physics of ultrasound

1. Physics Of Ultrasound And
Echocardiography

2. History of Ultrasound Imaging
▫ 1760 - Abbe Lazzaro Spallanzani – Father of ultrasound
▫ 1912 - First practical application for rather unsuccessful search
for Titanic
▫ 1942 - First used as diagnostic tool for localizing brain tumors
by Karl Dussik
▫ 1953 - First reflected Ultrasound to examine the heart, the
beginning of clinical echocardiography – Dr.Helmut Hertz , a
Swedish Engineer and Dr. Inge Edler a cardiologist
▫ 1970s - Origin of TEE ,Lee Frazin, a cardiologist from Chicago
mounted M-mode probe on a Transoesophageal probe.

3. Topic outline
1. Echo
basics
• Tips on Ultrasound waves / interaction with
tissues
• Ultrasound transducers /probes
• Image Resolution
2. Imaging
modes
• 2-D Imaging & Imaging planes (normal 2D
Echo)
• M-Mode (Normal M-Mode Echo)
3. Doppler Echo
• Basic principles
• Doppler Imaging Modalities
• CW Doppler
• Pulsed Doppler
• CF Doppler
• Relationship between Doppler velocity and
pressure gradient

4. Sound
Mechanical vibration transmitted through an
elastic medium
Pressure waves when propagate thro’ air at
appropriate frequency produce sensation of
hearing
Vibration Propagation
Surface Vibration Pressure Wave Ear

5. As sound propagates through a
medium the particles of the medium
vibrate
Air at equilibrium, in the
absence of a sound
wave
Compressions and
rarefactions that
constitute a sound
wave

6. “Sine wave”
 Amplitude - maximal
compression of
particles above the
baseline
 Wavelength - distance
between the two
nearest points of equal
pressure and density
One Compression and rarefaction constitute one sound wave . It can
be represented as “Sine wave”.

7. Velocity = frequency x
Wavelength
 Frequency – No. of wavelengths per unit time. 1
cycle/ sec = 1 Hz
 So, Frequency is inversely related to wavelength
 Velocity – Speed at which waves propagate
through a medium
– Dependent on physical properties of the medium
through which it travels
– Directly proportional to stiffness of the material
– Inversely proportional to density within a
physiological limit

8. Sound velocity in different
materials
Material Velocity ( m/s)
Air 330
Water 1497
Metal 3000 - 6000
Fat 1440
Blood 1570
Soft tissue 1540

9. ULTRASOUND
Ultrasound is sound with a frequency
over 20,000 Hz, which is the upper limit
of human hearing.
The basic principles and properties are
same as that of audible sound
Frequencies used for diagnostic
ultrasound are between 1 to 20 MHz

10.  Medical ultrasound imaging typically uses sound waves at frequencies of
1,000,000 to 20,000,000 Hz (1.0 to 20 MHz). In contrast, the human
auditory spectrum (between 20 and 20,000 Hz)
 Frequency and wavelength are mathematically related to the
velocity of the ultrasound beam within the tissue:
Velocity = Wavelength (mm) x frequency (Hz)
 The speed with which an acoustic wave moves through a medium is
dependent upon the density and resistance of the medium.
 Media that are dense will transmit a mechanical wave with greater
speed than those that are less dense.
 The resolution of a recording, ie, the ability to distinguish two objects
that are spatially close together, varies directly with the frequency
and inversely with the wavelength
High frequency, short wavelength ultrasound can separate objects
that are less than 1 mm apart.

11.  Imaging with higher frequency (and lower wavelength) transducers
permits enhanced spatial resolution
 However, because of attenuation, the depth of tissue
penetration or the ability to transmit sufficient ultrasonic energy
into the chest is directly related to wavelength and therefore
inversely related to transducer frequency
 As a result, the trade-off for use of higher frequency
transducers is reduced tissue penetration
 The trade-off between tissue resolution and penetration guides the
choice of transducer frequency for clinical imaging.
 As an example, higher frequency transducers can be used
in echocardiography for imaging of structures close to the
transducer.

12. Interaction of ultrasound wave with
tissues
1. Attenuation
2. Reflection
3. Scattering
4. Absorption

13. Attenuation
 Loss of intensity and amplitude of ultrasound wave as it
travels through the tissues
 Due to reflection, scattering and absorption
 Proportional to Frequency and the distance the wave
front travels –
 Higher frequency , more attenuation
 Longer the distance (Depth), more the attenuation
 And also on the type of tissue through which the beam
has to pass
 Expressed as “Half – power distance”
 For most of soft tissues it is 0.5 – 1.0 dB/cm/MHz

14. Reflection
Basis of all ultrasound imaging
From relatively large, regularly shaped objects with
smooth surfaces and lateral dimensions greater than one
wavelength – Specular Echoes
These echoes are relatively intense and angle
dependent.
From endocardial and epicardial surfaces, valves and
pericardium
Amount of ultrasound beam that is reflected depends on
the difference in Acoustic impedance between the
mediums

15.  The resistance that a material offers to
the passage of sound wave
 Velocity of propagation “v” varies between
different tissues
 Tissues also have differing densities “ρ”
 Acoustic impedance
“Z = ρv”
 Soft tissue / bone and soft tissue / air
interfaces have large “Acoustic Impedance
mismatch”
Acoustic Impedance

16. Scattering
Type of reflection that occurs when
ultrasound wave strikes smaller(less than
one wavelength) , irregularly shaped
objects - Rayleigh Scatterers ( e.g.. RBCs)
Are less angle dependant and less intense.
Weaker than Specular echoes
Result in “Speckle” that produces the
texture within the tissues

18. Interaction Of Ultrasound Waves With
Tissues
When an ultrasonic wave travels through a homogeneous medium, its path is a straight line.
However, when the medium is not homogeneous or when the wave travels through a
medium with two or more interfaces, its path is altered; either of the ff:
 Scattering:
 Small structures, eg, less than 1 wavelength in lateral dimension, result in scattering of
the ultrasound signal
 Unlike a reflected beam, scattering results in the US beam being radiated in all
directions, with minimal signal returning to the transducer
 Refraction:
 Attenuation:
 Signal strength is progressively reduced due to absorption of the US energy by
conversion to heat (frequency and, wavelength dependent)
 The depth of penetration:
 30 cm for a 1 MHz transducer,
 12 cm for 2.5 MHz transducer, and
 6 cm for a 5 MHz transducer
 Air has a very high acoustic impedance, resulting in significant signal attenuation
when imaging through lung tissue, especially emphysematous lung, or pathologic
conditions such as pneumomediastinum or subcutaneous emphysema
 In contrast, filling of the pleural space with fluid, generally enhances ultrasound
imaging

19. How is ultrasound imaging done?
“From sound to image”

20. Pierre Curie
(1859-1906),
Nobel Prize in
Physics, 1903
Jacques Curie
(1856-1941)
PIEZOELECTRIC
EFFECT

21. Piezoelectric effect
Crystals of tourmaline, quartz, topaz, cane
sugar, and Rochelle salt have the ability to
generate an electric charge in response to
applied mechanical stress
“Piezoelectricity" after the Greek word
Piezein, which means to squeeze or press.
“Converse” of this effect is also true

22. Construction of a Transducer
Backing
Material
Electrodes
Piezoelectric
crystal

24. US transducers use a piezoelectric crystal to generate and receive
ultrasound waves
Image formation:
is related to the distance of a structure from the transducer,
based upon the time interval between ultrasound transmission and
arrival of the reflected signal
The amplitude is proportional to the incident angle and acoustic
impedance, and timing is proportional to the distance from the transducer
Ultrasound Transducers

25. Production of ultrasound
1. Piezoelectric crystal
2. High frequency electrical signal with continuously
changing polarity
3. Crystal resonates with high frequency
4. Producing ULTRASOUND
5. Directed towards the area to be imaged
6. Crystal “listens” for the returning echoes for a given
period of time
7. Reflected waves converted to electric signals by the
crystal
8. processed and displayed

26. Schematic representation of the recording
and display of the 2-D image

28. Electronic Phased Array
which uses the principle
of Electronic Delay
Phased Array Transducers

30. Electronic Focusing
Electronic beam
steering

31. Characteristics of ULTRASOUND BEAM

32. Length of near field = ( radius)2 / wavelength
of emitted ultrasound

33. TEE workstation

34. Resolution
Ability to distinguish two points in space
Two components –
Spatial – Smallest distance that two targets
can be separated for the system to
distinguish between them.
Two components – Axial and Lateral
Temporal

35. • Axial Resolution
▫ The minimum separation
between structures the
ultrasound beam can
distinguish parallel to its
path.
▫ Determinants:
▫ Wavelength – smaller the
better
▫ Pulse length – shorter the
train of cycles greater the
resolution

36. • Lateral Resolution
▫ Minimum separation
between structures the
ultrasound beam can
distinguish in a plane
perpendicular to its path.
▫ Determinants:
▫ Depends on beam width –
smaller the better
▫ Depth
▫ Gain

37. Temporal resolution
Ability of system to accurately track
moving targets over time
Anything that requires more time will
decrease temporal resolution
Determinants:
Depth
Sweep angle
Line density
PRF

38. The Trade off ..

39. To visualize smaller objects shorter
wavelengths should be used which can be
obtained by increasing frequency of U/S
wave.
Drawbacks of high frequency –
More scatter by insignificant inhomogeneity
More attenuation
Limited depth of penetration
For visualising deeper objects lower frequency
is useful, but will be at the cost of poor resolution
So..

40. The reflected signal can be
displayed in four modes..
A- mode
B- mode
M- mode
2-Dimensional

41. A. Twodimensional (2-D) imaging :
– A 2D image is generated from data obtained mechanically (mechanical
transducer) or electronically (phased-array transducer)
– The signal received undergoes a complex manipulation to form the final
image displayed on the monitor including signal amplification, time-gain
compensation, filtering, compression and rectification.
B. M-mode:
 Motion or "M"-mode echocardiography is among the earliest forms of
cardiac ultrasound
 The very high temporal resolution by M-mode imaging permits:
– identification of subtle abnormalities such as fluttering of the anterior
mitral leaflet due to aortic insufficiency or movement of a vegetation.
– dimensional measurements or changes, such as chamber size and
endocardial thickening, can be readily appreciated
2-D & M Mode

42. B- Brightness
mode shows
the energy as
the brightness
of the point
M- Motion mode
the reflector is
moving so if the
depth is shown in a
time plot, the
motion will be seen
as a curve
A
B
C

43. M- mode
• Timed Motion display ; B – Mode with time
reference
• A diagram that shows how the positions of
the structures along the path of the beam
change during the course of the cardiac
cycle
• Strength of the returning echoes vertically
and temporal variation horizontally

44. M – Mode uses..
Great temporal resolution- Updated
1000/sec. Useful for precise timing of
events with in a cardiac cycle
Along with color flow Doppler – for the
timing of abnormal flows
Quantitative measurements of size ,
distance & velocity possible with out
sophisticated analyzing stations

46. 2 – D MODE
 Provides more structural and functional
information
 Rapid repetitive scanning along many different
radii with in an area in the shape of a fan
 2-D image is built up by firing a beam , waiting
for the return echoes, maintaining the
information and then firing a new line from a
neighboring transducer along a neighboring line
in a sequence of B-mode lines.

47. 2-D imaging by steering the transducer over an
area that needs to be imaged

48. Mechanical Steering of the Transducer

49. Electronic Phased Array Transducers for 2-D imaging
Linear Array Curvilinear Array

50. A single ‘FRAME’ being formed
from one full sweep of beams
A ‘CINE LOOP’ from multiple FRAMES

51. Resembles an anatomic section – easy to
interpret
2-D imaging provides information about
the spatial relationships of different
parts of the heart to each other.
Updated 30- 60 times/sec ; lesser
temporal resolution compared to M-mode

53. OPTIMIZATION OF 2-D IMAGES
Technical Factors I
 TRANSDUCER:
 High frequency increases backscatter and resolution but lacks depth
penetration
 Low-frequency transducers permit good penetration but reduced image
resolution
 DEPTH:
 The deeper the field of the image, the slower the frame rate
 The smallest depth that permits display of the region of interest should be
employed
 FOCUS:
 Indicates the region of the image in which the ultrasound beam is narrowest
 Resolution is greatest in this region
 GAIN:
 This function adjusts the displayed amplitude of all received signals

55. Study of blood flow dynamics
Detects the direction and velocity
of moving blood within the heart.
Doppler Study

56. Comparison between 2-D and
Doppler
2-D Doppler
Ultrasound
target
Tissue Blood
Goal of
diagnosis
Anatomy Physiology
Type of
information
Structural Functional
So, both are complementary to each other

57. Christian Andreas Doppler
(1803 – 1853)
DOPPLER
EFFECT

58. DOPPLER EFFECT-
Certain properties of light emitted from stars depend upon
the relative motion of the observer and the wave source.
Colored appearance of some stars as due to their motion
relative to the earth, the blue ones moving toward earth and
the red ones moving away.

59. OBSERVER 2
Long wavelength
Low frequency
OBSERVER 1
Small wavelength
High frequency

60. Doppler Frequency Shift - Higher returned
frequency if RBCs are moving towards the and lower
if the cells are moving away
Doppler principle as applied in Echo..

61. The Doppler equation
Velocity is given by Doppler
equation..
V = c fd / 2 fo cos 
V – target velocity
C – speed of sound in tissue
fd –frequency shift
fo –frequency of emitted U/S
 - angle between U/S beam & direction of
target velocity( received beam , not the
emitted)

62. Doppler Equation

63. Doppler blood flow velocities are
displayed as waveforms

64. When flow is perpendicular to U/S beam
angle of incidence will be 900/2700 ;
cosine of which is 0 – no blood flow detected
Flow velocity measured most accurately
when beam is either parallel or anti parallel
to blood flow.
Diversion up to 200 can be tolerated( error of
< or = to 6%)
Important consideration !

65. “Twin Paradoxes of Doppler”
Best Doppler measurements are made
when the Doppler probe is aligned
parallel to the blood flow
High quality Doppler signals require low
Doppler frequencies( < 2MHz)

66. Importance of being parallel to flow when
detecting flow through the aortic valve

67. Velocity is directly proportional to
frequency shift and for clinical use
it is usual to discuss velocity
rather than frequency shift (
although either is correct)
V a fd / cos V = c fd / 2 fo cos V a fd

68. BASIC PRINCIPLES:
 utilizes ultrasound to record blood flow within the cardiovascular
system (While M-mode and 2D echo create ultrasonic images of the
heart)
 is based upon the changes in frequency of the backscatter signal from
small moving structures, ie, red blood cells, intercepted by the
ultrasound beam

69.  A moving target will backscatter an ultrasound beam to the
transducer so that the frequency observed when the target is
moving toward the transducer is higher and the frequency
observed when the target is moving away from the transducer
is lower than the original transmitter frequency
 This Doppler phenomenon is familiar to us as the sound of a train
whistle as it moves toward (higher frequency) or away (lower
frequency) from the observer
 This difference in frequency between the transmitted frequency (F[t]) and
received frequency (F[r]) is the Doppler shift:
 Doppler shift (F[d]) = F[r] - F[t]

70. Doppler effect(Pairs of transmitting (T) and receiving (R) transducers):
• With a stationary target (panel A): the carrier frequency [f(t)] from the
transmitting transducer strikes the target and is reflected back to the
receiving transducer at the reflected frequency [f(r)], which is unaltered
• with a target moving toward the transducer (panel B): An increase in
f(r) is seen
• with a target moving away from the transducer (panel C): f(r) is
reduced
•In all cases, the extent to which f(t) is increased or reduced is proportional to
the velocity of the target

71. A flow moving toward the transducer has a higher observed
frequency than a flow moving away from the transducer.

72.  Blood flow velocity (V) is related to the Doppler shift by the
speed of sound in blood (C) and ø (the intercept angle between
the ultrasound beam and the direction of blood flow)
 A factor of 2 is used to correct for the "round-trip" transit time to and from
the transducer.
 F[d] = 2 x F[t] x [(V x cos ø)] ÷ C
 This equation can be solved for V, by substituting (F[r] - F[t]) for
F[d]:
 V = [(F[r] -F[t]) x C] ÷ (2 x F[t] x cos ø)

73.  the angle of the ultrasound beam and the direction of blood flow are
critically important in the calculation
 For ø of 0º and 180º (parallel with blood flow), cosine ø = 1
 For ø of 90º (perpendicular to blood flow), cosine ø = 0 and the Doppler shift is 0
 For ø up to 20º, cos ø results in a minimal (<10 percent) change in the Doppler shift
 For ø of 60º, cosine ø = 0.50
 The value of ø is particularly important for accurate assessment of
high velocity jets, which occur in aortic stenosis or pulmonary
artery hypertension
 It is generally assumed that ø is 0º and cos ø is therefore 1
•Ideally, the beam should be placed
parallel to blood flow
When the beam does not lie parallel,
it is possible to introduce a correction
into the calculation of flow velocity by
measuring the cosine of the angle of
interrogation and introducing this value
into the Doppler equation

74. SPECTRAL ANALYSIS
 When the backscattered signal is
received by the transducer, the
difference between the transmitted
and backscattered signal is
determined by comparing the two
waveforms with the frequency
content analyzed by:
fast Fourier transform (FFT)
 The display generated by this
frequency analysis is termed
spectral analysis
 By convention, time is
displayed on the x axis and
frequency shift on the y axis
 Shifts toward the transducer
are represented as
"positive" deflections
from the "zero" baseline, and
shifts away from the
transducer are displayed as
"negative" deflections

75. • Spectral information can be displayed in real time (Doppler figure)
The Doppler signal portrays the entire period of flow, ie:
acceleration (a),
peak flow (pf), and
deceleration (d).

80. Applications of Doppler - Different
modes to measure blood velocities
Continuous wave
Pulsed wave
Colour Flow Mapping

81. Modern echo scanners combine Doppler
capabilities with 2D imaging capabilities
Imaging mode is switched off (sometimes with
the image held in memory) while the Doppler
modes are in operation

82. CONTINUOUS WAVE DOPPLER
Continuous generation of ultrasound waves coupled with
continuous ultrasound reception using a two crystal
transducer

83. CWD at LVOT in Deep TG
Aortic Long axis view

84. Can measure high velocity flows ( in
excess of 7m/sec)
Lack of selectivity or depth discrimination
-Region where flow dynamics are being
measured cannot be precisely localized
Most common use – Quantification of
pressure drop across a stenosis by applying
Bernoulli equation

85. 1/2 PV2 Pressure
Kinetic
Energy
Potential
Energy
P = 4V2
Bernoulli Equation
Balancing Kinetic and Potential
energy
This goes down..As this goes up..

86. Doppler Velocity And Pressure Gradient
 Doppler echo can estimate the pressure difference across a stenotic
valve or between two chambers
 This r/n ship is defined by the Bernoulli equation and is dependent on :
 velocity proximal to a stenosis (V1)
 velocity in the stenotic jet (V2)
 density of blood (p), acceleration of blood through the orifice
(dv/dt), and viscous losses (R[v]):
 The pressure gradient (Δ P) can be calculated from:
 Δ P = [0.5 x p x (V2 x V2 - V1 x V1)] + [p x (dv/dt)] + R[v]
(If one assumes that the last two terms (acceleration and viscous losses) are small,
and then enter the constants, the formula is simplified to):
 Δ P (mmHg) = 4 x (V2 x V2 - V1 x V1)
 Thus, the Bernoulli formula may be further simplified:
 Δ P (mmHg) = 4V2

88. PULSED WAVE DOPPLER
Doppler interrogation at a particular depth
rather than across entire line of U/S beam.
Ultrasound pulses at specific frequency -
Pulse Repetition Frequency (PRF) or
Sampling rate
RANGE GATED - The instrument only
listens for a very brief and fixed time after
the transmission of ultrasound pulse
Depth of sampling by varied by varying the
time delay for sampling

89. Transducer alternately transmits and
receives the ultrasound data to a sample
volume. Also known as Range-gated
Doppler.

90. PWD at LVOT in Deep TG
aortic long axis view

91. PRF for a given transducer of a given
frequency at a particular depth is fixed; But
to measure higher velocities higher PRFs
are necessary
Drawback – ambiguous information
obtained when flow velocity is high
velocities (above 1.5 to 2 m/sec)
This effect is called Aliasing

92. ALIASING
Aliasing will occur if low pulse repetition
frequencies or velocity scales are used and high
velocities are encountered
Abnormal velocity of sample volume exceeds the
rate at which the pulsed wave system can record it
properly.
Blood velocities appear in the direction opposite to
the conventional one

93. Full spectral
display of a high
velocity profile
fully recorded by
CW Doppler
PW display is
aliased, or cut
off, and the top
is placed at the
bottom

94. Aliasing occurs if the
frequency of the
sample volume is more
than the Nyquist limit
Nyquist limit = PRF/2

95. To avoid Aliasing - PRF = 2 ( Doppler shift
frequency or Maximum velocity of Sample
volume)
Can be achieved by – Decreasing the
frequency of transducer, decrease the
depth of interrogation by changing the view
( this increases the PRF)

96. Color Flow Doppler
Displays flow data on 2-D
Echocardiographic image
Imparts more spatial information to
Doppler data
Displays real-time blood flow with in the
heart as colors while showing 2D images
in gray scale
Allows estimation of velocity, direction
and pattern of blood flow

97. Multigated, PW Doppler in which blood flow velocities
are sampled at many locations along many lines
covering the entire imaging sector

98. Echo data is processed through two channels
that ultimately combine the image with the
color flow data in the final display.

99. Color Flow Doppler..
Flow toward transducer – red
Flow away from transducer – blue
Faster the velocity – more intense is
the colour
Flow velocity that changes by more
than a preset value within a brief
time interval (flow variance) – green
/ flame

100. CFM v/s Angiography
CFM Angiography
Records velocity not flow; So
in MR, CFM jet area consists
of both atrial and ventricular
blood – Billiard Ball Effect
Records flow
Larger regurgitant orifice
area there will be smaller jet
area
Larger regurgitant orifice
area there will be larger jet
area

101. Instrumentation factors in Color Doppler
Imaging
Eccentric jets appear smaller than equivalently
sized central jets – Coanda Effect
High pressure jet will appear larger than a low-
pressure jet for the same amount of flow
As gain increases, jet appears larger
As ultrasound output power increases, jet area
increases
Lowering PRF makes the jet larger
Increasing the transducer frequency makes the
jet appear larger

102. Advantages & disadvantages Doppler methods used for cardiac
evaluation :
A. continuous wave doppler
B. Pulsed wave doppler
C. color flow doppler

103. CONTINUOUS WAVE DOPPLER
 employs two dedicated ultrasound crystals, one for
continuous transmission and a second for continuous
reception
 This permits measurement of very high frequency Doppler
shifts or velocities
 Limitations of this technique:
 It receives a continuous signal along the entire length of
the US beam
 Thus, there may be overlap in certain settings, such as:
 stenoses in series (eg, left ventricular outflow tract gradient
and aortic stenosis) or
 flows that are in close proximity/alignment (eg, AS and MR)

106. PULSED DOPPLER
 permits sampling of blood flow velocities from a specific region
 In contrast to continuous wave Doppler which records signal along
the entire length of the ultrasound beam
 is always performed with 2D guidance to determine the sample
volume position
 Particularly useful for assessing the relatively low velocity flows
associated with:
1) transmitral or transtricuspid blood flow,
2) pulmonary venous flow,
3) left atrial appendage flow, or
4) for confirming the location of eccentric jets of aortic
insufficiency or mitral regurgitation

108. COLOR FLOW IMAGING
• With CF imaging, velocities are displayed using a color scale:
 with flow toward the transducer displayed in orange/red
 flow away from the transducer displayed as blue

109. SECOND HARMONIC IMAGING
(Improving Resolution)
 An ultrasound wave traveling through tissue becomes distorted,
which generates additional sound frequencies that are harmonics of the
original or fundamental frequency
 produces more harmonics the further it travels through tissue
 uses broadband transducers that receive double the transmitted
frequency
 When compared to conventional imaging, it reduces variations in
ultrasound intensity along endocardial and myocardial surfaces,
enhancing these structures
 of particular benefit for patients in whom optimal echocardiographic
images are technically difficult to obtain
 harmonic imaging improves interphase definition