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DR.SHAMIM RIMA MBBS,DMU,FCGP M.PHIL RADIOLOGY & IMAGING BSMMU.

Image Artifact

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Image artifact in Ultrasound

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Page 1: Image Artifact

DR.SHAMIM RIMAMBBS,DMU,FCGP

M.PHILRADIOLOGY & IMAGING

BSMMU.

Page 2: Image Artifact

Figure : Diagram of an ultrasound beam. The main ultrasound beam narrows as it approaches the focal zone and then diverges. Grating lobes and side lobes are forms of off-axis energy

Figure : Diagram of an ultrasound beam. The main ultrasound beam narrows as it approaches the focal zone and then diverges. Grating lobes and side lobes are forms of off-axis energy

US Beam US Beam

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ARTIFACTS

An image artifact is any image attribute, which is not present in the original imaged

object. An image artifact is sometime the result of an improper operation of the imager,

and in other times a consequence of natural processes or properties of the human

body.

Artifacts in diagnostic ultrasound are a reflection or an echo, which appears on the display and represents the real anatomical structure not correctly.

An artifact can be a false, multiple or misleading information introduced by the imaging system or by interaction of ultrasound with the adjacent tissue.

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Artifacts in ultrasound can be classified as to their source like e.g.:

Physiologic (motion, different sound velocities, acoustical impedances of tissue);

Hardware (dimension of the ultrasound beam and the transducer array);

Imaging technique (B-mode, Spectral Doppler , color-Doppler, 3D ultrasound).

Image artifacts can occur in each medical ultrasound. Then an interpretation of the image is complicated and can eliminate the structural information of objects looking for.

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Technique-dependent artifacts:  Noise

Noise caused by excess gain

Resolving noise caused by excess gain- Decrease overall gain

An undesirable background interference or disturbance that affects image quality.

The noise is commonly characterized by the standard deviation of signal intensity in

the image of a uniform object (phantom) in the absence of artifacts.

Overall increase in echogenicity.

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Low signal

Low signal caused by inappropriate transducer selection or low gain settings

Resolving low gain artifacts:

- Increase overall gain

- Increase far gain

- Apply more acoustic coupling gel

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ARTIFACTS ASSOCIATED WITH ATTENUATION ERRORS

 Shadowing

Shadowing Increased through transmission

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Shadowing

Caused by sound striking a strong reflective interface example;

areas behind ribs or gall stones

Shadowing corrective technique:

- Scan around object causing shadowing

Shadowing corrective technique:

- Scan around object causing shadowing

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Shadowing.(a) Diagram shows the ultrasound beam encountering a strongly

attenuating material. The echoes received from points distal to this material are

significantly lower in intensity than echoes received from a similar

depth. (b)Longitudinal US image of the gallbladder shows shadowing (arrow)

posterior to echogenic gallstones.

Shadowing.(a) Diagram shows the ultrasound beam encountering a strongly

attenuating material. The echoes received from points distal to this material are

significantly lower in intensity than echoes received from a similar

depth. (b)Longitudinal US image of the gallbladder shows shadowing (arrow)

posterior to echogenic gallstones.

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Enhancement

Caused by sound travelling through fluid-filled structures without attenuation (example: area

behind cysts, area behind gallbladder)

Enhancement corrective technique:- Reduce overall gain- Decrease far gain

Enhancement corrective technique:- Reduce overall gain- Decrease far gain

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Enhancement artifacts occur if decreasing of the echo amplitude is not equal

with penetration depth caused by inhomogeneous tissue layers and fluids like

cysts or air-filled regions.

The enhanement artifacts appears as a hyperintense (hyperechoic) signal. The

attenuation of the ultrasound wave in fluids is much lower as the attenuation in

other tissues, therefore tissues distal to fluid are enhanced.

Artificial enhancement may also be found distal to a homogeneous solid tumor

surrounded by adipose tissue, due to the comparatively high attenuation in fat

Enhancement artifacts occur if decreasing of the echo amplitude is not equal

with penetration depth caused by inhomogeneous tissue layers and fluids like

cysts or air-filled regions.

The enhanement artifacts appears as a hyperintense (hyperechoic) signal. The

attenuation of the ultrasound wave in fluids is much lower as the attenuation in

other tissues, therefore tissues distal to fluid are enhanced.

Artificial enhancement may also be found distal to a homogeneous solid tumor

surrounded by adipose tissue, due to the comparatively high attenuation in fat

Enhancement artifacts

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Figure: Increased through transmission. (a)Diagram shows the ultrasound beam encountering a

focal weakly attenuating material. The echoes received from points distal to this material are

higher in intensity than echoes received from a similar depth in the imaging

plane. (b)Transverse US image of the liver shows hypoechoic and weakly attenuating hepatic

cysts. The hepatic parenchyma distal to the cysts is falsely displayed as increased in intensity

(arrow) secondary to increased through-transmission artifact.

Figure: Increased through transmission. (a)Diagram shows the ultrasound beam encountering a

focal weakly attenuating material. The echoes received from points distal to this material are

higher in intensity than echoes received from a similar depth in the imaging

plane. (b)Transverse US image of the liver shows hypoechoic and weakly attenuating hepatic

cysts. The hepatic parenchyma distal to the cysts is falsely displayed as increased in intensity

(arrow) secondary to increased through-transmission artifact.

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 Reverberation

Caused by sound interfacing with two structures of markedly differing acoustic properties;

example: inadequate amount of gel, obstructing bowel gas

Reverberation corrective technique:- Change angle of transducer- Apply more gel- Rotate patient

Reverberation corrective technique:- Change angle of transducer- Apply more gel- Rotate patient

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Reverberation occurs when sound encounters two highly reflective layers. The sound

is bounced back and forth between the two layers before traveling back. The probe

will detect a prolonged traveling time and assume a longer traveling distance and

display additional ‘reverberated’ images in a deeper tissue layer

Reverberation occurs when sound encounters two highly reflective layers. The sound

is bounced back and forth between the two layers before traveling back. The probe

will detect a prolonged traveling time and assume a longer traveling distance and

display additional ‘reverberated’ images in a deeper tissue layer

Reverberation

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Figure. Reverberation artifact. (a) Diagram shows ultrasound echoes

being repeatedly reflected between two highly reflective

interfaces. (b) The display shows multiple equally spaced signals

extending into the deep field.

Figure. Reverberation artifact. (a) Diagram shows ultrasound echoes

being repeatedly reflected between two highly reflective

interfaces. (b) The display shows multiple equally spaced signals

extending into the deep field.

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Transverse US image obtained over a palpable mass in a neonate shows reverberation

artifact (arrow).

Longitudinal US image of the gallbladder shows comet tail artifact (arrow) caused by

cholesterol crystals in Rokitansky-Aschoff sinuses. This finding is diagnostic of

adenomyomatosis. Shadowing gallstones are also identified.

Transverse US image obtained over a palpable mass in a neonate shows reverberation

artifact (arrow).

Longitudinal US image of the gallbladder shows comet tail artifact (arrow) caused by

cholesterol crystals in Rokitansky-Aschoff sinuses. This finding is diagnostic of

adenomyomatosis. Shadowing gallstones are also identified.

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Mirror Image Artifacts

If a structure is located close to a highly reflective interface (such as the diaphragm),

it is detected and displayed in its normal position. However, the strong reflector

causes additional sound waves to bend towards the neighboring anatomy, from

where they are bounced back towards the strong reflector and return to the

transducer. These sound waves have a longer travel time and are perceived as an

additional anatomic structure. The image is duplicated on the other side of the strong

reflector .

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Figure . Mirror image artifact. (a) In this diagram, the gray arrows represent the expected reflective path of the ultrasound beam. These echoes are displayed properly. The black arrows show an alternative path of the primary ultrasound beam. In this path, the primary ultrasound beam encounters the deeper reflective interface first. (b) The echoes from the deeper reflective interface take longer to return to the transducer and are misplaced on the display. (c) Longitudinal US image obtained at the level of the right hepatic lobe shows an echogenic lesion in the right hepatic lobe (cursors) and a duplicated echogenic lesion (arrow) equidistant from the diaphragm overlying the expected location of lung parenchyma.

Figure . Mirror image artifact. (a) In this diagram, the gray arrows represent the expected reflective path of the ultrasound beam. These echoes are displayed properly. The black arrows show an alternative path of the primary ultrasound beam. In this path, the primary ultrasound beam encounters the deeper reflective interface first. (b) The echoes from the deeper reflective interface take longer to return to the transducer and are misplaced on the display. (c) Longitudinal US image obtained at the level of the right hepatic lobe shows an echogenic lesion in the right hepatic lobe (cursors) and a duplicated echogenic lesion (arrow) equidistant from the diaphragm overlying the expected location of lung parenchyma.

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Mirror Image Artifacts

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Comet Tail artifacts

A comet tail artifact is similar to reverberation. It is produced by the front and back of a very

strong reflector (air bubble, BB gun pellet, suture) , createsa dense echogenic line extending

through the image.

The reverberations are spaced very narrowly and blend into a small band.

A comet tail artifact is similar to reverberation. It is produced by the front and back of a very

strong reflector (air bubble, BB gun pellet, suture) , createsa dense echogenic line extending

through the image.

The reverberations are spaced very narrowly and blend into a small band.

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Ring Down Artifacts

The artifact is caused by a resonance phenomenon from a collection of gas bubbles. A

continuous emission of sound occurs from the ‘resonating’ structure causing a long and

uninterrupted echo. It appears very similar to the comet tail artifact

Diagram shows the main ultrasound beam encountering a ring of bubbles with fluid trapped centrally. Vibrations from the pocket of fluid cause a continuous source of sound energy that is transmitted back to the transducer for detection. The display shows a bright reflector with an echogenic line extending posteriorly. Left lateral decubitus US image of the gallbladder shows air and fluid in the duodenum causing ring-down artifact (arrow).

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Figure : Ring-down artifact. (a) Diagram shows the main ultrasound beam encountering a ring of bubbles with fluid trapped centrally. (b) Vibrations from the pocket of fluid cause a continuous source of sound energy that is transmitted back to the transducer for detection. (c) The display shows a bright reflector with an echogenic line extending posteriorly.

Figure : Ring-down artifact. (a) Diagram shows the main ultrasound beam encountering a ring of bubbles with fluid trapped centrally. (b) Vibrations from the pocket of fluid cause a continuous source of sound energy that is transmitted back to the transducer for detection. (c) The display shows a bright reflector with an echogenic line extending posteriorly.

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Side Lobe Artifacts

This artifact is caused by low energy ‘side lobes’ of the main ultrasound beam. When an

echo from such a side lobe beam becomes strong enough and returns to the receiver, it is

‘assigned’ to the main beam and displayed at a false location. Side-lobe artifacts are

usually seen in hypoechoic or echo-free structures and appear as bright

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Lobe Artifacts

Side lobes and grating lobes are sound waves that are created peripheral to the

main beam

Are lower energy, so generally are lost to attenuation

Provide distracting echoes when misinterpreted as being from the main beam

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Side lobes are multiple beams of low-amplitude ultrasound energy that project radially from the

main beam axis .

Side lobe energy is generated from the radial expansion of piezoelectric crystals and is seen

primarily in linear-array transducers

Strong reflectors present in the path of these low-energy, off-axis beams may create echoes

detectable by the transducer.

These echoes will be displayed as having originated from within the main beam in the side lobe

artifact . As with beam width artifact, this phenomenon is most likely to be recognized as

extraneous echoes present within an expected anechoic structure such as the bladder

Side lobesSide lobes

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 Refraction artifact.

 Refraction Artifact.

A change in velocity of the ultrasound beam as it travels through two adjacent tissues

with different density and elastic properties may produce a refraction artifact.

In refraction, nonperpendicular incident ultrasound energy encounters an interface

between two materials with different speeds of sound. When this occurs, the incident

ultrasound beam changes direction.

The degree of this change in direction is dependent on both the angle of the incident

ultrasound beam and the difference in velocity between the two media.

The ultrasound display assumes that the beam travels in a straight line and thus misplaces

the returning echoes to the side of their true location .

In clinical imaging, this artifact may be recognized in pelvic structures deep to the junction

of the rectus muscles and midline fat.

Refraction artifact may cause structures to appear wider than they actually are or may

cause an apparent duplication of structures

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Figure: Refraction artifact. (a) Diagram shows the refraction or change in direction of the

obliquely angled incident ultrasound beam as it travels between two adjacent tissues with

different sound propagation velocities (C1and C2). The incident ultrasound beam with

refraction encounters two structures. (b)The object in the path of the refracted portion of

the beam is misplaced because the processor assumes a straight path of the beam.

Figure: Refraction artifact. (a) Diagram shows the refraction or change in direction of the

obliquely angled incident ultrasound beam as it travels between two adjacent tissues with

different sound propagation velocities (C1and C2). The incident ultrasound beam with

refraction encounters two structures. (b)The object in the path of the refracted portion of

the beam is misplaced because the processor assumes a straight path of the beam.

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Figure: Refraction artifactFigure: Refraction artifact

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Edge Shadowing

The lateral edge shadow is a thin acoustic shadow that appears behind edges

of cystic structures. Sound waves encountering a cystic wall or a curved

surface at a tangential angle are scattered and refracted, leading to energy

loss and the formation of a shadow.

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Figure . Side lobe artifact. (a) Diagram shows multiple beams of off-axis side lobe ultrasound

energy encountering an object (black circle). (b) The display assumes that the echoes

returning from this off-axis object came from the main beam and misplaces and duplicates the

structure.

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ARTIFACTS ASSOCIATED WITH VELOCITY ERRORS

Speed Displacement Artifact

Refraction Artifact.

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Speed Displacement Artifact

When sound travels through material with a velocity significantly slower than the

assumed 1540 m/sec, the returning echo will take longer to return to the

transducer.

The image processor assumes that the length of time for a single round trip of an

echo is related only to the distance traveled by the echo.

The echoes are thus displayed deeper on the image than they really are . This is

referred to as the speed displacement artifact; in clinical imaging, it is often

recognized when the ultrasound beam encounters an area of focal fat.

When sound travels through material with a velocity significantly slower than the

assumed 1540 m/sec, the returning echo will take longer to return to the

transducer.

The image processor assumes that the length of time for a single round trip of an

echo is related only to the distance traveled by the echo.

The echoes are thus displayed deeper on the image than they really are . This is

referred to as the speed displacement artifact; in clinical imaging, it is often

recognized when the ultrasound beam encounters an area of focal fat.

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Figure. Speed displacement artifact. (a)In this diagram, the gray arrows represent the expected reflected

path of the ultrasound beam. The echoes returning from the posterior wall of the depicted structure will

be displayed properly. The black arrows represent the path of an ultrasound beam that encounters an

area of focal fat. The dashed lines indicate that the sound beam travels slower in the focal fat than in the

surrounding tissue. (b) Because the round trip of this echo is longer than expected, the posterior wall is

displaced deeper on the display. (c, d) Transverse US image of the liver (c) and close-up detail

image (d) show that the interface between the liver and the diaphragm (arrow in c) is discontinuous and

focally displaced (arrows in d). This appearance may be explained by areas of focal fat within the liver.

Figure. Speed displacement artifact. (a)In this diagram, the gray arrows represent the expected reflected

path of the ultrasound beam. The echoes returning from the posterior wall of the depicted structure will

be displayed properly. The black arrows represent the path of an ultrasound beam that encounters an

area of focal fat. The dashed lines indicate that the sound beam travels slower in the focal fat than in the

surrounding tissue. (b) Because the round trip of this echo is longer than expected, the posterior wall is

displaced deeper on the display. (c, d) Transverse US image of the liver (c) and close-up detail

image (d) show that the interface between the liver and the diaphragm (arrow in c) is discontinuous and

focally displaced (arrows in d). This appearance may be explained by areas of focal fat within the liver.

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Slice Thickness

Remember that the image is a two dimension representation of a volume of tissue, not

a true two dimensional plane

Results in false echoes, especially in anechoic structures

Thick slices result in an inability to see objects substantially smaller than beamThickness

This is also referred to as elevational resolution

Generally not controllable by the user

Generally the poorest resolution in the system.

Remember that the image is a two dimension representation of a volume of tissue, not

a true two dimensional plane

Results in false echoes, especially in anechoic structures

Thick slices result in an inability to see objects substantially smaller than beamThickness

This is also referred to as elevational resolution

Generally not controllable by the user

Generally the poorest resolution in the system.

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Slice Thickness Artifacts

When the interface between a fluid-filled & soft tissue is acutely angled, the beam, which is relatively wide (2-3mm), may strike both tissue & fluid simultaneously.

Low-level artifactual echoes will be displayed within the fluid.

When the interface between a fluid-filled & soft tissue is acutely angled, the beam, which is relatively wide (2-3mm), may strike both tissue & fluid simultaneously.

Low-level artifactual echoes will be displayed within the fluid.

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TGC Banding

Occurs when the user has not appropriately adjusted the TGC curve

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Aliasing

Occurs only in Doppler imaging

If the pulse repetition frequency is set at too fast a speed, signals return to

transducer after the transmission of next signal.

With color flow, this artifact induces specks of color at the opposite end of the

color spectrum from the rest of the color related to high-velocity areas.

If the velocity exceeds the range, it will be depicted as going the other direction

Occurs only in Doppler imaging

If the pulse repetition frequency is set at too fast a speed, signals return to

transducer after the transmission of next signal.

With color flow, this artifact induces specks of color at the opposite end of the

color spectrum from the rest of the color related to high-velocity areas.

If the velocity exceeds the range, it will be depicted as going the other direction

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Aliasing of color doppler imaging and artefacts of color. Color image shows regions of aliased flow (yellow arrows).

Aliasing of color doppler imaging and artefacts of color. Color image shows regions of aliased flow (yellow arrows).

Reduce color gain and increase

pulse repetition frequency.

Reduce color gain and increase

pulse repetition frequency.

Aliasing

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Figure (a,b): Example of aliasing and correction of the aliasing. (a) Waveforms with

aliasing, with abrupt termination of the peak systolic and display this peaks bellow the

baseleineSonogram clear without aliasing. (b) Correction: increased the pulse

repetition frequency and adjust baseline (move down)

Figure (a,b): Example of aliasing and correction of the aliasing. (a) Waveforms with

aliasing, with abrupt termination of the peak systolic and display this peaks bellow the

baseleineSonogram clear without aliasing. (b) Correction: increased the pulse

repetition frequency and adjust baseline (move down)

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Can control aliasing by increasing PRF or changing baseline

It’s important to correct in pulse Doppler so accurate measurements can be made

It’s important to correct in color Doppler because it is easily mistaken for turbulence

Can control aliasing by increasing PRF or changing baseline

It’s important to correct in pulse Doppler so accurate measurements can be made

It’s important to correct in color Doppler because it is easily mistaken for turbulence

Aliasing

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Poor Angle For Color

As with pulsed Doppler, the best visualization of a vessel is a with the transducer at an

angle of less than 600 to the vessel.

If a 900 angle is used. Flow within vessels will be poorly seen.

As with pulsed Doppler, the best visualization of a vessel is a with the transducer at an

angle of less than 600 to the vessel.

If a 900 angle is used. Flow within vessels will be poorly seen.

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Figure : Effect of high vessel/beam angles. (a) and (b) A scan of fetal aortic flow is undertaken at a high

beam/vessel angle. Beam/flow angles should be kept to to 60° or less. A hudge discrepancy is observed

when use unapropiate angles > 60°. If absolute velocities are to be measured, beam/flow angles should

be kept to 60° or less.

Figure : Effect of high vessel/beam angles. (a) and (b) A scan of fetal aortic flow is undertaken at a high

beam/vessel angle. Beam/flow angles should be kept to to 60° or less. A hudge discrepancy is observed

when use unapropiate angles > 60°. If absolute velocities are to be measured, beam/flow angles should

be kept to 60° or less.

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ADVERSE BIOLOGICAL EFFECTS OF ULTRASOUND ON BODY TISSUES

There are several known effects of ultrasound on tissues, but at the low doses used in

diagnostic imaging these are not thought to create any adverse effect. At higher therapeutic

doses the effects are used to influence tissues.

There are several known effects of ultrasound on tissues, but at the low doses used in

diagnostic imaging these are not thought to create any adverse effect. At higher therapeutic

doses the effects are used to influence tissues.

The various effects of ultrasound can be categorised into three main types : The various effects of ultrasound can be categorised into three main types :

Mechanical - no adverse effect on cell structure or chromosomes have been reported Mechanical - no adverse effect on cell structure or chromosomes have been reported

Vibration of tissues

Forms small cavities in fluids during "suction" phase which disperse during the

"pressure" phase

This phenomenon is called cavitation in gas-free fluids, and pseudocavitation in

fluids containing gas

Vibration associated with high intensities of ultrasound result in heat production

Vibration of tissues

Forms small cavities in fluids during "suction" phase which disperse during the

"pressure" phase

This phenomenon is called cavitation in gas-free fluids, and pseudocavitation in

fluids containing gas

Vibration associated with high intensities of ultrasound result in heat production

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ADVERSE BIOLOGICAL EFFECTS OF ULTRASOUND ON BODY TISSUES

Heat Heat

At usual frequencies and intensities the heat produced by ultrasound is dispersed by

the local blood circulation

High intensity ultrasound is used to induce local hyperthermia for therapeutic purposes

At usual frequencies and intensities the heat produced by ultrasound is dispersed by

the local blood circulation

High intensity ultrasound is used to induce local hyperthermia for therapeutic purposes

Chemical Chemical

Depolymerisation - experimentally ultrasound can breakdown polysaccharides and

polypeptides including DNA. These effects have not been reported to occur in vivo

following diagnostic ultrasound procedures.

Oxidation

Reduction

Depolymerisation - experimentally ultrasound can breakdown polysaccharides and

polypeptides including DNA. These effects have not been reported to occur in vivo

following diagnostic ultrasound procedures.

Oxidation

Reduction

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ADVERSE BIOLOGICAL EFFECTS OF ULTRASOUND ON BODY TISSUES

High intensity, high frequency ultrasound has been shown to result in adverse effects

including :

High intensity, high frequency ultrasound has been shown to result in adverse effects

including :

Tissue necrosis

Chromosomal damage

Genetic mutations

Teratogenic changes

Tissue necrosis

Chromosomal damage

Genetic mutations

Teratogenic changes

So far these adverse effects have not been reported following diagnostic

ultrasonography but safety limits to minimise potential risks have been set as follows

So far these adverse effects have not been reported following diagnostic

ultrasonography but safety limits to minimise potential risks have been set as follows

Intensities over 100mW/cm2 (spatial peak/temporal average) should only be applied

for a few seconds/minutes.

For human use 10 mW/cm2 is approved for commercial 2-D ultrasound.

Intensities over 100mW/cm2 (spatial peak/temporal average) should only be applied

for a few seconds/minutes.

For human use 10 mW/cm2 is approved for commercial 2-D ultrasound.

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