usg artifacts

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USG ARTEFACTS moderator; Dr.pavan kumar k presenter; bhaskara rao

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Page 1: Usg artifacts

USG ARTEFACTS

moderator; Dr.pavan kumar k

presenter; bhaskara rao

Page 2: Usg artifacts

What is an Artifact?

Artifact is used to describe any part of an image that does not accurately represent the anatomic structures present within the subject being evaluated.

In ultrasonography (US), artifacts may cause structures to appear in an image that are not present anatomically or a structure that is present anatomically may be missing from the image. US artifacts may also show structures as present but incorrect in location, size, or brightness.

US is prone to numerous imaging artifacts, and these are commonly encountered in clinical practice. Artifacts have the potential to interfere with image interpretation.

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Ultrasound display equipment relies on physical assumptions to assign the location and intensity of each received echo.

These assumptions are that

the echoes detected originated from within the main ultrasound beam,

an echo returns to the transducer after a single reflection,

the depth of an object is directly related to the amount of time for an ultrasound pulse to return to the transducer as an echo,

the speed of sound in human tissue is constant, the sound beam and its echo travel in a straight path,

the acoustic energy in an ultrasound field is uniformly attenuated.

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In clinical sonography, these assumptions are often not maintained; when this occurs, echoes may be displayed erroneously and perceived as artifact. Artifacts thus arise secondary to errors.

inherent to the ultrasound beam characteristics,

the presence of multiple echo paths,

velocity errors, and

attenuation errors

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Artifacts Associated with Ultrasound Beam Characteristics Beam width artifacts

Side lobe artifacts

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Artifacts Associated with Ultrasound Beam Characteristics

Diagram of an ultrasound beam. The main ultrasound beam narrows as it approaches the focal zone and then diverges and appears as a complex three-dimensional bow-tie shape with additional off-axis low-energy beams, which are referred to as side lobes and grating lobes

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BEAM WIDTH ARTEFACT

The distal beam may widen beyond the actual width of the transducer.

A highly reflective object located within the widened beam beyond the margin of the transducer may generate detectable echoes. The ultrasound display assumes that these echoes originated from within the narrow imaging plane and displays them as such.

Clinically, beam width artifact may be recognized when a structure that should be anechoic such as the bladder contains peripheral echoes.

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Beam width artefact

The ultrasound image localization software assumes an imaging plane as indicated by the dotted lines.

Echoes generated by the object located in the peripheral field (gray circle) are displayed as overlapping the object of interest (white square).

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If this artifact is recognized during scanning, image quality may be improved by adjusting the focal zone to the level of interest and by placing the transducer at the center of the object of interest

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Adjusting the focal zone and placing the object of interest within the center of the focal zone will eliminate the misplaced echoes on the display

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Longitudinal US image of a partially filled bladder shows echoes (arrow) in the expected anechoic urine. The focal zone is improperly set too shallow.

Longitudinal US image obtained after adjustment of the focal zone and optimal placement of the transducer shows resolution of the intravesical echoes.

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SIDE LOBE ARTIFACT

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.

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Side Lobe Artifact

Diagram shows multiple beams of off-axis side lobe ultrasound energy encountering an object (black circle).

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 Multiple Echoes

Reverberation artifact

Comet tail artifact

Ring down artifact

Mirror image artifact

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REVERBERATION ARTIFACT

US assumes that an echo returns to the transducer after a single reflection and that the depth of an object is related to the time for this round trip.

In the presence of two parallel highly reflective surfaces, the echoes generated from a primary ultrasound beam may be repeatedly reflected back and forth before returning to the transducer for detection . When this occurs, multiple echoes are recorded and displayed. The echo that returns to the transducer after a single reflection will be displayed in the proper location. The sequential echoes will take longer to return to the transducer, and the ultrasound processor will erroneously place the delayed echoes at an increased distance from the transducer.

At imaging, this is seen as multiple equidistantly spaced linear reflections and is referred to as reverberation artifact

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Diagram shows ultrasound echoes being repeatedly reflected between two highly reflective interfaces.

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)

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COMET TAIL ARTIFACT

Comet tail artifact is a form of reverberation. In this artifact, the two reflective interfaces and thus sequential echoes are closely spaced.

On the display, the sequential echoes may be so close together that individual signals are not perceivable. In addition, the later echoes may have decreased amplitude secondary to attenuation; this decreased amplitude is displayed as decreased width .

The result is an artifact caused by the principle of reverberation but with a triangular, tapered shape

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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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RING DOWN ARTIFACT Ring-down artifact has been thought to be a variant of comet tail

artifact. This assumption was based on the often similar appearanceof the two artifacts.

In ring-down artifact, the transmitted ultrasound energy causesresonant vibrations within fluid trapped between a tetrahedron ofair bubbles. These vibrations create a continuous sound wave thatis transmitted back to the transducer .

This phenomenon is displayed as a line or series of parallel bandsextending posterior to a gas collection. Despite the similarsonographic appearance, these two artifacts have separatemechanisms

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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.

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The display shows a bright reflector with an echogenicline 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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MIRROR IMAGE ARTIFACT

Mirror image artifacts are also generated by the false assumptionthat an echo returns to the transducer after a single reflection.

In this scenario, the primary beam encounters a highly reflectiveinterface. The reflected echoes then encounter the “back side” of astructure and are reflected back toward the reflective interfacebefore being reflected to the transducer for detection. The displayshows a duplicated structure equidistant from but deep to thestrongly reflective interface .

In clinical imaging, this duplicated structure is commonly identifiedat the level of the diaphragm, with the pleural-air interface acting asthe strong reflector. At this location, the artifact is easily recognizedas hepatic parenchyma present in the expected location of lung

Page 24: Usg artifacts

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

The echoes from the deeper reflective interface take longer to return to the transducer and are misplaced on the display.

Page 25: Usg artifacts

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.

Page 26: Usg artifacts

Artifacts Associated with Velocity Errors

Speed displacement artifact

Refraction artefact

Page 27: Usg artifacts

Speed displacement artifact

The speed of sound within a material is dependent on its densityand elastic properties. US image processing assumes a constantspeed of sound in human tissue of 1540 m/sec. In clinicalsonography, the ultrasound beam may encounter a variety ofmaterials such as air, fluid, fat, soft tissue, and bone. The velocity ofsound in these materials is listed in Table

Page 28: Usg artifacts

Then 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

Page 29: Usg artifacts

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.

Because the round trip of this echo is longer than expected, the posterior wall is displaced deeper on the display.

Page 30: Usg artifacts

Transverse US image of the liver and close-up detail image show that the interface between the liver and the diaphragm (arrow ) is discontinuous and focally displaced (arrows ). This appearance may be explained by areas of focal fat within the liver.

Page 31: Usg artifacts

REFRACTION ARTIFACT

A change in velocity of the ultrasound beam as it travels throughtwo adjacent tissues with different density and elastic propertiesmay produce a refraction artifact.

In refraction, nonperpendicular incident ultrasound energyencounters an interface between two materials with different speedsof sound. When this occurs, the incident ultrasound beam changesdirection. The degree of this change in direction is dependent onboth the angle of the incident ultrasound beam and the differencein velocity between the two media. This relationship is described bySnell’s law:

sin r/sin I = c2/c1 r=refracted angle I=incident angle

c2=velocity in 2nd medium c1=vel. In 1st medium

Page 32: Usg artifacts

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 .

Page 33: Usg artifacts

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 (C1 and C2). The incident ultrasound beam with refraction encounters two structures.

The object in the path of the refracted portion of the beam is misplaced because the processor assumes a straight path of the beam.

Page 34: Usg artifacts

Artifacts Associated with Attenuation Errors

Shadowing

Increased through transmission

Page 35: Usg artifacts

Shadowing

When the ultra- sound beam encounters a strongly attenuating or highly reflective structure, the amplitude of the beam distal to this structure is diminished .

The echoes returning from structures beyond the highly attenuating structure will also be diminished. In clinical imaging, this phenomenon is recognized as a dark or hypoechoic band known as a “shadow” deep to a highly attenuating structure

Page 36: Usg artifacts

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.

Longitudinal US image of the gallbladder shows shadowing (arrow) posterior to echogenicgallstones.

Page 37: Usg artifacts

INCREASED THROUGH TRANSMISSION

When the ultrasound beam encounters a focal weakly attenuatingstructure within the imaging field, the amplitude of the beambeyond this structure is greater than the beam amplitude at thesame depth in the rest of the field . The echoes returning fromstructures deep to the focal weak attenuator will be of higheramplitude and will be falsely displayed as increased in echogenicity.

On the display, we identify this “increased through transmission” asa bright band extending from an object of low attenuation

Page 38: Usg artifacts

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.

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.

Page 39: Usg artifacts

With an understanding of the attenuation characteristics of materials encountered in human anatomy, these “artifacts” can be used by the clinician to determine the composition of a structure on the basis of US appearance and can be used to narrow a differential diagnosis

Transverse US image of the breast shows a small hypoechoic nodule with increased through transmission (arrow). The nodule was stable over a 2-year period in a patient with multiple cystic breast lesions. (b) Transverse US image of the breast shows a small hypoechoic nodule with posterior shadowing (arrow). The lesion was a pathologically proved breast cancer.

Page 40: Usg artifacts

Attenuation is also dependent on the frequency of the ultrasound. Attenuation increases with increase in frequency.

In soft tissues, the relationship between attenuation and frequency is linear. In bone and water, attenuation increases as the square of the frequency .

In clinical imaging, the different tissues an ultrasound beam encounters attenuate the beam differently. If the attenuation coefficient for a material is great, such as with fat, then the beam may not fully penetrate the imaging field. In this situation, deep structures may not be visualized. An appropriate-frequency transducer should be selected to optimize penetration

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Use of knowledge of ultrasound attenuation to improve image quality. (a) On a longitudinal US image of the liver obtained with a high-frequency transducer, the deep hepatic parenchyma is not well visualized (angle bracket). Disease in this portion of the liver may not be detected. (b) On a longitudinal US image of the liver obtained in the same patient with a lower-frequency transducer, the deep hepatic parenchyma is now well visualized.

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Conclusion

Often, these errors in image display are unavoidable and occursecondary to intrinsic physical properties of the ultrasound beamand its echo and to limitations of the display equipment.Recognition of these unavoidable artifacts is important becausethey may be clues to tissue composition and aid in diagnosis.

The ability to recognize and remedy potentially correctable artifactsis important for image quality improvement and optimal patientcare.

Page 43: Usg artifacts

Tissue harmonic imaging

Tissue harmonic ultrasonography (US) is based on the phenomenon ofnonlinear distortion of an acoustic signal as it travels through the body.

Imaging begins with insonation of tissue with ultrasound waves of aspecific transmitted frequency.

Harmonic waves are generated within the tissue and build up withdepth to a point of maximal intensity before they decrease because ofattenuation .

In comparison, conventional ultrasound waves are generated at thesurface of the transducer and progressively decrease in intensity asthey traverse the body .

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Harmonic wave frequencies are higher integer multiples of thetransmitted frequency, much like the overtones of a musical note.

Current technology uses only the second harmonic (twice thetransmitted frequency) for imaging.

The processed image is formed with use of the harmonic-frequencybandwidth in the received signal after the transmitted frequencyspectrum is filtered out

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Potential advantages of harmonic imaging include improved axialresolution due to shorter wavelength, better lateral resolution due toimproved focusing with higher frequencies, and less artifact than withconventional US.

Reduced artefact in harmonic imaging is due to the relatively smallamplitude of the harmonic waves, which reduces detection of echoesfrom multiple scattering events.

Additionally, side lobes are less likely to occur and degrade the image.

Furthermore, harmonic beams are produced beyond the body wall,thereby reducing the defocusing effect of the body wall. This is atheoretic advantage in imaging of obese patients .

Page 46: Usg artifacts

Harmonic Generation

Nonlinear effects in tissue distort the sound wave as it goes through tissue

Tissue is somewhat compressible

Sound traveling through the body is a pressure wave

At the wave peaks tissue is more dense so the wave travels faster

The higher the pressure, the greater the distortion

Any change from a perfect sinusoidal wave creates harmonics

Page 47: Usg artifacts

Nonlinear Propagation Propagation speed increases with

density (pressure)

The wave moves faster when the pressure is higher (peaks) and slower where it is lower (troughs)

Any change from a perfect sinusoid creates harmonics

The higher the amplitude, the more the effect

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Page 48: Usg artifacts

Tissue Harmonic Imaging

Conventional

processing

Acoustic

field

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TissueFundamental

Beam

2nd Harmonic Beam(from nonlinear propagation)Transmitted signal

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Harmonic

processing

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Page 49: Usg artifacts

Reverberation Artifacts

Sound reflects between lungs, ribs and skin

Reflected sound is mostly fundamental

Low energy reverberations appear deeper in image as haze and clutter artifacts

They are low energy so they never develop harmonics

Receiver filters tuned at harmonic frequency remove reverberations at fo and clean up the image

Page 50: Usg artifacts

Fundamental Processing

Acoustic

field

After

processing

Reverberations

here...

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clutter here

Receive filters

Transmitted signal

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Harmonic Processing

Acoustic

field

After

processing

Reverberations

here...

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Receive filters

Transmitted signal

Page 52: Usg artifacts

Tissue Aberrations

Shallow fat and skin layers distort sound beam

Distortions are low energy but send sound in random directions

Main beam generates harmonics during passage through surface layers

Distortion energy not strong enough to generate harmonics

Harmonic processing removes distortions and cleans image

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Conventional Processing

Acoustic

field

After

processing

Tra

nsd

ucer

Shallow fat layers

Tra

nsd

ucer

Shallow fat layers

Haze and clutter

Distorted sound beam

Receive filters

Transmitted signal

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Harmonic Processing

Acoustic

field

After

processing

Tra

nsd

ucer

Shallow fat layers

Distorted sound beam

Receive filters

Transmitted signal

Clean image

Tra

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Shallow fat layers

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Clinical Benefits of THI

Reduced nearfield artifacts

Reduced haze and clutter from tough acoustic windows and tissue aberrations

Improved border delineation and contrast resolution

Difficult to image patients addressed (!!)

Surprising since 90% or more of the scattered energy is filtered out with harmonic processing

Narrower beam sharpens up easy exams, but aberration reduction is main benefit

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Clinical Examples

Conventional imaging Tissue Harmonic Imaging

liver liver

aortaaorta

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Clinical Examples: Left Adrenal

Mass

Conventional imaging Tissue Harmonic Imaging

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Tissue Harmonic Imaging Summary

Tissue harmonic imaging benefits from the nonlinear propagation process in tissue

Harmonic beam is generated only where fundamental is strong

Weak fundamental scattered energy does not generate much harmonics

Clutter from tissue aberrations is low in harmonic content

Harmonic beam is low close to transducer where most aberrations occur

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SPATIAL COMPOUND IMAGING

In spatial compound imaging electronic steering of ultrasoundbeams from an array transducer is used to image the same tissuemultiple times by using parallel beams oriented In differentdirections .

Data from many such beam lines is acquired and averagedtogether in to a single image.

The appearance of speckle varies according to the beam linedirection ,averaging echoes from different directions tends toaverage and smooth speckle making the images look less grainy.

since speckle noise is random it cancels out during compoundingwhile signal is reinforced.

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CODED PULSE EXCITATION Higher frequency us pulses produce images with better spatial

resolution, however increased attenuation with increasing frequency limits evaluation of deep structures with high frequencies.

Coded pulse excitation is a means of overcoming this limitation providing good penetration at higher frequencies necessary for high spatial resolution.

This technique uses longer ultrasound pulses instead of the routinely used short us pulses.

The longer pulses carry greater ultrasound energy which increases the energy of echos that return from the deeper tissues allowing higher frequencies.

Coded pulse produced with a very characteristic shape and the returning echos have a similar shape,which makes their identification from background noise easier.

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ADAPTIVE IMAGE PROCESSING

EXTENDED FIELD OF VIEW IMAAGING

B-MODE FLOW IMAGING(NOT DOPPLER TECHNIQUE)

3D-4D ULTRSOUND

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THANK YOU