This document provides an overview of ultrasound physics concepts including: - How ultrasound waves interact with tissue through attenuation, reflection, scattering, refraction, and diffraction. - Key properties of ultrasound waves like wavelength, frequency, amplitude, and acoustic impedance. - Factors that determine image resolution such as transducer frequency and beam focusing. - Common artefacts that can occur like reverberation, side lobes, and multi-path artefacts. - The importance of understanding ultrasound physics principles to optimize image quality and avoid misdiagnosis.

1. AACA Pre-conference Workshop perioperative ultrasound
Nov 5-6th 2006
P ERI O P ERATI V E
U LTRA S O U N D
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Ultrasound Physics
Vascular Access
Regional Anaesthesia
Haemodynamics
An introduction
Ultrasound-guided central and
Ultrasound-guided brachial
FAST scan, Bedside Limited
Dr Lenore George
peripheral vascular access
plexus blockade
Echocardiography
Dr James Lai
Dr Chris Nixon
Dr Neil MacLennan
Dr James Lai
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Page 7
Page 10
Page 13
AACA Pre-conference Workshop
Perioperative Ultrasound
November 5-6th 2006
Ultrasound Physics
The sound waves used diagnostically in ultrasound have a frequency of >
2MHz, whereas the audible range for humans is 20 Hz to 20 kHz.
Sound waves are propagated through a medium by the vibration of molecules
(longitudinal waves). Within the wave, regular pressure variations occur with
alternating areas of Compression, which correspond to areas of high pressure
and high amplitude, and with areas of Rarefaction or low pressure zones
where widening of particles occurs. Sound waves are expressed as sine waves
with the following properties:
Wavelength (λ) is the distance between two areas of maximal compression
(or rarefaction). The importance of wavelength is that the penetration of the
ultrasound wave is proportional to wavelength and image resolution is no more
than 1-2 wavelengths.
Frequency (f) is the number of wavelengths that pass per unit time. It is
measured as cycles (or wavelengths) per second and the unit is hertz (Hz). It is
a specific feature of the crystal used in the ultrasound transducer. It can be varied by the operator within set limits – the higher the frequency, the better the
resolution but the lower the penetration.
A basic knowledge of ultrasound physics is vital to the
correct application of ultrasound for diagnostic and
therapeutic interventions
Image acquisition is highly
operator dependent
A knowledge of the physical
attributes of ultrasound
waves and image generation
is critical to recognition of
artefacts and prevention of
misdiagnosis
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2. AACA Pre-conference Workshop perioperative ultrasound
Nov 5-6th 2006
P H Y S I CS
Interaction of Ultrasound with tissue
Can be described by attenuation, reflection, scattering, refraction and diffraction.
Attenuation
Propagation Velocity (v) is the speed that sound
waves propagate through a medium and depends on
tissue density and compressibility.
The relationship between these variables is expressed
by the Wave Equation
v = λf
In soft tissue propagation velocity is relatively constant at 1540 m/sec and this is the value assumed by
ultrasound machines for all human tissue. Hence
wavelength is inversely proportional to frequency.
Amplitude is the height above the baseline and represents maximal compression. It is expressed in decibels which is a logarithmic scale.
Acoustic Power is the amount of acoustic energy
generated per unit time.
Energy is measured in joules (J) with joules being the
amount of heat generated by the energy in question.
The unit is the Watt (W) with
1W = 1J/sec. The biological effects of ultrasound in
terms of power are in the milliwatt range.
Intensity is the power density or concentration of
power within an area expressed as Watts/m2 or mW/
cm2. Intensity varies spacially within the beam and is
greatest in the centre. In a pulsed beam it varies temporally as well as spacially
The loss of ultrasound as a medium is traversed. Occurs due to absorption of ultrasound energy by conversion to heat as well as reflection, refraction and
scattering. Attenuation is increased (and hence penetration of beam reduced) by:
• Increased distance from the transducer
• Less homogenous medium to traverse due to
increased acoustic impedance mismatch.
• Higher frequency (shorter wavelength) transducers
Air forms a virtually impenetrable barrier to ultrasound, while fluid offers the least resistance.
Reflection
Ultrasound waves are reflected at tissue boundaries
and interfaces. These reflected echoes return to the
transducer and form the basis of all ultrasound imaging. The amount reflected depends on the difference
in acoustic impedance between the two tissues traversed by the beam.
The Acoustic Impedance (Z) is a measure of how
ultrasound traverses that tissue and depends on:
• density of the medium (p)
• propagation velocity of ultrasound through
the medium (v) such that
Z=pv
A large difference in acoustic impedance is referred
to as acoustic impedance mismatch. The greater
the acoustic mismatch the greater the % of ultrasound reflected and the less transmitted. Examples
include soft tissue/ bone and soft tissue/ air interfaces. Indeed, the acoustic impedance of gas or air is
such that it forms a virtually impenetrable barrier to
ultrasound.
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3. AACA Pre-conference Workshop perioperative ultrasound
Nov 5-6th 2006
P H Y S I CS
Refraction
When an ultrasound beam encounters media of different velocities, the proportion of the beam that is
not reflected but is transmitted undergoes refraction
or bending. These phenomena are responsible for
some artefacts such as double image artefact. It also
can allow for improved image quality by the use of
acoustic lenses that can focus the ultrasound beam and
improve resolution.
Diffraction
The ultrasound beam spreads out with distance from
the transducer. This has the effect of lessening the
intensity of the beam.
Transducers
Ultrasound waves are generated by piezoelectric
crystals. Piezoelectric = “pressure electric” effect.
When an electrical current is applied to a quartz crystal its shape changes with polarity. This causes expansion and contraction that in turn leads to the production of compression and rarefaction of sound waves.
The reverse is also true and an electrical current is
generated on exposure to returning echoes that are
processed to generate a display. Hence the crystals
are both transmitter (small proportion of the time)
and receiver (most of the time). The frequency of
the generated wave is a specific feature of the crystal
used.
Modern transducers use multiple small elements to
generate the ultrasound wave. If a single small element transducer is used the waves radiate from it in a
circular fashion as do ripples in a pool. If multiple
small elements fire simultaneously however the individual curved wave fronts combine to form a linear
wave front moving perpendicularly away from the
transducer face. This system, that is multiple small
elements fired individually, is termed phased array.
Near field and Focusing
In a standard disc shaped transducer the beam shape
is cylindrical. Initially the beam is of comparable diameter to the transducer as the series of ultrasound
waves that make up the beam travel parallel to each
other. This is the near-field or Fresnel zone. At
some point distal to the transducer however the
beam begins to diverge which will reduce the ability
to distinguish two objects close together (resolution).
Beyond the point at which the beam begins to diverge is the far-field or Fraunhofer zone.
It is possible to focus the ultrasound beam to cause
convergence and narrowing of the beam thus improve (lateral) resolution. Focusing can be achieved
by either mechanical or, in a phased array element, by
electronic means. If the transducer face is concave or
a concave acoustic lens is placed on the surface of
the transducer, the beam can be narrowed at a predetermined distance from the transducer. The point at
which the beam is at its narrowest is the focal point
or focal zone and is the point of greatest intensity
and best lateral resolution.
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4. AACA Pre-conference Workshop perioperative ultrasound
Nov 5-6th 2006
P H Y S I CS
Resolution
This is the ability to distinguish / identify two objects. It includes spatial and temporal resolution. Spatial resolution is the ability to distinguish two separate
objects that are close together and is itself divided
into Axial Resolution, which is this ability along the
axis of the ultrasound beam, and Lateral Resolution,
which is in the direction perpendicular to the beam's
axis. Temporal resolution is the ability to accurately
locate structures or events at a particular instant in
time
Lateral Resolution: The ability to distinguish objects that are side by side. It is generally poorer than
axial resolution and is dependent on beam width. As
the ultrasound machine assumes that any object visualised originates from the centre of the beam then
two objects side by side cannot be distinguished if
they are separated by less than the beam width.
The determinants of beam width Transducer frequency (beam width increases with
lower frequency transducers)
Focusing of the beam
Gain (increased gain will increase the beam width
and reduce resolution)
To optimise lateral resolution therefore:
• Use highest frequency transducer (but note that
this occurs at the expense of penetration).
• Optimise the focal zone.
• Use the minimum necessary gain (see the knobology section).
Narrowing the Sector to the area of interest - narrowing the sector angle.
Minimise Line density (but at the expense of lateral
resolution).
Artefacts
The ultrasound machine makes various assumptions
in generating an image. These include:
• The ultrasound beam only travels in a straight
line with a constant rate of attenuation.
• The speed of sound in all body tissues is
1540m/s.
• The ultrasound beam is infinitely thin with all
echoes originating from its central axis.
• The depth of a reflector is accurately determined by the time taken for sound to travel
from the transducer to the reflector and return.
Reverberation artefact
This occurs when ultrasound is repeatedly reflected
between two highly reflective surfaces. If for example, the transducer acts as another reflective surface
to a returning echo, this echo will then be re-reflected
and retrace itself resulting in an artefactual image
identical, but at twice the distance from the transducer, as the real image. Due to the process of attenuation, each subsequent echo is weaker than the
first.
Axial Resolution: The ability to recognise two different objects at slightly different depths from the
transducer along the axis of the ultrasound beam.
Axial resolution = spatial pulse length (SPL)
2
where SPL = λ * no. of cycles
It is improved by higher frequency (shorter wavelength) transducers but at the expense of penetration.
Higher frequencies therefore are used to image structures close to the transducer.
Temporal Resolution is dependent on frame rate. It
is improved by:
Minimising Depth - the maximum distance from the
transducer as this affects the PRF.
A is the true anatomical structure strongly
reflective of ultrasound.
A1 is the artefact generated by the returning
ultrasound beam being re-reflected from the
transducer and again reflected from the true
anatomical structure at A. As this doubly reflected beam has travelled twice as far
it is seen at twice the distance from the transducer. It is weaker but otherwise identical
to the real structure at A.
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5. AACA Pre-conference Workshop perioperative ultrasound
Nov 5-6th 2006
P H Y S I CS
A Mirror Image artefact is a type of reverberation
artefact occurring at highly reflective air / fluid interfaces such as the lung and descending aorta. The first
image is displayed in the correct position while a false
image is produced on the other side of the reflector
due to its mirror like effect.
Side Lobe artefacts
Side lobe beams are generated from the edges of the
transducer element and project in a different direction from the main beam. These echoes are much
weaker than those of the main beam but if a very
strong reflector is encountered they may be strong
enough on returning to the transducer to be displayed prominently on the image. Because any returning echoes are always assumed by the machine to
have been generated from the main beam, their position on the display is incorrect (although at the right
depth as time taken to and from transducer is the
same. Multiple side lobe echoes as from a rapidly oscillating beam, are displayed as a curved line equidistant from the transducer along its length.
The two most important features that distinguish a
side lobe artefact from an anatomical structure are
that it is equidistant to the transducer along its length
and it passes through anatomical structures.
Multipath artefact arises when the path of the ultrasound beam to and from a reflector is different.
Part of the original echo returns to the transducer
while part is reflected off a second interface before
returning. As the second echo takes longer to return
it is displayed deeper, but along the same line as the
first.
Beamwidth artefacts
We assume the ultrasound beam is razor thin but in
fact it has a measurable thickness. Any echoes generated from the beam’s edge will be displayed as
though they have arisen from its centre. This is most
apparent when most of the beam travels through
fluid but part of the beam interacts with adjacent
soft tissue. Echoes from this interface will be displayed as having arisen from inside the chamber. The
intensity of the artefact decreases with distance.
Propagation speed errors
Occur when the media through which the ultrasound
beam passes does not propagate at 1540 m/s result-
ing in these echoes appearing at incorrect depths on
the display. These speed errors can make the image
appear “split” or “cut”.
Acoustic shadowing
A shadow results when an ultrasound beam is unable
to pass through an area deep to a strongly reflecting
or attenuating structure. It occurs deep to a region of
high acoustic impedance mismatch such as soft tissue
/ gas or soft tissue / calcification interfaces. It is seen
for example, deep to areas of calcification and prosthetic material. Tissue dropout occurs due to poor
beam penetration and may result in misdiagnoses of
defects such as septal defect.
Incorrect gain
Excess gain can create or obscure information and
reduce lateral resolution. This is of great importance
when attempting to planimeter calcified valves as increased gain will artificially reduce the valve area. Incorrect gain or focal zone setting may give an erroneous appearance of spontaneous echo contrast
(SEC). True SEC can be distinguished by a “swirling”
pattern as distinct from the grainy appearance of
high gain settings.
Near field clutter
This is due to acoustic noise near the transducer, resulting from high amplitude oscillations of the piezoelectric elements, and can make distinguishing
structures within 1 cm of the transducer very difficult.
In general, artefacts change their appearance and appear or disappear depending on the view. Real structures will remain constant and can be seen in multiple
views. By using appropriate settings, understanding
the imaging principles and knowing echocardiographic anatomy, it should be possible to readily distinguish artefacts and pitfalls from real structures.
Reference
This article is based on “Ultrasound Physics”, a chapter in the Westmead TOE Manual by Dr Lenore
George, Consultant Anaesthetist, Westmead Hospital, Sydney.
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