Basics

1. BASICS OF ULTRASOUND
 BY : DR. CHANDNIWADHWANI
Resident doctor
Department of Radiology
SSG Hospital Vadodara.
 References:
 Radiographics
 Springer
 “The fundamentals of x-ray and radium physics ”-by Joseph Selman
 Rumack

2.  Topics to be covered:
1. History of ultrasound
2. Principle
3. Ultrasound wave and its interactions
4. Ultrasound machine and its parts
5. Image display
6. Artifacts and their clinical importance
7. Doppler ultrasound
8. Elastography
9. Recent advances
10. Safety issues

3. Ultrasound:
 Sound waves with frequency greater than
range of human hearing.(>20,000Hz)
 Audible sound range is 20 to 20,000 Hz.
 The sound with frequency < 15 Hz is called
Infrasound.

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.

11. ULTRASOUND MACHINE

17. ECHOLOCATION

21.  Wavelength: is the distance between
sucessive wave crests.
 Frequency(f) : is the number of cycles per
second( 1 hertz= 1 cycle/second)
 Velocity (c)= wavelength x frequency
 Velocity of sound depends on density and
compressibility of the medium.

22. Sound waves travel faster in solids and slower in gases.
Higher in bone and metal and lower in lungs and air

23. PRINCIPLE OF ULTRASOUND
 Ultrasound works on the principle of
piezoelectric effect.
 When a crystal is subjected to mechanical
pressure, electric voltage is generated and
vice versa.

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

26. 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 property of material-
“ACOUSTIC IMPEDENCE”
 If two materials have same impedence , no
echo produced.

27.  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.

34. 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.

37. ULTRASOUND TISSUE INTERACTION

38. Ultrasound machine:

39. ULTRASOUND MACHINE

42. 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.

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

45. 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.

47. CRYSTAL LAYER
 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.

48. 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).

49. Function of damping block:
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

51. ULTRASOUND BEAM

57. Ultrasound beam characters:
 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)

58.  The length of near field and divergence of the
far field depend upon:
A. FREQUENCY: higher the frequency longer
the near fiels and less divergent the far field
 Depth resolution increases with higher
frequencies
B. CRYSTAL DIAMETER: increasing diameter
increases the near field length but worsens
the lateral and depth resolution.

64. TIME GAIN COMPENSATION
 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.

67. -

79. APPLICATIONS OF A-MODE:
 Opthalmology-distance measurements
 Echoencephalography
 Echocardiography
 Detecting a cyst in breast
 Studying midline displacement in brain

86. DOPPLER mode

87. DOPPLER EFFECT:
 The increasing frequency of a fire engine
siren as it approaches and its decreasing
frequency as it recedes , is known as “doppler
effect”.
 It is caused by the compression of sound
waves ahead of the siren and rarefaction of
sound waves behind the siren.
 This thought made Austrian physicist John
Doppler to discover the doppler effect.

88. Doppler shift:

89. Aliasing:

91.  Artifacts are the errors in images produced by
physical processes that affect ultrasound
beam.
 They are potential pitfalls that might confuse
the examiner.
 Some artifacts provide useful information for
novel interpretation.

92. REVERBERATION

93. RING DOWN ARTIFACTS

100. 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

102. 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.

106. Harmonic imaging:
 Technique of US which provides images of
better quality compared to conventional
imaging.
 Advantages:
 Reduces reverberation side lobe artefacts.
 Increased axial and lateral resolution
 Increased signal to noise ratio
 Improved resolution in patients with large
body habitus.

107. RESOLUTION
 It is the ability to appreciate two closely
placed objects as separate.
 3 types:
1. Axial
2. Lateral
3. temporal
 AXIAL : Ability to resolve objects in the line
of beam. Factors affecting axial resolution
include SPL(spatial pulse length) and
frequency

108. Lateral resolution
 Resolution at 90 degree to the direction of
beam.
 Factors affecting
 Width of the beam
 Distance from the transducer
 Frequency
 Side and grating lobe levels

109. Temporal resolution
 Ability to detect moving objects in the field of
view in their true sequence.
 It is determined by:
 Frame rate(number of frames generated per
unit second)

110. ELASTOGRAPHY
 Also called ‘sonoelastography’(palpation
imaging) is a non invasive technique to
depuct relative stiffness or
displacement(strain) in response to imparted
force.
 Basis of elastography is analogous to manual
palpation.
 Stiff tissue deform less and exhibit less strain
than complaint tissue when same force is
applied.

111. Used to evaluate tissues:
 Breast
 Lymph nodes
 Prostate
 Liver
 Blood vessels
 Thyroid
 Musculoskeletal structures
 Kidney transplant monitoring
 Cardiac
 Deep vein thrombosis

112. TECHNIQUES OF ELASTOGRAPHY
1. Quasi-static elastography
2. Dynamic elastography

113. STRAIN ELASTOGRAPHY
 Uses a uniform compression(applied by the user)
at the surface of body to cause deformation of
tissue.
 Scanner calculates and displays the induced
deformation.
 Limitations:
 Operator dependent
 Limited to superficial organs such as breast ,
thyroid
 Absence of quantification.

116. DYNAMIC ELASTOGRAPHY
1. Transient elastography
2. ARFI ( acoustic radiation force imaging)
technique
3. Supersonic shear wave elastography

117. TRANSIENT ELASTOGRAPHY
 Probe (3.5 MHz) contains a vibrator and an
US transducer.
 Principle: mechanical pulse induced at skin
surface by vibrator generates a transient
wave that propagates longitudinally.
 velocity and amplitude of shear wave are
measured.
 This velocity is converted to kPa, and reflects
tissue stiffness.

119. FIBROSCAN(Transient hepatic
elastography)
 Fibroscan device works by measuring shear
wave velocity.
 A 50 MHz wave is passed into liver from a
small transducer.
 The shear wave velocity can be converted
into liver stiffness which is expressed in kilo
pascals.

120. Beyond staging of liver
fibrosis;
 Fibroscan also used to evaluate patients with
portal hypertension
 Assess recurrence of disease after liver
transplantation
 Predict survival by evaluation of liver texture
 Also role in evaluation of breast cancer,
prostate cancer, and diseases in which
fibrosis(desmoplastic reaction) play a crucial
role.

121.  Advantages:
 Gold standard to stage fibrosis
 Non invasive as compared to biopsy
 Easy to use
 Quantification of tissue elasticity
 Rapid , painless
 Good reproducibility
 Limitations:
 Difficult in obese and ascites
 Left sided liver cannot be examined

122. ACOUSTIC RADIATION FORCE
IMAGING (ARFI)
 Uses a focused ultrasound pulse.
 Provides a estimate of stiffness of deep
tissues, non accessible by external
compression.
 Advantages:
 Less user dependent
 Better resolution than strain elastography
 Better transfer of shear modulus contrast to
image contrast

123. ULTRAFAST SHEAR WAVE
ELASTOGRAPHY(SWE)
 This technique developed by supersonic
imaging allows generation of multiple shear
waves along same longitudinal axis leading to
propagation of plane shear wave.
 It allows measurement of velocity of this
plane at each point of image in real time.

126.  Advantages:
 Displayed in real time like conventional US
image.
 Good reproducibility
 Quantitative value of stiffness
 Short acquisition time(~30 ms)
 Limitations:
 Expensive
 Requires a complex software

129. PRACTICAL APPLICATIONS:
 In breast tissues:
 Helps in characterization of benign/malignant
 Characterization of micro calcification
 Elastography of lymph nodes(metastatic
nodes appear stiffer and larger )
 Monitoring treatment response to neo-
adjuvant chemotherapy

132. In liver:
 Assessment of fibrosis(increased stiffness)
 Prediction of cirrhosis related
complications(correlation between FibroScan
values and development of esophageal varices)
 Assessment of response to anti-viral
therapy(stiffness falls with response and increases
with relapse)
 Characterization of liver tumors
 Biopsy site from stiffest region
 Much larger volume of liver assessed than biopsy

133.  Among benign tumors, hepatic adenoma has
greatest stiffness.
 Hemangioma – less shear stiffness than
fibrotic liver
 Cholangiocarcinoma-most stiff
 HCC less stiffer than cholangiocarcinoma.

135. In thyroid:
 Main indication of elastography in thyroid
disease is:
 Nodule characterization-
Example: papillary cancers are hard, follicular
cancers do not increase stiffness.
 Detect metastatic nodes.

137. In Prostate:
 Normal: elasticity values below 30 kPa.
 BPH: central and transition zone becomes
hard and stiff, give increased values of
elasticity
 Carcinoma: peripheral zone, elasticity
values>35 kPa
 Therefore 35 kPa is considered cut off value
to d/d benign and malignant prostatic lesions.

138. Elastography for staging
prostatic cancer
 Allows excellent visualisation of prostatic
capsule as soft rim artifact.

140. In renal application:
 Assessment of fibrosis
 Characterization of tumors

143. AIUM statement:
 “No confirmed biological effects on patients
or operators caused by exposure at intensities
typical for diagnostic ultrasound..
 Current data indicate that the
benefits…outweigh the risks.”

144. Summary:
 Ultrasound> 20,000Hz
 Piezoelectric effect= pulse-echo principle
 Frequency and wavelength are inversely
proportional
 Attenuation and frequency are inversely related.
 Broad bandwidth enables multiHertz probr
 Resolution determines image clarity
 Electronic arrays may be linear or sector
 Shadowing and enhancement are the most useful
artifacts in diagnosis
 Diagnostic ultrasound is safe!