Ultrasound physics- Artifacts, Spi -E

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Ultrasound artifact for ultrasound physics – SPI

 Some of the Ultrasound artifacts are commonly encountered and a knowledge is necessary to avoid false diagnosis.

          acoustic shadowing

 Acoustic shadowing on an ultrasound image is characterized by a signal void behind structures that strongly absorb or reflect ultrasonic waves. This happens most frequently with solid structures, as sound conducts most rapidly in areas where molecules are closely packed, such as in bone or stones. 

 Acoustic enhancement, also called posterior enhancement orenhanced through transmission, refers to the increased echoes deep to structures that transmit sound exceptionally well.

 Acoustic enhancement

 This is characteristic of fluid filled structures such as cysts, theurinary bladder and the gallbladder. The fluid only attenuates the sound less than the surrounding tissue. The time gain compensation(TGC) overcompensates through the fluid-filled structure causing deeper tissues to be brighter. Simply it is seen as increased echogenicity (whiteness) posterior to cystic area. The presence of acoustic enhancement aids in the identification of cystic masses but some solid masses, especially lymphoma, may also show acoustic enhancement posteriorly. 

 Ultrasound beam width artifact ;

Occurs when a reflective object located beyond the widened ultrasound beam, after the focal zone, creates false detectable echoes that are displayed as overlapping the structure of interest.

To understand this artifact, it is important to remember that the ultrasound beam is not uniform with depth, the main beam leaves the transducer with the same width as it, then narrows as it approaches the focal zone and widens again distal to this zone . 

Usually, it occurs when scanning an anechoic structure and some peripheral echoes are identified, i.e. gas bubbles in the duodenum simulating small gallstones and peripheric echoes in the bladder. 

It is possible to avoid this artifact adjusting the focal zone to the depth level of interest and by placing the transducer at the center of the object being studied. 

 The comet-tail artifact;

It is a grey-scale ultrasound finding seen when small calcify / crystalline / highly reflective objections are interrogated, and is believed to be a special form of reverberation artifact. 

It is similar to the colour comet-tail artifact and is seen in similar situations, although is in general less sensitive than the later. 

  comet tail . = it can be seen with  Adenomyomatosis.

 The color comet-tail artifact

It is an ultrasonographic sign seen in a number of situations, when colour Doppler scanning is performed. 

Typically the artifact, which resembles the grey-scale comet-tail artifact, is seen in situation when a small highly refractive (usually calcify) object is interrogated with color Doppler. Twinkle artifact occurs and immediately deep to the object a tail linear aliased band of colour extends away from the probe.

The following are situations in which this sign may be present and helpful:

 it can be seen with =  Vesicouretric junction stone

 Mirror image artifact 

Mirror image in ultrasonography is seen when there is a highly reflective surface (e.g. diaphragm) in the path of the primary beam.

The primary beam reflects from such a surface (e.g. diaphragm) but instead of directly being received by the transducer, it encounters another structure (e.g. a nodular lesion) in its path and is reflected back to the highly reflective surface (e.g. diaphragm). It then again reflects back towards the transducer.

There is a technical false presumption that the returning echo has been reflected once and hence the delayed echoes are judged as if being returned from a deeper structure, thus giving a mirror artifact on the other side of the reflective surface. it can be seen in  =  liver mass behind strong reflector the diaphragm, also  =  mirror image  behind the bladder wall.

 Reverberation artifact occurs when an ultrasound beam encounters two strong parallel reflectors.

When the ultrasound beam reflects back and forth between the reflectors (“reverberates”), the ultrasound transducer interprets the sound waves returning from the reverberation as deeper structures since it took longer for the wave to return to the transducer.

Reverberation artifacts can be improved by changing the angle of insonation so that reverberation between strong parallel reflectors cannot occur.

                There are two sub types of reverberation artifact:

  • comet-tail artifact: a short train of reverberations from an echogenic focus which has strong parallel reflectors within it (e.g. cholesterol crystals in adenomyomatosis)
  • ring down artifact: a type of continuous sound wave returning to the transducer, often caused by fluid trapped between gas bubbles

 The  ring down artifact; “Ring-down” is an ultrasound artifact that appears as a solid streak or a series of parallel bands radiating away from abdominal gas collections. Using an in vitro system of bubbles in water or gelatin, it was found that the ring-down artifact originated from the center of a cluster of four bubbles (bubble tetrahedron), three on top and one nestled beneath. Entrapped between the bubbles is a horn- or bugle-shaped fluid collection that we theorize emits a continuous sound wave back to the transducer when struck by an ultrasound pulse. Electronic processing by the scanner converts this continuous sound wave into the series of bands seen in the ring-down artifact.

             COMET TAIL AND RING DOWN ARTIFACTS 

usually are used as synonyms, due to their similar appearance on ultrasound. But their physical basis are completely different and the mechanism that produce them:

 – Comet tail artifact is a form of reverberation; therefore it has the same production basis, i.e, repeated trips between the transducer and two reflective surfaces. -Ring-down artifact occurs when the ultrasound beam excites the liquid trapped between gas bubbles, producing the liquid to vibrate or resonate.– Metallic elements/calcifications cause comet tail artifact, being thus useful for diagnosing from foreign bodies, metal clips of suture, catheters… to calculus or granulomas.

 Gas is responsible for Ring-Down artifact. Its display can translate from gas than can be found in physiological situations (for example inside bowel), after surgical procedures (aerobilia after cholecystectomy or hepaticojejunostomy) or pathological processes (pneumatosis intestinalis, emphysematous cholecystitis). It is necessary to emphasize that knowledge of clinical data and patient history are essential to guide in the etiology .

                                  Speckle artifact 

It may be encountered in ultrasound. It is caused by the scattering of waves from the surface of small structures within a certain tissue. The artifact produces a textured appearance.

                              Speed displacement artifact

It is a gray scale ultrasound finding that can be identified as an area of focal discontinuity and displacement of an echo deeper than that its actual position in an imaged structure.  The artifact is due to the image processor’s assumption that the velocity of the ultrasound beam within the imaged anatomy is uniform and that distance of the structure from the transducer is the sole factor determining that structure’s location.  However, objects imaged by portions of the ultrasound beam slowed by tissue the beam traverses, will appear discontinuous and artificially deep.  A commonly encountered scenario is speed displacement artifact due to slowing of the ultrasound beam by focal fat.

 Another artifact is the  “side-lobe-artifact” Artifice; Side lobes are multiple beams of low-amplitude ultrasound energy that project radically 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. 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 .

Twinkle artifact is the result of intrinsic machine noise seen with colour Doppler ultrasound . It occurs as a focus of alternating colors on Doppler signal behind a reflective object (such as calculi), which gives the appearance of turbulent blood flow . And it appears with or without an associated colour comet-tail artifact . 

Twinkle artifact is more sensitive for detection of small stones (e.g.urolithiasis) than is acoustic shadowing, and is highly dependent on machine settings and is most pronounced when the reflecting surface is rough. One of these settings is the location of the focal zone: i.e. when the focal zone is located below a rough reflecting surface, the twinkling artifact becomes more obvious than when it is above it =  twinkle artifact. Mosaic .

 In radiologic imaging, the term 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. To avoid confusion, the radiologist should be able to recognize artifacts when they occur. Physicians who understand the physical explanation of these artifacts will be able to use this knowledge to improve both the image quality of their scans and the diagnostic power of their interpretations.

The creation of a US image is based on the physical properties of ultrasound pulse formation, the propagation of sound in matter, the interaction of sound with reflective interfaces, and echo detection and processing. 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, and the acoustic energy in an ultrasound field is uniformly attenuated.

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 . In this article, these errors and the types of artifacts they create are discussed. For each artifact, emphasis is placed on the physical explanation, imaging appearance, diagnostic importance, and when applicable, scanning technique modifications that can be applied to improve image quality.

                            Ultrasound Artifacts – B .

Beam width artifact  =can be identified by understanding the shape of the ultrasound beam. The main ultrasound beam exits the transducer at approximately the same width as the transducer, then narrows as it approaches the focal zone and widens again distal to the focal zone .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. 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 .

In the past, ring-down artifact has been thought to be a variant of comet tail artifact. This assumption was based on the often similar appearance of the two artifacts, the theory that in ring-down artifact, the transmitted ultrasound energy causes resonant vibrations within fluid trapped between a tetrahedron of air bubbles. These vibrations create a continuous sound wave that is transmitted back to the transducer . This phenomenon is displayed as a line or series of parallel bands extending posterior to a gas collection. Despite the similar sonographic appearance, these two artifacts have separate mechanisms .

Mirror image artifacts are also generated by the false assumption that an echo returns to the transducer after a single reflection. In this scenario, the primary beam encounters a highly reflective interface. The reflected echoes then encounter the “back side” of a structure and are reflected back toward the reflective interface before being reflected to the transducer for detection. The display shows a duplicated structure equidistant from but deep to the strongly reflective interface .

In clinical imaging, this duplicated structure is commonly identified at the level of the diaphragm, with the pleural-air interface acting as the strong reflector. At this location, the artifact is easily recognized as hepatic parenchyma present in the expected location of lung .. US image processing assumes a constant speed of sound in human tissue of 1540 m/sec. In clinical sonography, the ultrasound beam may encounter a variety of materials such as air, fluid, fat, soft tissue, and bone.

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 .

Relationship is described by Snell’s law:

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, non perpendicular 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. This relationship is described by Snell’s law:

where c = velocity, i = incidence, and r = refraction. 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 .

                        “compensation amplification”

As an ultrasound beam travels through the body, its energy becomes attenuated secondary to absorption and scatter . An echo that travels a greater distance in the body will be attenuated more than an echo of similar energy that travels a shorter path. Ultrasound processing incorporates“compensation amplification” of echoes that take longer to return to the transducer. In this process, the echoes that return later are amplified more than earlier returning echoes. This serves to make the image appear more uniform in the deep field. “Time gain compensation” refers to a user-adjustable form of compensation . The attenuation coefficient expresses the loss of ultrasound intensity per distance traveled and varies in different mediums .

When the ultrasound beam encounters a focal material that attenuates the sound to a greater or lesser extent than in the surrounding tissue, the strength of the beam distal to this structure will be either weaker or stronger than in the surrounding field. Thus, 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 .

Similarly, when the ultrasound beam encounters a focal weakly attenuating structure within the imaging field, the amplitude of the beam beyond this structure is greater than the beam amplitude at the same depth in the rest of the field . The echoes returning from structures deep to the focal weak attenuator will be of higher amplitude and will be falsely displayed as increased in echogenicity. On the display, we identify this “increased through transmission” as a bright band extending from an object of low attenuation . 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 .

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 .

Steve Ramsey, PhD ; Calgary .Alberta    

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