This presentation is about physics of ultrasonography.

1. PHYSICS OF
ULTRASONOGRAPHY
PRESENTED BY
DR. ABHILASHA SINGH
DNB RESIDENT

2. Applications Training for Service – Ravindran Padmanabhan 2
What is Sound ?
• Sound is a mechanical, longitudinal wave that travels in
a straight line
• Cannot travel through Vacuum
• Velocity of sound depends on the nature of medium.

3. Compression (Longitudinal) Waves
■ Particle motion parallel to direction of
wave travel
1
2
1
2
Wave Travel
Motion of
Individual Coil

4. 3
What is Ultrasound?
• Ultrasound is a mechanical, longitudinal
wave with a frequency exceeding the upper
limit of human hearing, which is 20,000 Hz
or 20 kHz.
• Typically at 2 – 20 Mhz.

7. • Ultrasound was first used for clinical
purposes in Glasgow in 1956.
• Obstretician Ian Donald and engineer
Tom Brown developed first prototype
systems based on an instrument used to
detect industrial flaws in ships.

9. 4
Basic Ultrasound Physics
Velocity
Frequency
Amplitude
Wavelength

10. 5
Velocity of sound
- Speed at which a sound wave travels through a medium(cm/s
- Independent of frequency and depends primarily on the
physical makeup of material through which sound is being
transmitted
- Determined by
1. Compressibility 2.Density
- Velocity – Slowest in air/gas
– Fastest in solids
- Average speed of ultrasound in body is1540m/sec

11. 6
Velocity
Near Field Imaging FarField Imaging
Tissues closer appear on
top and faster the waves
return
Tissuesfurther appearat
the bottom & slower the
waves return

12. 7
Frequency
• Number of cycles per second
• Units are Hertz
• Ultrasound imaging frequency range 2-20Mhz
• In ultrasonic frequency range, the velocity of sound is
constant in any particular medium, when frequency is
increased the wavelength must decrease

13. 8
Frequency:
Higher the freq Lower the penetration and Higher
the resolution
Low the freq higher the penetration and lower the
resolution

14. 9
Wavelength
• Distance over which one cycle occurs

15. 10
Velocity (v), Frequency (ƒ), & Wavelength ( )
λ
Given a constant velocity, as frequency increases
wavelength decreases
V = ƒ λ

16. Intensity OR Loudness
• Determined by the length of oscillation of the particles
conducting the wave
• Greater the amplitude of oscillation, the more intense
the sound
• Ultrasonic intensities are expressed in watts per squa
centimeter

17. Relative Sound Intensity
• Sound intensity is measured in decibels
• A bell is comparison of relative power of two
sound beams expressed log-arithmically using
the base 10
• The number of decibels is obtained by
multiplying the number of bels by 10

20. TRANSDUCER:
 It is a device that converts energy from one form to
another.
 ULTRASOUNDTRANSDUCER converts electric energy
into sound energy and sound energy back into electric
energy.

21. Ultrasound Transducer
 Acts as both speaker & microphone
 Emits very short sound pulse
 Listens a very long time for returning echoes
 Can only do one at a time
Speaker
transmits sound pulses
Microphone
receives echoes

22. How Ultrasound Image is constructed
1. Electrical Energy
converted to Sound
waves
2. The Sound waves are
reflected by tissues
3. Reflected Sound waves are
converted to electrical signals and
later to Image
AcousticAcoustic ImpedanceImpedance
Velocity
Frequency
Velocity
Frequency
Reflection
Amplification
Reflection
Amplification

23. 15
Pulse-Echo Method
• Ultrasound transducer produces
“pulses” of ultrasound waves
• These waves travel within the body and
Interact with various tissues
• The reflected waves return to the transducer
and are processed by the ultrasound
machine
• An image which represents these reflections
is
formed on the monitor

24. TRANSDUCER DESIGN
1. Matching layer
2. Piezoelectric crystal
3. Backing block
4. Acoustic absorber
5. Metal shield
6. Signal cable

26. 1. MATCHING LAYER
 It minimizes the acoustic impedence differences
between transducer and the patient.
 Its impedence is intermediate to that of the soft tissue
and the transducer.
 Its thickness is equal to one-forth of the wavelength,
which is known as quarter wave matching
 Matching layer is made of perspex or plexiglass loaded
with aluminium powder.

27. 2. PIEZOELECTRIC CRYSTALS
 Some naturally piezoelectrc occurring materials
include Berlinite(structurally identical to
quartz),cane sugar,quartz,Rochelle
salt,topaz,tourmaline,and dry bone
 An example of man-made piezoelectric materials
include barium titanate and lead zirconate titanate
(PZT)
 They can be designed to vibrate in either the
thickness or radial mode.

28. Piezoelectric Principle :
■ Voltage generated when certain
materials are deformed by pressure
■ Reverse also true!
– Some materials change
dimensions when voltage applied
■ dimensional change causes
pressure change
– when voltage polarity reversed, so
is dimensional change
V

29. Crystallayer:
 Molecules of piezoelectric crystal are polarized,
one end is positive and other negative.
 When high frequency current is applied, it
alternatively thickens and thins in its short axis, and
generates ultrasound waves as a beam in air infront
and back of the crystal face.

30. 3. DAMPING BLOCK
 Located on the backside of the crystal , made up of
tungsten particles suspended in epoxy resin
 It absorbs backward US pulse and attenuates stray US
signals.
 Transducer and damping block are separated from the
casing by an insulator(rubber cork).

31. Function of dampingblock:
In B- Mode operation,
 It must stop the vibration within a microsecond so
that the transducer becomes ready to immediately
receive the reflected echoes from the body

32. RESONANT FREQUENCY

33. TRANSDUCER Q FACTOR
 Refers to two characteristics of piezoelectric crystals:
-purity of sound
-length of time that the sound persists
 High Q transducer:- Nearly pure sound made up of narrow
range of frequencies.
 Low Q transducer:-Whole spectrum of sound covering a
much wider range of frequencies
 Ring down-time – Interval between initiation of the wave and
complete cessation of vibrations.

34. Q = fo
f2--- f1
Where, Q = Q factor
fo = resonance frequency
f2 = frequency above resonance at which intensity reduced by half
f1 = frequency below resonance at which intensity reduced by half
 Narrow range of sound frequencies and long ring down-time :
Useful for Doppler ultrasound transducer
 Broad range of sound frequencies and short ring down-time : Useful
for organ imaging because it can furnish short ultrasound
pulses and will respond to a broad range of returning frequencies.

35. Spatial Pulse Length
 Depends on source & medium
 as wavelength increases, spatial pulse length increases
Spat. Pulse Length = # cycles per pulse X wavelength
(dist. / pulse) (cycles / pulse) (dist. / cycle)
 Distance in space traveled by ultrasound during one pulse

36.  Calculate SPL for 5 MHz sound in soft tissue, 5 cycles
per pulse
(Wavelength=0.31 mm/cycle)
SPL = 0.31 mm / cycle X 5 cycles / pulse = 1.55 mm / pulse
Spat. Pulse Length = # cycles per pulse X wavelength

37.  as # cycles per pulse increases, spatial
pulse length increases
 as frequency increases, wavelength
decreases & spatial pulse length
decreases
– speed stays constant
 Spatial pulse length determines axial
resolution
Spat. Pulse Length = # cycles per pulse X wavelength
Wavelength = Speed / Frequency

38. 40
Ultrasound - Internals

39. CHARACTERISTICS OF AN ULTRASONIC BEAM

44. Ultrasoundbeamcharacters:
 An unfocused ultrasound beam leaving a flat
crystal has 2 parts:
1. Initial cylindrical segment(near field or
frensnal zone)
2. Diverging conical portion ( far field or
fraunhofer zone)

45. x’ = r2
λ
Where, x’ = length of Frensel zone (cm)
r = radius of the transducer(cm)
λ. = wavelength(cm)
 Zone longest with largest transducer and high frequency
sound
 Zone shortest with small transducer and low frequency
sound

46. The length of near field and divergence of the far field depend upon:
A. FREQUENCY: higher the frequency longer the near fields and
less divergent the far field
Depth resolution increases with higher frequencies
Major drawback-Tissue absorption increases with increasing
frequency
B. CRYSTAL DIAMETER: increasing diameter increases the near
field length but worsens the lateral and depth resolution.

47. FOCUSED TRANSDUCER

49. Ultrasound tissue interaction
1. Reflection
2. Refraction
3. Absorption
4. Attenuation
5. Scattering

50. 1.REFLECTION
 A reflection of a beam is called ECHO.
 The production and detection of echoes forms the basis
of ultrasound.
 Reflection occurs at the interface between two materials.
 It depends on the
1. tissue’s- “ACOUSTIC IMPEDENCE”
2. beam’s angle of incidence
 If two materials have same impedence , no echo produced.

51. Acoustic Impedance
 Definition
Acoustic Impedance = Density X Prop. Speed
(rayls) (kg/m3) (m/sec)
 increases with higher
– Density
– Stiffness
– propagation speed
 independent of frequency

52. Acoustic Impedance of SoftTissue :
Density:
– 1000 kg/m3
Propagation speed:
– 1540 m/sec
Acoustic Impedance = Density X Prop. Speed
(Rayls) (kg/m3) (m/sec)
1000 kg/m3 X 1540 m/sec = 1,540,000 rayls

53.  Differences in acoustic impedance determine fraction of
intensity echoed at an interface
 If the difference in acoustic impedence is:
 Small –weak echo is produced and most of the sound waves
will continue in second medium
 Large- strong echo is produced
 Very large- all sound waves will be totally reflected back.
Example: tissue-air interface 99% of beam is reflected back.

55. Angle of incidence
 The amount of reflection is determined by the angle of incidence between the
sound beam and reflecting surface
 The higher the angle of incidence(i.e., the closer it is to a right angle),the less the
amount of reflected sound
R = Z2 – Z1
2 X 100
Z2 + Z1
Where, R = percentage of beam reflected
Z1=acoustic impedence of medium 1
Z2=acoustic impedence of medium 2

56.  Lung-chest wall interface : 99.9%
 Kidney-fat interface : 0.64%
 Skull-brain interface : 44 %
T = 4Z1Z2
(Z2 + Z1)2
Where, R = percentage of beam transmitted
Z1=acoustic impedence of medium 1
Z2=acoustic impedence of medium 2
 The sum of reflected and transmitted portions of sound beam
must be 100%

60. 2.REFRACTION
Incident
reflective
refraction
Angle of incidence = angle of reflection
Scattered
echoes

61. REFRACTION

62.  The angle of refraction is governed by Snell’s law, which is
sin θi = V1
sin θt V2
where, θi = incidence angle
θt = transmitted angle
V1 = velocity of sound for incident medium
V2 = velocity of sound for transmitting medium
 Refraction can cause artifacts, which cause spatial
distortion (real structures are imaged in wrong location)

63. 3.ABSORPTION

64.  Three factors determine the amount of absorption :
1) the frequency of the sound
2) the viscosity of conducting medium
3) the “relaxation time” of the medium
 Liquids – Low viscosity – Little absorption
Soft tissues – High viscosity – Medium absorption
Bones – Very high viscosity – High absorption
 Relaxation time is the time that it takes for a molecule to return to
its original position after it has been displaced

65.  The relaxation time is a constant for any particular material
 A molecule with a longer relaxation time may not be able to
return completely before a second wave arrives
 Compression wave is moving in one direction and molecule in
opposite direction and hence more energy required to reverse
the direction of molecule and converted to heat.
 In soft tissues there is linear relationship between absorption of
ultrasound and frequency
 The proper frequency is a compromise between the best
resolution (higher frequency) and the ability to propagate the
energy into the tissues(lower frequency)

66. QUARTER WAVE MATCHING
 Method of improving energy transfer is that of
mechanical impedance matching
 A layer of material of suitable thickness and
characteristic impedance is placed on the front surface of
the transducer, the energy is transmitted into the patient
more efficiently
 The thickness of matching layer must be equal to one
fourth the wavelength of sound in the matching layer
Zmatching layer= Ztransducer x Zsoft tissue
 Use of quarter-wave matching will also improve the
transmission of ultrasound pulses returning from tissues
back into the transducer

67. 4.ATTENUATION

68. 5.SCATTERING
 Not all echoes are reflected back to probe.
 Some of them are scattered in all directions in a
non uniform manner.
 More so with very small objects or rough
surfaces.
 Part of scattering goes back to transducer and
generate images is called BACKSCATTER.

72. A-MODE (Amplitude mode)

74.  No memory is built into the display mechanism,so it discards
previous pulses as it receives new ones.
 A permanent record is made by photographing the electronic
display.
Applications ofA-MODE:
 Opthalmology-distance measurements
 Echoencephalography
 Echocardiography
 Detecting a cyst in breast
 Studying midline displacement in brain

75. TM MODE
 For the TM mode spikes are converted into dots,the dots
move back and forth as indicated by arrows.
 To make a permanent record,the motion must be
recorded over a period of time.This is accomplished by
moving the line of dots to the top of scope and then
gradually dropping them to bottom
 A record of sweep time is made with a camera using an
exposure time longer than sweep time
 Disadvantage – Short time can be recorded
 A strip chart record can be as long as the operator
desires and this method is increasing in popularity for
echocardiography.

76. B-MODE (Brightness Mode)

78.  Contact scaning – transducer is placed on patients skin with
mineral oil on skin acting to exclude air and to ensure good
acoustic coupling between transducer and skin.
 If the angle between the perpendicular from the tranducer
surface surface and the interface to be imaged is greater than 5o
the amount of reflected ultrasound returning to the transducer
will be too little to produce an image.
 Compound scanning motion is required to present the surface of
the transducer to the wide variety of interface angles require lung
imaging

79.  Localization of one echo relative to another is accomplished with
a small computer that is fed information by an arm containing
three joints.
 Scanning arm serves:
1) it determines the spatial orientation of sound beam.
2) it constrains the motion of the transducer so that all
components of a single image slice through the same plane in the
patient

80. Gray scale imaging
 The purpose is to display the great variation of the amplitudes of
echoes arising from tissues as varying shades of gray on a
television monitor
 Possible by the development of the scan conversion memory
tube (“scan converter”)
 It is similar to a cathode ray tube,the electron beam is used
alternately to write the information on the target read the
information to generate the signal sent to a television monitor
and erase the target in preparation for receiving a new set of
information
 Target –Silicon backplate about 25mm in diameter on which
more than a million tiny squares of silicon “wafers” are placed.

81.  Final picture is composed only of the strongest echo
detected from each point of the scan ,rather than of a
random addition of numerous signals(called
“overwriting”)
 Two types of scan conversion memory tube:
1. Analog scan converter
2. Digital scan converter
 In analog scan converter tube there is an objectionable
flicker of the image viewed on the monitor, it is due to
fact that tube must simultaneously store the image and
transmit the image to the television monitor.
 Once stored image can be viewed for about 10 min
before image deterioration begins.

82.  Digital scan converter converts variation in amplitude of
echo signal received by the transducer into binary
numbers.
 They are free of gray scale drift
 Have much faster speed,eliminate flicker on monitor and
can be viewed indefinitely
 Control of TV monitor can be used to adjust contrast and
brightness

83. Controls
They are designed to regulate the intensity of echos from
various depths :
1.Time gain compensation
2.Delay
3.Intensity
4.Coarse gain
5.Reject
6. Near gain
7.Far gain
8.Enhancement

84. 1.TIME GAINCOMPENSATION
 TGC amplifies the signal proportional to the time
delay between transmission and detection of US
pulses.
 It amplifies and brings the signal in the range of 40-
50 dB.
 This process compensates for tissue attenuation and
makes all equally reflective boundaries equal in
amplitude irrespective of depth.

86. 2. DELAY CONTROL : Regulates depth at which the TGC begins to augment
weaker signals
3. INTENSITY CONTROL : Determines the potential difference across the
transducer. Increasing intensity produces more energetic ultrasonic beams
and thus stronger echoes at all level.
4. COARSE GAIN : Regulates the height of echoes from all depths
5. REJECT : It discriminate echoes below a minimum amplitude. Cleans up
the image by removing small useless signals.
6. DELAY CONTROL : Regulates depth at which TGC begins to augment the
weaker signal.
7. NEAR GAIN CONTROL : Used primarily to diminish and not to enhance
near echoes.
8. FAR GAIN CONTROL : Used to enhance all distant echoes
9. ENHANCEMENT CONTROL : Augment a localised portion of TGC curve.It
gates a specific depth and enhances echoes within the gate to any desired
level.

90. Ultrasound Beam Profile
 Beam comes out as a slice
 Beam Profile
Approx. 1 mm thick
Depth displayed – user controlled
 Image produced is “2D”
tomographic slice
assumes no thickness
 You control the aim
1mm

91. Accomplishing this goal depends
upon...
■ Resolving capability of the system
– axial/lateral resolution
– spatial resolution
– contrast resolution
– temporal resolution
■ Processing Power
– ability to capture, preserve and display the
information

92. 29
Types of Resolution
• Axial Resolution
– specifies how close together two objects can
be along the axis of the beam, yet still be
detected as two separate objects
– frequency (wavelength) affects axial resolution

93. • Lateral Resolution
– the ability to resolve two adjacent objects that
are perpendicular to the beam axis as
separate objects
– beamwidth affects lateral resolution

94.  Factors affecting
 Width of the beam
 Distance from the transducer
 Frequency
 Side and grating lobe levels

95. • Spatial Resolution
– also called DetailDetail Resolution
– the combination of AXIAL and LATERAL
resolution
– some customers may use this term

96. Temporal Resolution
– the ability to accurately locate the position of moving
structures at particular instants in time
– also known as frame rate

97. Contrast Resolution
– the ability to resolve two adjacent objects of similar
intensity/reflective properties as separate objects -
dependant on the dynamic range

98. Liver metastases

101. -

109. COUPLING AGENTS

111. • Artifacts are the errors in images producedby
physical processes that affect ultrasound beam.
• They are potential pitfalls that might confuse the
examiner.
• Some artifacts provide useful information for novel
interpretation.

112. REVERBERATION

113. RING DOWNARTIFACTS

120. ACOUSTIC SHADOWING
 Tissues deeper to strongly attenuating objects like
calcification, appear darker because the intensity of
transmitted beam is lower.
Example:
 Strong after shadowing due to gall stones.
 Rib shadow

122. ENHANCEMENT
 Seen as abnormally high brightness.
 Occurs when sound travels through a medium with
attenuation rate lower than surrounding tissue.
Example:
 Enhancement of tissues below cyst or ducts.
 Tissues deeper to gall and urinary bladder.

125. THANK YOU….