
 1 Hz = 1 cycle per second
 High frequencies expressed in kHz and MHz
 1 kHz = 1000Hz
 1 MHz = 1,000kHz

13. Speed of sound
 Determined by properties of medium
 i.e. Its resistance to compression
 C α-1 COMPRESSIBILITY
• FASTER IN SOLID
• SLOWER IN GAS
 Propagation velocity of sound (c) = f x λ

14. VELOCITY
BONE AND
METAL
FASTER
AVERAGE
SOFT
TISSUE
1540m/s
LUNG AND
AIR
SLOWER

16. REFRACTIONREFLECTION
ECHO
ABSORPTION
Interaction of ultrasound with matter

17. Reflection
 Reflection occurs at a boundary/interface between
two adjacent tissues
 Z = ρc

18. Z1
Z2
The difference in acoustic impedance (z)
between the two tissues causes reflection
of the sound wave

19.  angle of incidence α-1 amount of reflected sound

21. soft tissue - air
interface
reflects almost the entire
beam,
transducer must be
directly coupled to the
patient skin without an
air gap
bone - tissue
interface
Bone transmits about
70% of sound energy
due to its impedance
difference - hence not
possible to image
through it
air/gas
interface
causes total reflection -
hence they cast a
shadow and structures
underneath cannot be
imaged
E.g Bowel wall can be
imaged and not its
lumen

22. Specular reflectors are smooth so
when incident beam strikes, the
reflected beam leaves in the same
angle as the incident beam
(bright fibrous structures like)
Diaphragm,
Bladder wall
Tendons
Diffuse reflectors tend to be
rough so when the incident beam
strikes, echoes from the interfaces
are scattered in all directions
kidney,
liver,
small blood vessels
walls of heart chambers
Specular reflectors Diffuse reflectors

24. Interference
If two sound waves of the same wavelength cross each other, the pressure waves combine.
Constructive interference
waves in step/phase,
their amplitudes add up -
Destructive interference
waves out of phase, they
tend to cancel out each other

25. Refraction
 When sound passes from a tissue with one acoustic
propagation velocity to a tissue with a higher or lower
sound velocity, there is a change in the direction of the
sound wave
 Causes artefacts

26. Absorption
 As the ultrasound pulse moves through matter, it continuously
loses energy - attenuation
 There is absorption of the ultrasound energy by the material
and gets converted into HEAT.
 Scattering and refraction interactions also remove some of the
energy from the pulse and contribute to its overall attenuation,
but absorption is the most significant
 Reduction of intensity of beam as it traverses through matter.

27. Attenuation
 Attenuation of sound energy influences the depth
in tissue, from which useful information can be
obtained  Affects transducer selection and
operator controlled instrument settings
 The deeper the wave travels in the body, the
weaker it becomes
 The amplitude/strength of the wave decreases
with increasing depth
 Decibel

28. TISSUE
absorption + reflection + scattering
Attenuation

29.  frequencies a attenuation
 Attenuation determines the efficiency with which ultrasound penetrates
a specific tissue

30. Instrumentation
1. Transmitter - energize transducer
2. Transducer
3. Receiver and processor - detect and amplify
backscattered energy
4. Display – presents the ultrasound image/data
5. Recording/Storage

31. Transmitter
 Pulsed ultrasound
 Produces precisely timed high amplitude voltage as
pulses to energize the transducer
 Controls PRF
 Pulse repetition frequency – rate of pulses emitted by
transducer

32. Pulse repetition frequency
 Determines time interval between pulses
 Determines depth
 PRF – 1 to 10 kHz for diagnostic imaging
 Pulse should travel to depth and return before next
pulse is send
 Highest USG frequency permitting penetration to
depth of interest should be selected

33. Pulse echo principle
 To determine the depth of the tissues.
 T = 2D / c ---- principle of pulse echo
 Generation of sound pulse
 Reflection of pulse from targets
 Detection of reflected echoes
 Calculation and display of range of
targets

35. Transducer
 Device that converts one form of energy into another
 Ultrasonic transducers convert an electric signal  ultrasonic
energy  reflected back from the tissues into electrical signal
 Transducer functions as both transmitter and receiver
 Used in both pulsed and continuous wave modes

37.  Piezoelectric element located near the face of
transducer.
 Front and back faces coated with thin conducting film
 Surfaces of crystal coated with gold or silver electrodes
 Outside electrode grounded to protect the patient
and its surface is coated with water tight electric
insulator
 Inside electrode rest against a thick backing block
that absorbs sound waves transmitted back into
transducer
 Housing – strong plastic
 An acoustic insulator of rubber or cork prevents the
sound from passing into the housing

38. Piezoelectricity
 Discovered by Pierre and Jacques Curie in 1880
 Greek word “piezein” - to press
 Piezoelectric effect – application of an electric field to
certain materials causes a change in their physical
dimensions
 Piezoelectric material - innumerable dipoles arranged
in a geometric pattern
 Each dipole – positive charge at one end and
negative charge at other

39. Generates ultrasound waves
Crystal expands and contracts
Alignment of dipoles
Electric field
Generation of voltage
When crystal gets compressed

40. Serves as ultrasonic signal for display
Voltage amplified
Induces voltage between the electrodes
Forces dipoles to change orientation
Physical compression of crystal element
Transmits their energy to transducer
Echoes reflect back to transducer from each tissue
interface
Sound waves pass though the body

41.  Compression force and associated voltage – piezoelectric effect
 In response to a compressive force on a piezoelectric slab, a net
electric dipole moment  detection of a voltage in response to
this strain
 Reverse piezoelectricity - In response to the application of an
electric field, a deformation of the piezoelectric slab detected.

42. Piezoelectric material
 Quartz – original transducer material
 Ferroelectrics ( artificial piezoelectric material ) – barium titanate,
lead zirconate titanate
 To produce polarization the ceramic is heated at high
temperature in a strong electric field
 At high temperature, the dipoles are free to move, and the
electric field brings them into the desired alignment
 The crystal is then gradually cooled while subjected to a constant
high voltage

43. Curie temperature
 It is the temperature above which this polarization is
lost
 PZT - 365˚C
 Quartz - 573˚C

44. Resonant frequency
 Frequency at which the wavelength = 2 x thickness of
piezoelectric disc
 Constructive interference / resonance of waves emitted from
forwards and back face of disc occurs
 Transducer most efficient as transmitter and receiver of sound
 Depends on thickness of disc and material of disc
(determines velocity)

45. • The thicker the piezoelectric element, the lower the resonant
frequency
• Conversely thinner the element higher the operating frequency
• Range of frequencies produced by a transducer – bandwidth
• Broad band technology produces medical transducers that
contain more than one operating frequencies

46. 2.5 MHz deep abdomen, obstetric and
gynaecological imaging
3.5MHz general abdomen, obstetric and
gynaecological imaging
5.0MHz vascular, breast, pelvic imaging
7.5MHz breast, thyroid
10.0MHz breast, thyroid, superficial veins, superficial
masses, musculoskeletal imaging.
15.0MHz superficial structures, musculoskeletal
imaging.

47. Pulse duration/length
 Transducer continues to vibrate for a short time after it is
stimulated  ultrasound pulse will be several cycles long
 Damping block is used to stop the crystal vibration so that
transducer is ready to receive the reflected waves from tissue
interfaces within the body
 Damping materials used – tungsten and rubber powder
suspended in epoxy resin(backing block)

50. Q factor
 Mechanical coefficient ( Q ) –
mean frequency : bandwidth
 Greater Q –
 narrower bandwidth,
 lesser damping,
 longer ring down time/ pulse duration
 Lesser Q –
 broader bandwidth,
 heavier damping,
 smaller ring down time/ pulse duration

51. Coupling agents
 Commonly known as GEL
 Fluid medium needed to provide a link between the transducer
and the patient
 Composition –
 Carbomer – 10gm
 Propylene glycol – 75gm
 Trolamine – 12.5gm
 EDTA – 0.25gm
 Distilled water – upto 550ml

52. Ultrasound beam characteristics
 Unfocused beam – An ultrasound beam leaving a flat
crystal element has an initial cylindrical segment, followed
by a diverging conical portion
 Frequency –
 Higher the frequency, longer will be the cylindrical
segment or near field (Fresnel zone) and far field
(Fraunhofer zone) becomes less divergent
 Lateral resolution – junction of near and far fields
 Depth resolution improves at higher frequencies
 As frequency is increased, greater absorption of sound
energy  weakens beam intensity

54.  Near field / Fresnel zone
 Interference of pressure waves near the
transducer results in great amplitude
variation
 Far field / Fraunhofer zone
 Farther from the transducer, the waves
diverge and amplitude decreases at steady
state
 Higher frequency transducer, larger diameter - increases length
of near zone and decreases divergence of far zone, hence beam
becomes more directional
Crystal diameter –Increase in crystal diameter
increases the near zone length but worsens
lateral and depth resolution

55. Focused beam
 Improves both lateral and depth resolution
 Concave crystal face – Greater the concavity, nearer the beam
will focus relative to the crystal
 Acoustic lenses – Shorter the focal length, the nearer the lens
will focus the beam relative to the lens face

57. Receiver
 Detects and amplifies weak signals
 Compensates the differences in echo strength, which
results from attenuation by different tissue thickness
 Compression of wider range of amplitudes returning
to transducer into a range that can be displayed to the
user

58. Time gain compensation
 Amplitude of sound pulse diminishes as it travels through body
due to attenuation and echo pulse similarly attenuated as it
travels back to transducer
 Selective amplification of signals received from greater depth
 TGC is under operator control

65. Dynamic range
 Equivalent to number of shades of gray
 Ratio of the largest to the smallest echoes processed by
components of ultrasound device
 Dynamic range decreases as signals pass through the imaging
system as operations such as TGC eliminate small and large
signals  smaller change in echo amplitude that will effect the
gray scale brightness  image contrast is enhanced for smaller
dynamic range
 Echoes returning from tissues can have dynamic ranges of 100 to
150dB
 Increasing dynamic range yields more shades of gray in
ultrasound signals

67. Modes of display
 A mode
 B mode
 M mode

68. A mode
 ‘A’ - amplitude
 Earliest incarnation of ultrasound system
 Shows signals as spikes and height of the spike represents the
amplitude of the reflected signal
 Only shows position of tissue interfaces
 Ophthalmology studies – to detect optic nerve

69. M mode
 ‘M’ – motion
 Detection of motion of a structure or structures over time along
scan line of interest
 A B mode image is frozen on the screen and used to direct the
beam from a stationary transducer along a line of interest
 Echoes are displayed as a line of moving bright dots
 Displays echo amplitude and shows position of moving reflectors
 Evaluation of rapid motion of cardiac valves and of cardiac
chamber and vessel walls

71. B mode
 ‘B’ – brightness
 Involves the pulse echo sequence with the buildup of
an image from innumerable echo blips
 Brightness corresponds to received echo amplitude
 Greatest intensity signal – white
 Absence of signal – black
 Signs of intermediate intensity – gray

73.  Static B mode ultrasound -
 Image is compiled as sound beam is scanned
across patient
 Larger field of view, restricted transducer
movements and single-depth focusing
• Real time ultrasound -
 Image is automatically scanned in a
succession of frames sufficiently rapidly to
demonstrate the motion of tissues
 Frame rate of 15-60/second
 Assessment of both anatomy and motion

74. Gray scale imaging
 Display great amplitudes of echoes arising from
tissues as varying shades of gray on monitor
 Scan convertor –
 Improve grayscale images
 Enable freeze frame
 Manipulate data and archive if necessary
 Write, read and erase
 Analog and digital scan convertors

75. 3D Ultrasound
 Images obtained from a set
of 2D scans
 3D scanners used for fetal,
gynaecological and cardiac
scanning
 4D ultrasound is the term
used for real time 3D
imaging

76. To produce real-time
images
Mechanical scanners
Conventional /group of single
element transducers is mechanically
moved to form images in real time
Oscillating Rotating
electronic scanners
Other uses an array of transducers,
which do not move but are activated
electronically –
Linear Curvilinear Phased

77. Mechanical – Oscillating transducers
With unenclosed crystal –
single element caused to
oscillate through an angle,
which defines the field of view;
Sector scan
With enclosed crystal –
transducer enclosed within a
fluid filled container; use of an
electromagnet and trapezoidal
image produced

79. Mechanical – Rotating transducers
 Employs 3 or 4 crystals mounted symmetrically on a
rotating wheel
 The wheel is driven by an electric motor to perform
circular motion in one direction only
 The crystal elements are excited one at a time to
provide the ultrasound beam
 Replaced by annular array transducers

80. Linear Array
 Array element is pulsed sequentially to produce a
rectangular scan pattern
 At any time, only one element or subset of elements
is in operation. The echoes are received before the
next element is excited
 Used for small parts, vascular and
obstetric applications

82. Curvilinear array
 One element or a subset of
elements at a time scanned
sequentially
 Produces trapezoid scan
pattern

83. Phased array
 All elements are scanned at the same time
 Ultrasound beam is caused to sweep back and forth
across the patient by electronically controlled steering
and focusing
 Ideal for cardiac imaging

84. Transducer selection
 For general purpose – convex with 3.5MHz
 For obstetric purpose – convex or linear with 3.5MHz
 For superficial structures – linear with 5MHz
 For paediatric or thin people – 5MHz

85. Tissue harmonics imaging
 Based on nonlinear propagation of ultrasound wave propagating
through tissue causing distortion and generating harmonics
 It decreases “clutter” noise
 It yields sharper overall images with improved contrast, axial and
lateral resolution
 Used in obstetrics, cardiology and abdominal applications

86. Spatial Resolution
 Ability to distinguish between two closely situated
structures from one another
 Axial or lateral resolution

87. Axial (depth) resolution
 Minimum reflector resolution along axis of ultrasound
beam
 Determined by pulse length
 Higher frequencies  lesser pulse length  higher
axial resolution

88. Lateral (horizontal) resolution
 Minimal reflector resolution perpendicular to beam and parallel
to transducer
 Determined by beam width
 Beam width is a function of wavelength, piezoelectric
transducer diameter, distance from
transducer
 Can be controlled by focusing the beam

90. Image optimization
 Gain and TGC - Sensitivity controls
 Focus – Lateral resolution
 Frequency – Axial resoultion
 High  good axial resolution but poor penetration
 Low  less axial resolution but better penetration
 Depth – decrease in depth  increase in frame rate (temporal
resolution)
 Sector width  increase in frame rate

94. TGC gain freeze depth
focus

95.  Gain –
 Overall brightness of the image
 Amplification of returning ultrasound signal received from all
depths
 High gain  high amplitude  differentiation lost
 Adjust to minimum required to obtain a complete range from low
(dark gray) to high (white) amplitude signals
 Time Gain compensation –
 Adjustment of image brightness at a selective depth
 Corrects varying depths of intensities
 Adjust TGC to decrease in near field and increase in far field (‘’
shape)

96.  Focus –
 Allows focus of ultrasound beam to area of interest
 Optimizes ultrasound intensity in near and far field  improved
spatial resolution
 Depth –
 Adjustment of the depth of field of view
 Greater the depth, lesser resolution and frame rate
 Set the depth to minimum required to visualize all structures of
interest
 Frequency –
 Increased frequency  increase resolution
 Sector width –
 Wide sector  frame rate and temporal resolution

97. • Dynamic range –
• 100dB of ultrasound information available but
monitor can display a much smaller range
• Allows compression of wide spectrum of
amplitudes
• Compressed signals then displayed on monitor
varying shades of gray

98. Linear array
18-5MHz
Linear array
12-3MHz
Curved array
5-1MHz
Tightly curved array
10-3MHz

99. Bioeffects
 Thermal
 Mechanical

100. Thermal bioeffect
 Mechanism - with sound wave attenuation, energy is lost as
heat
 Longer use of ultrasound on the tissue  longer the
ultrasound has attenuated in the tissue
 More the tissue attenuates ultrasound  more the bioeffects
 More ultrasound attenuates at the surface and focal zone
 Higher the frequency/power/PRF/pulse duration , more the
attenuation and thus the heating

101. Thermal indices
 NCRP guidelines – in ultrasound examinations, no temperature
rise > 1°C
 TI – indicator of relative potential for increasing tissue
temperature
 TIS – Thermal index for soft tissue
 TIB – Thermal index for bone, at the focus
 TIC – Thermal index for bone, at surface

102. Mechanical bioeffects
 Current USG systems can produce cavitation
 Determines potential for bubble formation in vivo
 Cavitation -
 Stable
 Inertial / Transient

103. Stable cavitation
 A bubble forms as the peak low pressure region of an
ultrasound pulse passes through a nucleation site
 This bubble grows and shrinks with the high pressure
and low pressure region of the ultrasound pulses
 This growing and shrinking of the bubble causes fluid
to flow around the bubble - micro-streaming
 Causes mechanical damage, sometimes cell lysis

104. Inertial/transient cavitation
 Here, the bubbles formed in a similar way but under
high pressure or intensities, they expand very quickly
following which they collapse violently
 This collapse produces high temperature and
pressure, which damages cells in the region of the
collapse

106. Mechanical index
 It is an approximation of mechanical effects
 It is a calculated number based on ultrasound
parameters and facts about the machine that the
manufacturer built into the programming
 Mechanical index less than 1.9, no mechanical effects
is seen