Physics Lecture Series 2022 Dr L Sibanda.ppt

For learning purposes only

1. The Science of Medical Ultrasound Imaging
Dr Lidion Sibanda
14 September 2021
Contact 1: Introduction

2. Overview
►SRU5101: Ultrasound Physics and Instrumentation Electromagnetic
waves and Sound waves properties; acoustic variables, Power and
intensity. Interaction process of ultrasound energy with matter
(absorption, reflection, refraction, transmission and reflection
coefficients) Ultrasound transducers design, principles of operation and
current developments in ultrasound transducer designs, principles of
operation and current developments in ultrasound transducer designs,
the piezoelectric effect, Beam characteristics, Fresnel and Fraunhoffer
zones, focussing and image resolution, pulse echo principle, Image
display modes, Doppler principles, Signal processing and
instrumentation, pulsed versus continuous wave Doppler, Doppler
applications in Medical Imaging, Image Quality and manipulation in
Ultrasound (Knobology), Recent advances in Ultrasound Imaging,
Biological Effects of ultrasound, Thermal (TI) and non-thermal (MI)
effects, artefacts in ultrasound imaging and quality assurance.

3. General Objectives
►Wave motion
►Basic acoustics
►Ultrasound instrumentation
►Image display and storage
►Special imaging modes
►Image quality
►Imaging pitfalls
►Doppler sonography
►Operating modes: clinical implications
►Therapeutic applications: high-intensity focused
ultrasound

4. The Science of Medical Ultrasound Imaging
Dr Lidion Sibanda
14 September 2021
Lecture 1: Wave motion

5. Objectives
►Classical categories of energy
►Distinct ways of energy transfer
►Classification of wave motion
►4-essential aspects in wave motion
►Concept of wave packets
►Superposition concept
►General wave motion in 1D
(wave function)

6. Chaos & order
►Ethical requirement: health
professionals have an obligation
of diligence & professional duty to
offer optimal services
►Understanding the science of
imaging is fundamental in the
transformation of chaotic imaging
into orderly imaging

7. Classical categories of energy
►Two categories: 1. matter and 2.
fields
►Matter: generally discrete and
localised. Motion described by
newton’s laws of motion
►Fields: continuous and described by
field equations (e.g Maxwell's for
electromagnetic, Newton’s laws for
Gravitational)

8. Two Distinct ways of energy transfer
►1. Energy transfer by dynamicity of
matter. This entails kinetic energy
and therefore momentum of the
body
►2. Energy transfer by wave motion.
Energetic signals transfer by force
fields on medium particles without
net transfer of the mass

9. Wave motion is classified as
Mechanical
OR
Non-mechanical
Mechanical
►Requires medium for support ( Particle
displacement, momentum, energy, stress,
strain – all measured w.r.t equilibrium
position).
►Medium supports wave motion because
of its inertia (mass) and its elasticity
►Examples: sound, seismic, surface water
waves

10. Wave motion is classified as
Mechanical
OR
Non-mechanical
Non-mechanical
►Dispenses with (manages without) material
inertial & elasticity
►Propagates in free space
►Electrical wave- voltage & current
►Electromagnetic and light waves- electric &
magnetic fields
- e.g in free space, wave guides, material medium

11. Complete study of wave
motion entails four essential
aspects.
►1. Description of wave
►2. Excitation and sources
►3. Propagation
►4. Detection

12. Description of wave
►Stipulates those physical quantities of
the wave that are involved: (e.g. particle
velocity, medium stress, strain, pressure,
voltage, current, electric field, magnetic
field e.t.c), together with field equations
& polarization of the wave
- Stipulates polarization and relates
physical quantities to wave energy,
momentum, momentum, angular
momentum (where it is involved).

13. Excitation and sources
►Gives an account of the generation
process for the wave

14. Propagation
►Outlines manner by which the wave moves in
space
►Gives an account of what happens at interfaces
as well as when the wave encounters obstacles
►Deals with interaction processes -actions at
interfaces/obstacles
►Reflection, refraction, diffraction, interference,
dispersion, attenuation, absorption, dissipation
of wave energy by medium as well as any
Doppler effects

15. Detection
►Gives an account of how the wave
signal/energy is efficiently detected
and utilized
►Gives an appraisal/evaluation of
detection & utilization
►Accounts for choice of detectors

16. Superposition concept
►This is a general law of physics that applies whenever
there is a linear relationship between cause and effect
►Classical versus quantum approaches
►Wave Pencils & Wave Packets
►Waves in same medium: Same or Different directions
►Resultant disturbance at any given
Position = vector or linear (algebraic)
Sum of individual disturbances
Result may be enhancement or diminution at
Any given point in time

17. Key points
►Multiple waves propagate
simultaneously and independently in
same medium
►At any point in time there is an
algebraic sum to give net disturbance
►Net disturbance can be an
enhancement or diminution
►After crossing each wave continues in
its original direction without a record of
interaction with the other wave
►A given general wave may represent
superposition of simpler waves

18. General wave motion in 1D
(wave function)
►I-D wave is represented by a wave function
►Function of distance (x) and time (t)
►This wave function Ψ(x,t) is a dynamic variable
that describes the particular wave motion
►The variables x & t thus mark events
►A wave profile can be observed simultaneously
at all x positions at time t

19. The Science of Medical Ultrasound Imaging
Dr Lidion Sibanda
14 September 2021
Lecture 2: Basic Acoustics

20. Objectives
►Propagation of Sound
►Acoustic Impedance
►Reflection
►Refraction
►Attenuation
►Distance Measurement
►Wavelength and Frequency

21. Diagnostic Ultrasound
applications
►Applications pivoted on acoustic energy reflected
from interfaces
►Interpretation of interaction processes provide
both anatomical and physiological information
►USS technology is complex
►Must blend this complexity with high
Competence skills round the sphere
Of knowledge
►Understanding methods &
instrumentation is fundamental

22. Longitudinal (sound) waves
►Sound= mechanical energy travelling in a medium
(matter)
►Alternate compressions & rarefactions such that
►There is limited displacement of medium particles
about their equilibrium positions
►A plot of this pressure against time gives a
sinusoidal wave
►Thus physical quantities (basic units) for the
measurement of sound waves = pressure & time
►Remember physical quantities w.r.t. the wave equation

23. Recall
►Distance between corresponding points on the
wave = wavelength (λ) = single cycle
►Time to complete single cycle = periodic time
(T)
► Number of cycles per unit time = frequency (f)
►Frequency f = 1/T, Unit of acoustic frequency
= hertz (Hz) = sec−1
►In nature acoustic =1 Hz -100 KHz
►Human hearing 20 Hz-20 kHz
►Ultrasound for diagnostics typically = 2 -15 MHz
& these are generally higher than for Doppler
imaging
►Google application of 50-60 MHz

24. Propagation
►Waves travelling in a direction
perpendicular to particle displacement =
transverse waves
►Propagation along direction of particle
displacement = longitudinal wave
►Longitudinal dominant in tissue and
fluids (in conventional USS & Doppler)
►Transverse important in shear wave
elastography

25. Propagation velocity
►Velocity depends on the resistance of the
medium to compression
►Resistance influenced by density & elasticity.
►Velocity directly proportional to elasticity
(stiffness) & density.
►Longitudinal wave velocity generally regarded
as constant for a given tissue and not affected
by the frequency or wavelength of the sound
►However acoustic transverse (shear) wave
velocity is modelled by Young modulus which is
a measure of tissue stiffness or elasticity

26. At an interface: acoustic
impedance
►At interfaces we can have both reflection &
transmission
►At non-shearing & non-absorbing interface particle
velocity is continuous and energy is conserved
►By this concept we can derive amplitude reflection
and transmission coefficients in terms of Z
1 & Z
2
►Reflection (R) & transmission (T) coefficients for energy can also be written in
terms of Z
►Z = acoustic impedance and depends on all three medium parameters

27. Acoustic impedance
►USS imaging can be based on both transmitted
or reflected echoes/pulses
►To produce an echo an appropriate reflecting
interphase must be present
►Totally homogeneous medium has no
interfaces to reflect sound, and the medium
appears anechoic or cystic
►Acoustic interfaces produce echoes
►Amount of reflection or backscatter is
determined by the difference in the acoustic
impedances of the materials forming the
interface.

28. PROPAGATION OF SOUND WAVES
REFLECTION AT AN
INTERFACE
►Amplitude reflection
coefficient
►ΘR = (z1-z2)/(Z1+z2)
►Energy reflection
coefficient
►R = {(z1-z2)/(Z1+z2)} 2
TRANSMISSION AT AN
INTERFACE
►Amplitude transmission
coefficient
►ΘT =2z1/(Z1+z2) 2
►Energy transmission
coefficient
►T = {(4z1z2)/(z1+z2)} 2

29. Impedance matching
Total transmission
{T=1, R=0}

30. (z1 greater than z2)
Soft reflection
►Before
►After

31. (z2 greater than z1)
Hard reflection & Phase reversal
►Before
►After

32. (z2 = Zero)
Free interface, Total
reflection (R=1, T=0) & NO
phase reversal
►Before
►After

33. Class work
►List typical longitudinal wave propagation velocities in tissues
of the human body
►Discuss the clinical implications of the averaged (1540
metres per second) propagation velocity in USS imaging
►How & why would you expect the velocity to vary from lung
tissue to bone tissue?
► Expand what is termed measurement errors/ mis-
registration artifacts/ propagation velocity artefact
►The propagation velocity of sound, c= frequency x
wavelength. Explain the significance of this wavelength in
spatial resolution
►USS machine inherent formulary to compute resolution, but
why then is the user competence of primary importance.

34. Staging (distance measurement)
►Velocity is critical in determining depth of
an interface. Explain the concept of echo-
ranging
► What influences the accuracy of
distance measurements?
►Consider assumptions made in the
calculation: presumed velocity versus
true velocity; Interaction processes -is
reflection the only interaction?
►Does light really travel in a straight path?

35. What are the take home
points?
►Acoustic impedance Z = ρc
►Interfaces with large acoustic impedance
differences reflect almost all the incident energy.
►Interfaces with small differences in acoustic
impedance (muscle and fat interface) reflect only
part of the incident energy
►Propagation velocity & acoustic impedance are
determined by the properties of the tissues
involved and are independent of frequency.

36. Take home points
►Where Z
1 = Z
2 :Impedance matching (T=1 & R=0)
►Where Z
1 greater than Z
2 :reflected wave is in phase
with transmitted wave
►Where Z
1 less than Z
2 : hard reflection and phase
reversal at interface
►Where Z
2 tends to infinity = total reflection (R=1).
Phase reversal. Zero disturbance at terminal
point
►Where Z
2 tends to zero: free interface, disturbance
at terminal point is double the incident
disturbance

37. The Science of Medical Ultrasound Imaging
Dr Lidion Sibanda
14 September 2021
Lecture 3: Interaction processes

38. Objectives
►Reflection
►Refraction
►Diffraction
►Transmission
►Attenuation

39. Specular reflectors
►Ultrasound reflection is determined by the size and
surface features of the interface
►Specular reflectors: Large and relatively smooth
surfaces reflect like a mirror (Diaphragm, Vessel wall,
Wall of urine-filled, bladder, Endometrial stripe)
►Reflection coefficient (R) tells us about fraction of
the incident energy reflected
► If a specular reflector is perpendicular to the incident
sound beam, the amount of energy reflected is
determined by the relationship:
► Therefore, specular display depends on angle of
insolation

40. Diffuse reflectors
►Diffuse reflectors account for the echoes that form the
characteristic echo patterns seen in solid smaller
interfaces within organs
►involve structures with individual dimensions much
smaller than the wavelength
► The constructive and destructive interference of sound
scattered by diffuse reflectors results in the production of
ultrasound speckle, a feature of tissue texture of
sonograms of solid organs
►Important conflicts: most vessel walls behave as specular
reflectors that require insonation at a 90-degree angle for
best imaging, whereas Doppler imaging requires an
angle of less than 90 degrees between the sound beam
and the vessel

41. Misregistration due to
refraction
►Bending of light at interface
governed by Snell law
► Refraction means echo detected is
coming from different depth and
location
►Adjusting beam to be perpendicular
to surface minimizes the artifact

42. Attenuation
►As acoustic energy propagates work is performed
and heat energy is transferred to the medium
●This Acoustic power /Watts does not
account for spatial distribution of energy
●Intensity =power/area does account for
this
●Attenuation determines maximum depth
that can be visualized
●Hence: selection of transducer, machine
settings- Time Gain compensation (TGC),
power output attenuation & system gain
levels

43. Attenuation
►Attenuation is measured relative than absolute (decibel dB)
►This value is 10 times the log in base 10 of the ratio of the
power or intensity values being measured
►For example, if the intensity measured at one point in
tissues is 10 mW/cm2 and at a deeper point is 0.01
mW/cm2, the difference in intensity is???
►Attenuation is the result of the combined effects of
absorption, scattering, and reflection.
►Attenuation depends on the insonating frequency as well as
the nature of the attenuating medium
►High frequencies are attenuated more rapidly than lower
frequencies
►High frequency for superficial imaging!!

44. The Science of Medical Ultrasound Imaging
Dr Lidion Sibanda
14 September 2021
Lecture 4: USS Instrumentation

45. Objectives
To understand:
►The transmitter/pulser that energizes the
transducer
►The ultrasound transducer that produces US
waves
►The receiver and processor that detects and
amplifies the backscattered energy and manipulate
the reflected signals for display
►The display that presents the ultrasound image or
data in a form suitable for analysis and interpretation
►The method to record or store the ultrasound
image

46. Transmitter
►Focus on clinical applications of pulsed ultrasound
►Source of these pulses is called the ultrasound transducer
►Transducer is energized by application of precisely timed
high-amplitude voltage.
► The maximum voltage/maximum acoustic output of
diagnostic scanners is restricted upon manufacture
►Controls that permits attenuation of the output voltage are
common and these avoid exposure of patient to high
ultrasound energy
►Diligent, prudent, ethical use dictates reduction of power
levels to the lowest levels consistent with the diagnostic
problem (Ref ALARA)

47. Key points
►Multiple waves propagate
simultaneously and independently in
same medium
►At any point in time there is an
algebraic sum to give net disturbance
►Net disturbance can be an
enhancement or diminution
►After crossing each wave continues in
its original direction without a record of
interaction with the other wave
►A given general wave may represent
superposition of simpler waves

48. Transmitter
►The transmitter also controls pulse repetition
frequency (PRF) of the transducer
►PRF is important in determining the depth from
which unambiguous data can be obtained both
in imaging and Doppler modes
►Pulse spacing must allow enough time between
the pulses to permit the sound to travel to the
depth of interest and return before the next
pulse is sent

49. Transmitter
►NB Electrically stimulation of transmitter results in a
range or band of frequencies
►Preferential frequency of the pulses is modelled by
transducer material: propagation (vibration) speed of
the transducer material and its thickness
►In the pulsed wave operating modes (seen in most
clinical ultrasound applications), ultrasound pulses
contain additional frequencies- both higher and lower
than the preferential frequency.
►This range of frequencies is termed its bandwidth.
►The shorter the pulse the greater is the bandwidth

50. Transmitter
►Ultrasound bandwidth refers to the range of
frequencies produced and detected by the
ultrasound system
►Generally, modern digital ultrasound systems
employ broad-bandwidth technology
►NB Each tissue has a characteristic wave
speed of a given frequency
►The range of frequencies arising from a tissue
exposed to ultrasound is referred to as the
frequency spectrum bandwidth of the tissue,
or tissue signature

51. Transmitter
►Broad-bandwidth technology is
advantageous
 Provides a means to capture the
frequency spectrum of insonated
tissues,
Preserves acoustic information and
tissue signature
►Reduces speckle artifact by a process
of frequency compounding

52. Transmitter
►Length of an ultrasound pulse = length/duration/number of voltage
pulses
►Continuous wave (CW) ultrasound devices means a constant
alternating current is applied
►In imaging a single/brief voltage pulse causes the transducer to vibrate
at its preferential frequency
►The transducer continues to vibrate a short time after it is stimulated by
the voltage pulse (not mechanically damped!!) and the ultrasound
pulse will therefore, be several cycles long.
►Thus number of cycles of sound in each pulse determines the pulse
length
►Problem: longer pulses result in poorer axial resolution so damping
materials are used
►In clinical imaging applications highly efficient damping results in very
short pulses generally consisting of only two or three cycles

53. Transmitter
►Special transducer coating and
ultrasound coupling gel enhance
efficient transfer of energy from the
transducer to the body
►Subsequently, pulses are
propagated, reflected, refracted,
and absorbed consistent with basic
acoustic principles summarized
earlier

54. Transmitter
►Produced pulses result in a series
of wave fronts that form a three-
dimensional (3-D) beam of
ultrasound
►Features of this beam are
influenced by constructive and
destructive interference, curvature
of the transducer and acoustic
lenses applied to shape the beam

55. Transmitter
►Near field (or Fresnel zone): By interference of
pressure waves near the transducer, pressure
amplitude varies greatly
► Far field (or Fraunhofer zone): Further from
the transducer as determined by the radius of
the transducer and the frequency, the beam
diverges coupled with a steady decrease of the
pressure amplitude
►Modern multi-element transducer arrays, the
precise timing of the firing allows correction of
this divergence and focusing at selected depths

56. To Note: Transducer
►A transducer : converts one form of energy to another
►Ultrasound transducer converts electric energy to
mechanical energy, and vice versa (piezoelectricity)
►Mechanical energy (reflected echoes)
compress/decompress thus provide alternating pressure
changes that give AC voltage
►Electric energy is provided by the transmitter
►Piezoelectric materials respond to an electric field by
changing shape
►Alternating voltage (polarity) applied to the transducer
expands and contracts its thickness =generation of
mechanical pressure wave

57. Receiver
►Returning echoes strike the transducer face
producing small voltages
►The receiver detects, compensates for the
differences in echo strength, amplifies these
weak signals due to attenuation
►The attenuation of sound is proportional to
frequency and is constant for specific tissues
►This is by control of time gain compensation
(TGC) or depth gain compensation (DGC).

58. Receiver
►Echoes from deeper tissues are weaker than
those from superficial tissues= these must be
amplified more in order to produce a uniform
tissue echo appearance
►Thus TGC controls permit the user to
selectively amplify the signals from deeper
structures or to suppress the signals from
superficial tissues thereby compensating for
tissue attenuation
►Automatic TGC common but manual control
remains important in image quality