Ultrasound physics

Ultrasound Physics
Md Serajus Salekin Chowdhury
Senior Scientific Officer
Bangladesh Atomic Energy Commission

Waves
A wave can be described as a disturbance that travels through a medium from one location to another
location. For an example, when a pebble is thrown in a pond, it creates a disturbance and forms
ripples which travel together as a front in a straight-line direction, or the waves may be circular
waves that originate from the point where the disturbances occurs in the pond.
In physics, the wave is defined as the energy transferred through medium with regular vibration
or oscillating motion. A few examples of waves are: water wave, light wave, electromagnetic wave,
sound wave, seismic wave (earthquakes).
One way to categorize waves is on the basis of the direction of movement of the distinct particles of
the medium relative to the direction that the waves travel. Waves can be classified into two types
Longitudinal Wave and Transverse Wave.
Figure : Waves

Longitudinal Wave
Longitudinal waves occur when the oscillations are parallel to the direction of propagation. It is a
wave in which particles of the medium move in a direction parallel to the direction that the wave
moves. A sound wave traveling through air is a classic example of a longitudinal wave.
Figure: Longitudinal Wave

Transverse Wave
Transverse waves occur when a disturbance creates oscillations perpendicular at right angles to the
direction of energy transfer. It is a wave in which particles of the medium move in a direction
perpendicular to the direction that the wave moves. The electromagnetic radiations which include
light waves are a classic example of transverse waves.
Figure: Transverse Wave

Amplitude
Amplitude of a wave is defined as the distance from the rest position of the vibrating particle of the
medium to the highest point of the crest or the lowest point of the trough. It is denoted with a or A
and measured in meter.
Figure: Amplitude

Wavelength
Waves make characteristic patterns as they travel through space. The wavelength, λ, of a wave is the
distance from any point on one wave to the same point on the next wave along. (The symbol is a
Greek letter, 'lambda'.) To avoid confusion, it is best to measure wavelength from the top of a crest to
the top of the next crest, or from the bottom of a trough to the bottom of the next trough. Wavelength
is measured in meter.
Figure: Wavelength

Wavelength
Since a longitudinal wave does not contain crests and troughs, its wavelength must be measured
differently. A longitudinal wave consists of a repeating pattern of compressions and rarefactions.
Thus, the wavelength is commonly measured as the distance from one compression to the next
adjacent compression or the distance from one rarefaction to the next adjacent rarefaction.
Figure: Wavelength

Frequency
The frequency, f, of a wave is the number of waves passing a point in a certain time. We normally use
a time of one second, so we can say, “the number of waves passing a point in one second is called
frequency of the wave”. The unit of frequency is hertz (Hz), i.e., one hertz is equal to one wave per
second.
For water waves and sound waves the unit hertz is usually good enough but radio and TV waves have
such a high frequency that the kilohertz (kHz) or even the megahertz (MHz) are better units.
1 kHz = 1,000 Hz
1 MHz = 1,000,000 Hz

Time Period
Time taken for one complete cycle or vibration by the wave generating source is called time period.
In other word it can be defined as the time taken by a wave to travel the distance of a complete
wavelength. It is simply reciprocal of the frequency (1/f). Unit of time period is second.
𝑇𝑖𝑚𝑒 𝑃𝑒𝑟𝑖𝑜𝑑 𝑇 =
1
𝐹𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 (𝑓)

Wave Speed/ Wave Velocity
The wave speed/velocity is measured as the distance travelled by the crests or trough in a given time
interval. The relationship is similar to the relationship of speed, distance and time in the classical
physics.
Therefore, wave speed/wave velocity can be represented mathematically as,
SW = DW /T
Where,
SW = speed of the wave
DW = distance travelled by crest or trough of the wave
T = time taken to travel the distance DW
The wave speed varies for different types of waves. The Speed of light wave is 3 ×× 108108 m/s.

Frequency, Wavelength & Wave velocity Relationship
The diagrams at the right show several "snapshots" of the production of a wave within a
rope. The motion of the disturbance along the medium after every one-fourth of a period
is depicted. Observe that in the time it takes from the first to the last snapshot, the hand
has made one complete back-and-forth motion. A period has elapsed. Observe that
during this same amount of time, the leading edge of the disturbance has moved a
distance equal to one complete wavelength. So in a time of one period, the wave has
moved a distance of one wavelength. Combining this information with the equation for
speed (speed = distance/time), it can be said that the speed of a wave is also the
wavelength/period.
Since the period is the reciprocal of the frequency, the expression 1/f can be substituted
into the above equation for period. Rearranging the equation yields a new equation of
the form:
Speed = Wavelength • Frequency
The above equation is known as the wave equation. It states the mathematical
relationship between the speed (v) of a wave and its wavelength (λ) and frequency (f).
Using the symbols v, λ, and f, the equation can be rewritten as
v = f • λ

Sound
Sound is a mechanical wave that results from the back and forth vibration of the particles of the
medium through which the sound wave is moving. If a sound wave is moving from left to right
through air, then particles of air will be displaced both rightward and leftward as the energy of the
sound wave passes through it. The motion of the particles is parallel (and anti-parallel) to the
direction of the energy transport. This is what characterizes sound waves in air as longitudinal
waves.
Because of the longitudinal motion of the air particles, there are regions in the air where the air
particles are compressed together and other regions where the air particles are spread apart. These
regions are known as compressions and rarefactions respectively. The compressions are regions of
high air pressure while the rarefactions are regions of low air pressure. The diagram below depicts a
sound wave created by a tuning fork and propagated through the air in an open tube. The
compressions and rarefactions are labeled.
Figure: Propagation of sound

Types of Sound Wave
According to audibility, more precisely frequency limit, sound waves are grouped in three classes:
1. Infrasound,
2. Audible sound, and
3.Ultrasound.
Infrasound
Infrasound are the sound waves with frequencies below the lower limit of human audibility. Usually
hearing becomes gradually less sensitive as frequency decreases and below 20 Hz or cycle per
second our ear become insensitive to sound. The study of such sound waves is referred to sometimes
as infrasonics, covering sounds beneath 20 Hz down to 0.1Hz and rarely to 0.001 Hz. This frequency
range is utilized for monitoring earthquakes, charting rock and petroleum formations below the earth,
and also in ballistocardiography and seismocardiography to study the mechanics of the heart.

Audible Sound
Vibrations transmitted through an elastic solid or a liquid or gas, with frequencies in the approximate
range of 20 to 20,000 hertz, capable of being detected by human organs of hearing are known as
audible sound.
Ultrasound
Ultrasounds are sound waves with frequencies higher than the upper audible (<20,000 Hz) limit of
human hearing. Ultrasound has the same physical property as audible sound, the only difference is
we cannot hear it.

Basic Ultrasound
Medical Ultrasound, also known as diagnostic sonography or ultrasonography, is a diagnostic
imaging technique based on the application of ultrasound. It is used to see internal body structures
such as tendons, muscles, joints, vessels and internal organs. Its aim is often to find a source of a
disease or to exclude any pathology. The practice of examining pregnant women using ultrasound is
called obstetric ultrasound, and is widely used.
Ultrasonic images are made by sending pulses of ultrasound into tissue using a probe. The
sound echoes off the tissue; with different tissues reflecting varying degrees of sound. These echoes
are recorded and displayed as an image to the operator.
Types of Ultrasound Images
Many different types of images can be formed using sonographic instruments. The most well-known
type is a B-mode image, which displays the acoustic impedance of a two-dimensional cross-section
of tissue. Other types of image can display blood flow, motion of tissue over time, the location of
blood, the presence of specific molecules, the stiffness of tissue, or the anatomy of a three-
dimensional region.

Generation of Ultrasound
An ultrasound wave is usually generated and detected by a piezoelectric crystal that mounted on
transducer, popularly known as ultrasound probe. (We will discuss about it later)
Piezoelectric crystals have an unique vice versa phenomena. If you put it in any electric field it will
deforms with the variation of the field. On the other hand it produces electric field in response to
mechanical stress.
An ultrasound wave is generated when an electric field is applied to an array of piezoelectric
crystals located on the transducer surface. Electrical stimulation causes mechanical distortion of the
crystals resulting in vibration and production of sound waves (i.e. mechanical energy).
Each piezoelectric crystal produces an ultrasound wave. The summation of all waves generated by
the piezoelectric crystals forms the ultrasound beam. Ultrasound waves are generated in pulses
(intermittent trains of pressure waves) and each pulse commonly consists of 2 or 3 sound cycles of
the same frequency.

Generation of Ultrasound
The pulse length (PL) is the distance traveled per pulse. Waves of short pulse lengths improve axial
resolution for ultrasound imaging. The PL cannot be reduced to less than 2 or 3 sound cycles by the
damping materials within the transducer.
Pulse Repetition Frequency (PRF) is the rate of pulses emitted by the transducer (number of pulses
per unit time). Ultrasound pulses must be spaced with enough time between pulses to permit the
sound to reach the target of interest and return to the transducer before the next pulse is generated.
The PRF for medical imaging ranges from 1-10 kHz. For example, if the PRF = 5 kHz and the time
between pulses is 0.2 msec, it will take 0.1 msec to reach the target and 0.1 msec to return to the
transducer. This means the pulse will travel 15.4 cm before the next pulse is emitted (1,540 m/sec x
0.1 msec = 0.154 m in 0.1 msec = 15.4 cm).

Modes of Ultrasound
The principal modes of ultrasound in echocardiography are:
1. 2-D or 2 dimensional mode
2. M-mode or motion mode
3. Colour flow Doppler imaging
4. Pulse wave Doppler
5. Continuous wave Doppler
6. Tissue Doppler

Effect of Ultrasound on Foetus
Having an ultrasound won't affect your baby. Ultrasound sends sound waves through your womb
(uterus), which bounce off your baby's body. The echoes are turned into an image on a screen, so
your sonographer can see your baby’s position and movements. The frequency or length of the sound
waves depends on how far along your pregnancy is and the type of scan being carried out.
Studies have found no link between ultrasound and birth weight, childhood cancers, dyslexia
or hearing.
Almost all women want a scan during pregnancy, so it's hard to find women who haven't had one
make a comparison. That in itself may reassure you.

Effect of Ultrasound on Fetus
During an ultrasound scan, the equipment generates a small amount of heat which is absorbed by the
part of the body that's being scanned. Antenatal scans produce less than one degree C. This means
they're fine for you and your baby. It's only if the temperature of the scanned body tissue rises by four
degrees C that harm may be caused.
The most routine type of scanning is used to get 2D pictures of your baby. This uses a low intensity
of ultrasound spread over a large area, which causes very minimal heating. What's more, the fluid
around your baby and any movements she may make will help to spread any heat. This may help to
reassure you further.
3D and 4D scans are just as safe as 2D scans, because the image is made up of sections of two-
dimensional images converted into a picture. The power intensity is the same as it is for 2D scanning.

Types of Ultrasound Probe
Probes are generally described by the size and shape of their face ("footprint"). Selecting the right
probe for the situation is essential to get good images, although there may be times where more than
one probe may be appropriate for a given exam. The essential element of each ultrasound transducer
is a piezoelectric crystal, serving both to generate and to receive ultrasound waves.
The ultrasound transducers differ in construction according to:
- piezoelectric crystal arrangement,
- aperture (footprint),
- operating frequency (which is directly related to the penetration depth)
The following types of transducers/probes are most often used in the critical ultrasound imaging:
1. sector or phased array,
2. linear, and
3. convex (standard or micro-convex).

Sector or Phased Array Probe
A phased array probe generates an image from an electronically steered beam in a close array,
generating an image that comes from a point and is good for getting between ribs such as in cardiac
ultrasound.
Figure: Sector or Phased Array Probe

Sector or Phased Array Probe
Features:
 Piezoelectric crystal arrangement: phased-array (most commonly used)
 Footprint size: small and field of view will be spreaded widely at
 Operating frequency (bandwidth): 1-5 MHz (usually 3.5-5 MHz)
 Element radius: 32 ch – 128 ch
 Ultrasound beam shape: sector, almost triangular
Uses:
 Abdominal application.
 Transesophageal application.
 Brain diagnosis,
 Echocardiography,
 Gynecological ultrasound, etc.

Linear or Vascular Probe
A linear (also sometimes called vascular) probes are generally high frequency, better for imaging
superficial structures and vessels.
Figure: Linear or Vascular Probe

Features:
 Piezoelectric crystal arrangement: linear
 Footprint size: usually big (small for the hockey transducers)
 Operating frequency (bandwidth): 3-12 mhz (usually 5-7.5 mhz)
 Ultrasound beam shape: rectangular
Uses:
 Vascular application:
 Arteria carotis
 MT,
FMD measurement for early detection of arterial sclerosis

Uses:
 venipuncture, blood vessel visualization
 Breast, Thyroid
 Tendon, arthrogenous
 Intraoperative, laparoscopy
 The thickness measurement of Body fat and Muscles for daily healthcare check and
locomotive syndrome check
 Photoacoustic imaging, ultrasonic velocity change imaging.

Convex or curvilinear Probe
A convex or curvilinear probes may have a wider footprint and lower frequency for transabdominal
imaging, or in a tighter array (wider field of view) and higher frequency for endocavitary imaging.
Figure: Convex or Curvilinear Probe

Features:
 piezoelectric crystal arrangement: curvilinear, along the aperture
 footprint size: big (small for the micro-convex transducers)
 operating frequency (bandwidth): 1-5 MHz (usually 3.5-5 MHz)
 ultrasound beam shape: sector; the ultrasound beam shape and size vary with distance from the
transducer, that causes the lack of lateral resolution at greater depths
Uses:
 Typical abdominal application.
 Transvaginal and transrectal application.
 Pelvic and lung (micro-convex transducer) ultrasound
 Diagnoses organs.

Ultrasound Probe Care
An ultrasound transducer is the most important and usually the most expensive element of the
ultrasound machine, so it should be used carefully, which means the following:
• Do not throw, drop or knock the transducer,
• Do not allow to spoil the transducer`s duct,
• Wipe the gel from the transducer after each use,
• Do not sluice with alcohol-based confections.

Image Orientation
Ultrasound orientation is essential to understand what is being seen in point-of-care ultrasound. There
are two key aspects: how the indicator is oriented relative to the screen and how the probe and
indicator are placed and oriented relative to the patient. The conventional ultrasound image is a two-
dimensional plane composed of frames made up of scan lines that are updated many times a second
to create a moving image.
Ultrasound orientation can be challenging because it involves understanding how a two-dimensional
plane may cut through a three-dimensional object in any of the three standard planes (sagittal,
transverse, or coronal), as well as any oblique orientation between those planes. However, one of the
great advantages of ultrasound is the ability to obtain images from a variety of different approaches,
and a thorough understanding of orientation will allow you to take advantage of this.

Indicator‐to-Screen Orientation
There are two rules for indicator-to-screen orientation in standard emergency ultrasound imaging,
which uses the same convention as general radiology imaging:
1. The top of the screen is closer to the probe. Bottom of the screen shows structures farther
away from the probe.
2. The left side of the screen, as it is viewed, corresponds to the side of the probe marked with
an indicator.
Note: Imaging performed by cardiology specialists uses a different and opposite convention for rule
2.
The "indicator" may differ widely between manufacturers, and is typically a bump or a groove. It is
important to verify your orientation prior to beginning any exam. This should be done by placing a
small amount of gel on the side of the probe where you believe the indicator to be, and confirming
that the side with the gel corresponds to the left of the screen as it is viewed (Figs. a and b).

Indicator‐to-Screen Orientation
(a) Shows a thumb placed on the indicator, with gel placed on the face of the probe on that side.
(b) (b) Shows what should be seen on the screen in general imaging orientation.

Indicator‐to-Patient Orientation
Once the indicator-to-screen orientation is understood and verified, the probe is placed on the patient
and images are viewed on the screen.
The top of the screen will show structures closer to where the probe is placed, with the bottom of the
screen showing structures farther away from the probe face. The left of the screen will show
structures toward where the indicator is directed.

Image Planes
Longitudinal (Sagittal) - transducer is placed along the patient`s
long axis with the index directed cranial (the cranial structures
are displayed on the left side of the screen, i.e. the marked side of
the screen).
Transverse (Axial) - transducer is placed perpendicular to the
patient`s long axis with the index directed (usually) to the
patient`s right (the right structures are displayed on the left side
of the screen, i.e. the marked side of the screen).
Frontal (Coronal) - transducer is placed lateral with the index
directed cranial (the cranial structures are displayed on the left
side of the screen, i.e. the marked side of the screen; the
structures proximal to the probe are located at the top of the
screen).

Manipulation of Transducer
An image acquisition and optimization of the views is achieved the proper manipulation of the
transducer:
 tilting allows optimization of the image and imaging different structures in the same
tomographic plane,
 angulation of the transducer slides the ultrasound image from side to side,
 rotation the transducer at a single position changes the image plane.
An unexperienced sonographer should manipulate the probe carefully (limited movements) in one
image plane. The chaotic movements do not allow obtaining a reliable image views and proper data
acquisition.

Modes of Ultrasound
1. A-mode (A=amplitude)
The amplitude of reflected ultrasound is displayed on
an oscilloscope screen. It is just of historical
importance. The A-mode is now used only in
ophthalmology.
 Each pulse produces a new a one-dimensional
display or image
 Line of information on the display
 An uncommon display, except in
ophthalmologic sonography used
for precise intraocular length measurements Figure: A-mode Ultrasound

Modes of Ultrasound
2. B-mode (B=brightness) (2D in echocardiography)
This is now the essential imaging modality in the
diagnostic ultrasound. An amplitude of the reflected
ultrasound signals is converted into a gray scale image.
Owing to the wide gray scale (most of the ultrasound
machines use 256 shades of gray) even very small
differences in echogenicity are possible to visualize.
 Basis for gray scale,
two-dimensional (2D) imaging
 US unit tracks the position of the transducer to place a
dot on the screen corresponding to the transducer
position (X, Y locations), creating a 2D image
Figure: B-mode Ultrasound

Modes of Ultrasound
3. M-mode (M=motion)
It reflects a motion of the heart structures over
time. Nowadays, the integration of 2D and M-
mode images is possible. Due to its excellent
temporal resolution (high sampling rate), M-
mode is extremely valuable for accurate
evaluation of rapid movements.
• One-dimension image used to investigate
moving structures with respect to time
• Evaluates motion pattern of moving
structures such as in the heart
Figure: M-mode Ultrasound

Modes of Ultrasound
4. D-mode (D=Doppler)
This imaging mode is based on the Doppler
effect, i.e. change in frequency (Doppler shift)
caused by the reciprocal movement of the
sound generator and the observer. Diagnostic
ultrasound uses the change in frequency of
ultrasound signal backscattered from red blood
cells. The frequency of the reflected ultrasound
wave increases or decreases according to the
direction of blood flow in relation to the
transducer.
Figure: M-mode Ultrasound

Types of Doppler
Doppler Duplex
Doppler Duplex technique is based on the
simultaneous B-mode real-time and Doppler
imaging. A gray scale display serves for the
localization of flow measurement site. Both
spectral Doppler techniques (Continuous-Wave
and Pulsed) can be used in Doppler Duplex
imaging
Figure: Doppler Duplex Ultrasound

Types of Doppler
CW Doppler (Continuous-Wave Doppler)
(CWD)
This imaging mode requires two piezoelectric
crystals. One continuously transmits and the
other one receives the Doppler signals along the
scan-line. CWD is very useful in high velocity
signals recording. It defines blood flow
direction but has no value in Doppler signal
source identification.
Figure: CW Doppler Ultrasound

Types of Doppler
PW Doppler (Pulsed-Wave Doppler) (PWD)
The same piezoelectric crystal transmits and
receives the ultrasound signal. The reflected
signal returns to the transducer after a definite
delay that is known as pulse repetition
frequency (PRF). PWD allows measurement
from a small, specific blood volume (depth of
interest), which is defined by a sample volume.
It provides both blood flow direction and
precise determination of of Doppler signal
source. The main limitation is failure to display
the high velocity signals. The maximum
detectable frequency shift (the Nyquist limit) is
determined by the value of half the PRF. After
the velocity exceeds the limit, aliasing occurs Figure: PW Doppler Ultrasound

Types of Doppler
Color Doppler
Color Doppler imaging is based on the PWD
assumption. However, in Color Doppler
imaging, multiple sample volume are evaluated
along each scanning line. Velocities are
displayed using a color scale. Velocities toward
the transducer are red, and velocities away from
the transducer are blue.
Figure: Color Doppler Ultrasound

Types of Doppler
Power Doppler
Power Doppler is a variant of Color Doppler
technique which displays the magnitude of the
Doppler signal rather than the Doppler
frequency. It is often used to increase sensitivity
to low flows and velocities. Power Doppler
does not display flow direction or different
velocities.
Figure: Power Doppler Ultrasound

Artifacts
An ultrasound artifact is a structure in an image which does not directly similar with actual tissue
being scanned.
Artifact assumes different forms including :
 Structures in the image that are not actually present
 Objects that should be represented but are missing from the image.
 Structures which are misregistered on the image.

Common Artifacts
1. Reverberation
2. Acoustic Shadowing
3. Acoustic Enhancement
4. Edge Shadowing
5. Beam Width Artifact
6. Slice Thickness Artifact
7. Side Lobe Artifact
8. Mirror Image
9. Double Image
10. Equipment-generated Artifact
11. Refraction Artifact

REVERBERATION
 This is the production of false echoes due to repeated reflections between two interfaces with a
high acoustic impedance mismatch.
 The echo from the interface is received by the transducer and displayed on the image.
 Some of the energy in the returned echo is reflected at the transducer face, and return to the
reflecting interface as if it was a weak transmitted pulse, returning as a second echo.
 As the time taken for the second echo to arrive is twice that taken by the depth.
 This sequence of reflection and transmission can occur many times, with the third echo taking
three times as long to return to the transducer and being displayed at three times the depth, and so
on.
 The reverberation echoes will be equally spaced because the time for each additional echo is
multiple of the time of return of the first echo.
 These reverberation echoes will be strong because of the high acoustic mismatch.
 This artifact will be seen at the skin-transducer interface and behind bowel gas.

REVERBERATION
Rectification:
 Increase the amount of gel used.
 Use a stand-off pad.
 Reduce the gain.
 Move the position of the transducer.

ACOUSTIC SHADOWING
 This appears as an area of low amplitude echoes
behind an area of strongly attenuating tissue.
 It is caused by severe attenuation of the beam at
an interface, resulting in very little sound being
transmitted beyond.
 The attenuation can be due to either absorption
or reflection of the sound waves, or a
combination of the two.
 Acoustic shadowing will occur at interfaces with
large acoustic mismatch such as:
Soft tissue and gas
Soft tissue and bone or calculus

ACOUSTIC ENHANCEMENT
 This artifact appears as a localized area of increased echo amplitude behind an area of low
attenuation.
 On a scan it will appears as an area of increased brightness, and can commonly be seen distal to
fluid-filled structures such as the urinary bladder, GB or a cyst.
 The artifact arises due to the application of the time-gain compensation(TGC) to areas of low
attenuating structures such as fluid.
 It is caused by the low level of attenuation of the beam as it passes through fluid relative to the
greater attenuation of the beam in the adjacent more solid tissue.
 This artifact can often be an useful diagnostic aid, particularly when scanning a soft-tissue mass
or cyst containing low level echoes.
 These echoes may often cause the structure to disappear in the image as it blend into the
surrounding echo pattern.

EDGE SHADOWING
 A combination of refraction and reflection occurring at the edges of rounded structures will result
in edge shadowing artifact.
 It arises due to refraction of the beam caused by both the curvature of the rounded edges and
difference in speed of two materials.
 When the ultrasound beam reaches the rounded edge of a structure, reflection will occur, with an
angle of incidence equal to the angle of reflection.
 The outer part of the beam will be totally reflected, but the reminder of the beam passes through
the rounded structure and is refracted.
 This combination of reflection and refraction of the beam at the edges of a rounded structure
results in a thin strip of tissue behind the edge not being insonated and causes a shadow.

BEAM WIDTH ARTIFACT
 This artifact can be
demonstrated by scanning a
point reflector in a phantom,
where the display will clearly
portray this as a line.
 During routine scanning, the
artifact can be seen when
spurious echoes are displayed
in an echo-free area.
 Correct positioning of the focal
zone will help to reduce this
artifact.
 The focal zone is controlled by
electronically narrowing the
beam

SLICE THICKNESS ARTIFACT
 These occurs due to the thickness of the beam, and are similar to beam width artifacts.
 These artifacts will typically be seen in transverse views of the urinary bladder when structures
adjacent to the slice through the bladder being scanned will be incorporated into the image.
 These echoes are then displayed as if they were arising from within the bladder.
 Although the appearance of this artifact is similar to the beam width artifact, the differentiating
factors is that the reflector causing the slice thickness artifact will not be seen on the display.
 This artifact is a result of inherent characteristics of the transducer, and apart from trying a
different transducer, cannot be eliminated.

SIDE LOBE ARTIFACT
 The energy within the ultrasound
beam exists as several side lobes
radiating at a number of angles from
a central lobe.
 Echoes are generated by these side
lobes in addition to the main lobe,
but all the returning echoes are
assumed by the transducer to have
arisen from the central axis of the
main lobe.
 Side lobe echoes will therefore be
misregistered in the display.
 This artifact can often be seen in
area such as the urinary bladder and
may also arise within a cyst.

MIRROR IMAGE ARTIFACT
 These artifacts results in a mirror image of a structure occurring in an ultrasound display.
 They arise due to specular reflection of the beam at a large smooth interface.
 An area close to a specular reflector will be imaged twice, once by the original ultrasound beam
and once by the beam after it has reflected off the specular reflector.
 Mirror image artifacts are most commonly seen where there is a large acoustic mismatch, such as a
fluid-air interface.
 Typically this artifact can occur during the scanning of a full bladder, when air in the rectum
behind the bladder act as specular reflector and mirror image of the bladder is displayed
posteriorly.
 It will then have the appearance of a large cyst behind the bladder.
 It can also be seen when scanning the liver, and the diaphragm act as a specular reflector.

DOUBLE IMAGE ARTIFACT
 This image is caused by refraction of the beam
and may occur in areas such as the rectus
abdominis muscle on the anterior abdominal
wall.
 In the transverse plane the edges of the muscle
act as a lens and the ultrasound beam to be
refracted and this causes the single structure to
be interrogated by two separate refracted
beams.
 Two sets of echoes will therefore be returned
and these will cause display of two structures
in the image. This results in, for example, two
images of the transverse aorta side by side in
the abdomen.

EQUIPMENT GENERATED ARTIFACT
 Incorrect use of the equipment controls can lead to artifact appearing.
 Misuse of controls such as the gain or TGC can result in echoes being recorded as too bright or too
dark.
 Care must be taken when setting these controls, to ensure an even brightness throughout the image.
 If the dynamic range control is incorrectly set, this can lead to an image which has too much
contrast, and result in the loss of subtle echo information.
 Gain must be in medium level.
 Blurring of a moving image can occur if the frame rate is too low or if the persistence is too high.
 It is important to ensure that the frame rate is capable of recording a moving structure at the speed.
 Use of multiple focal zones can give rise to a prominent banding effect within the image.

REFRACTION ARTIFCT
 The refraction is the change of the sound direction on passing from one medium to another.
 In ultrasound, refraction is due to sound velocity mismatches combines with oblique angles of
incidence, most commonly with convex scanheads.
 When the ultrasound wave crosses at an oblique angle the interface of two materials, through
which the waves propagate at different velocities, refraction occurs, caused by bending of the wave
beam.
 Refraction artifact cause spatial distortion and loss of resolution in the image.

Ultrasound physics

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