Musculoskeletal Ultrasound Artifacts



Musculoskeletal Ultrasound Artifacts





Artifacts Due to Beam Characteristics


Anisotropy is one of the most common artifacts encountered in musculoskeletal US. The term anisotropy refers to a property being directionally dependent, as opposed to isotropy, which implies identical properties in all directions. Anisotropy can result in a structure appearing hypoechoic if the US beam is not directly perpendicular to the structure. It is seen particularly in tendons, ligaments, nerves, and, to a lesser degree, muscle due to their oblique course and parallel reflective surfaces. Anisotropy can occur when the US beam is angled as little as 5° off perpendicular. It can mimic pathology, such as tendinosis or tendon tear, or may make a biopsy needle less conspicuous. It is particularly noticeable when the structure being imaged is curving, e.g., the ankle tendons curving around the malleoli or the supraspinatus tendon fibers curving to their insertional area. Anisotropy is eliminated by angulating (toggling) the transducer head so that the US beam is aligned perpendicular to the area of interest. It is important to evaluate only the part of the structure that is perpendicular to the US beam and not be tempted to diagnose pathology in other adjacent parts. The anisotropic effect can frequently be used to more clearly demarcate a structure of interest, such as the median nerve in the carpal tunnel or the ankle ligaments.


Off-axis lobe artifact includes side lobe and grating lobe artifacts. Side lobes are multiple beams of low amplitude that project radially from the primary beam axis and are mainly seen with linear transducers. Grating lobes are additional off-axis beams that are stronger than side lobes. Both side and grating lobes are weaker than the primary lobe and, as such, do not normally give echoes of sufficient strength to be displayed. However, if a side lobe or a grating lobe encounters a more highly reflective structure outside the primary beam, their echoes may be incorrectly displayed on the image along the path of the main beam. Off-axis lobe artifacts in musculoskeletal imaging are usually seen as spurious echoes within an expected anechoic structure, such as a cyst or an effusion. Side lobes artifacts will be seen at the edge of Baker cysts. Much less commonly, off-lobe artifacts may give rise to multiple needle paths during US-guided intervention.


This artifact is due to incorrect focal zone positioning too superficial to the area of interest. A strong reflector located within the widened beam beyond the focal zone will generate detectable echoes. These echoes are assumed to have originated from the narrow primary beam and are displayed on the primary beam image even though they have not arisen with the primary beam area. Beam-width artifact is typically seen either as spurious echoes displayed at the edges of an anechoic structure or as reduced contrast at the borders of a lesion. Although the distinction can be very difficult, Beam-width artifact produces a more confined artifact than off-axis lobe and, of course, can be improved by optimizing the focal zone. Off-lobe axis artifact will not improve with optimization of the focal zone.


Slice-thickness artifact is similar to beam-width artifact but instead of being predominantly in a medial-lateral direction, it occurs due to an aberration in the anteroposterior direction. Slice-thickness artifact occurs due to the thickness of the beam, which is 90° to scan plane and equates to the thickness of the transducer head. The transducer will receive echoes from in front of and behind the assumed plane of origin, which will be included in the displayed image. Increasing the thickness of the transducer head will increase the propensity to develop slice-thickness artifact.



Artifacts Due to Multiple Echoes


Multiple reflections (reverberations) can occur between any 2 strong reflectors parallel to each other or between the transducer and a strong parallel reflector. As a result of these strong reflectors, echoes are reflected back and forth repeatedly. This leads to spuriously reflected echoes being displayed beneath the real reflector at intervals equal to either the distance between the transducer and real reflector or the distance between the 2 strong reflectors. Each subsequent reflection is weaker than the previous one due to sound beam attenuation. Reverberation artifact in musculoskeletal imaging is most commonly seen when the US beam encounters a strongly reflecting surface, e.g., a biopsy needle or a metal prosthesis. An advantage of seeing this artifact is that it ensures your transducer is at a right angle to the structure being imaged. Comet-tail artifacts are a particular type of reverberation artifact usually seen in echo-free regions and are composed of a dense tapering trail of echoes behind a highly reflective small structure, such as calcification, crystal aggregate in gout, or colloid aggregate in a ganglion cyst.


Ring-down (resonance) artifact is similar in its appearance to comet-tail artifact but is generated by a different mechanism. When a transmitted US beam passes through small gas locules or fluid bubbles, the US energy leads to vibration of these bubbles, which emit and return sound waves back to the transducer. The effect is a larger artifact than expected.


Mirror-image artifact occurs when an object is located directly in front of a highly reflective smooth surface. The US beam is totally reflected by the strong reflector back toward the object. When the beam hits the object, part of the energy is reflected back to the strong reflector, which then redirects this echo back toward the transducer. True and false images are created equidistant from but on opposite sides of the strong reflector. In musculoskeletal imaging, a mirror-image artifact most commonly manifests as an extraosseous structure being mirrored within bone. The mirror image is inverted and, rather than being a exact copy, is often a distorted view of the real image.



Artifact Due to Velocity Errors


The speed of sound varies within different tissues, though US technology assumes that all tissues have a constant speed of 1,540 m/sec. If the speed of sound through tissues is > 1,540 m/sec, the returning echo will return faster to the transducer leading to the structure being displayed more superficially on the image than expected. Conversely, if the average speed through the tissues is < 1,540 m/sec, the structure will appear deeper than it is in reality. Sound travels ~ 10% slower through fat, which has a sound transmission speed of 1,450 m/sec as opposed to lean tissue (1,540 m/sec).


Refraction of the US beam at a boundary between 2 media of different acoustic impedance will result in image misregistration. In refraction artifact, a nonperpendicular incident US beam will refract and change direction at a boundary between 2 materials of different acoustic impedance. Since US display technology assumes that the sound beam travels in a straight line, it will misplace any returning refracted echoes to the side of the true object location. Refraction artifact may result in a structure appearing longer than expected or a misplaced duplication of a structure (ghost image).


In defocusing artifact (edge-shadowing artifact), refraction produces shadowing at the edges of a relatively large (compared to the US beam width) curved structure. Refraction occurs at the edge of the structure due to the curved edge and also differences in acoustic impedance between the 2 tissue types involved. The US beam at the edge of a structure, such as a cyst or soft tissue mass, is deviated from its original path. This is commonly seen in musculoskeletal imaging. Edge-shadowing artifact is also the reason for shadowing seen commonly in full-thickness Achilles tendon tear.



Artifact Due to Attenuation Error


This is the most commonly encountered artifact in musculoskeletal US and is the reason why US, for example, cannot look deep into joints. Acoustic shadowing artifact is caused by severe attenuation of the US beam at an interface, either by reflection, absorption, or, less commonly, refraction, resulting in reduced sound being transmitted beyond that interface. This leads to an area of hypoechogenicity behind a strongly reflecting interface or strongly attenuating tissue. In musculoskeletal imaging, this is seen as shadowing posterior to calcification, bone, foreign bodies, or dense fibrous tissue. The main implication is that the tissues behind that structure are not visible on US. This can cause problems. For example, in examination of the knee, it may be difficult to tell whether or not a large osseous structure is attached to the bone. It is also an advantageous sign. For example, acoustic shadowing behind noncalcified tissue would make it very likely that this tissue is composed of dense fibrous tissue. Acoustic shadowing, in the absence of calcification, is a good clue to the presence of fibrosis. Objects with a small radius or a rough surface are more inclined to produce a clean posterior shadow, whereas objects with a large radius or smooth surface are more inclined to produce a dirty shadow as a result of reverberation echoes superimposed on the acoustic shadow.


This is caused by low-attenuating structures, such as fluid, resulting in no weakening of the US beam as it passes through these tissues relative to adjacent tissues. This leads to an area of high-amplitude echoes behind the area of low-attenuating tissue. This artifact is seen when examining fluid, myxoid tissue, or associated low-attenuating tissue structures, such as ganglion cysts, nerve sheath tumors, or giant cell tumor of tendon sheath. Tissues deep to the low-attenuation structure appear hyperechoic. Similar to acoustic shadowing, acoustic enhancement is a useful diagnostic sign and can be used, for example, to increase diagnostic accuracy of nerve sheath tumors that typically show mild to moderate acoustic enhancement.



Artifact Due to Improper Machine Setting


US technology assumes that each pulse of echoes is received by the transmitter before the next pulse is emitted. If returning 1st pulse echoes arrive after the 2nd pulse has been transmitted, returning 1st pulse echoes are interpreted incorrectly as having originated from the 2nd pulse and will be incorrectly placed nearer to the transducer. Range ambiguity artifact is not commonly seen now, as the US machine automatically reduces pulse repetition frequency during imaging of deeper structures.


Incorrect use of equipment controls, such as gain or time-gain compensation, can result in the visualized echoes being either too bright or too dark. To avoid these artifacts, the machine should be properly setup at the start of each examination and repeatedly readjusted during the course of the examination.



Artifact Seen During Doppler Imaging


Too much transducer pressure may obliterate vascular flow, particularly in soft superficial vascular structures. To avoid this happening, you should use a large amount of US gel and actively lift the transducer off the skin when assessing tissue vascularity. Noticing the effect of transducer pressure can be helpful for tissue characterization. For example, the vascularity of nerve sheath tumors typically blanches with only slight transducer pressure.


Noise is caused by increased color or power Doppler settings. Noise can be recognized as small, spurious, inconsistent color dots. Color gain settings should be adjusted to minimize noise. To achieve maximum color sensitivity, increase color gain until noise pixels are clearly seen in the Doppler box, then slowly decrease gain until nearly all of the noise has disappeared.


Aliasing artifact occurs when the velocity range exceeds the scale available to display and is only seen with color and spectral Doppler imaging. Aliasing is not seen with power Doppler, as it does not provide flow-directional information. The sample rate on color or pulsed Doppler is equal to the pulse repetition frequency, which limits maximum velocity scale. The Nyquist limit is the upper limit of Doppler shift that can be detected accurately and is equal to 1/2 the pulse repetition frequency. If Doppler shift signal is higher than the Nyquist limit, aliasing artifact occurs with inaccurate display of color or spectral Doppler velocity. In color Doppler, the display wraps around the scale and overwrites exiting data while in spectral Doppler, velocity peak is cut off at the top end of the scale and is displayed at the lower end of the scale, i.e., below baseline. Aliasing can be overcome by increasing the velocity scale (pulse repetition frequency). Increasing the pulse repetition frequency is disadvantageous in that low velocities cannot be accurately measured.


As expected, range-ambiguity artifact can occur in Doppler imaging with increasing pulse repetition frequency used to overcome aliasing artifact. This should be identified in order to avoid detection of frequency shifts deeper than the target object.


This artifact leads to the spectral waveform being displayed with nearly equal amplitude above and below spectral analysis baseline in a mirror-image pattern. It is caused by the US beam intercepting the vessel at a 90° angle, particularly for small vessels moving in and out of the imaging plane. True bidirectional flow is a rare occurrence and is only seen in the neck of a pseudoaneurysm or when there is reversed diastolic flow near a high-resistance vascular stenosis. True bidirectional flow is never simultaneously symmetrical above and below baseline and may be seen to vary across the cardiac cycle.


Since Doppler imaging is angle dependent, the angle formed between the interrogating sound beam and vascular flow should be optimized. The angle can be changed by electronic steering. When the angle between the US beam and the vessel being studied approaches 90°, no Doppler shift is detected, and color may not present even though flow is present. The color box should be steered to make the insonation angle to flow closer to 0° or 180° and not close to 90°. This Doppler angle effect artifact is seen in color, power, and spectral Doppler imaging. The ideal Doppler imaging angle is either 0° or 180° but is generally not anatomically achievable when imaging vasculature. A Doppler angle of 60° has traditionally been specified as a standard, though with improvements in US technology, a better criterion would be an angle of 45-55°, striving to measure velocities at 50° whenever possible.


This artifact is usually caused by abnormally high Doppler gain settings, leading to color signal extending beyond the true grayscale vessel margin. It leads to color display outside a vessel wall, making vessels appear larger, and also potentially masking luminal detail, such as the presence of a thrombus. Decreasing color Doppler gain will minimize blooming artifact, though it will also reduce sensitivity to flow detection in small vessels.


Movement of either the patient or the transducer during Doppler imaging gives rise to a random color mosaic that obscures the grayscale image. Power Doppler is most susceptible to this artifact.


This is caused by the same mechanism as that seen with grayscale imaging and is a feature of color, spectral, or power Doppler imaging. It most commonly occurs adjacent to highly reflective surfaces, such as the lung when, for example, imaging the supraclavicular region where one may see duplication of the subclavian artery or vein with the false images being displayed deeper than the real image.


The presence of fluid flow in nonvascular cystic collections can mimic real blood flow in color or power Doppler imaging. However, no arterial or venous spectral waveform will be seen if this apparent fluid flow is subjected to spectral Doppler analysis.


Twinkling artifact can be seen behind any strongly reflecting surface, such as calcification, bone, or foreign body. It is seen as a mosaic of rapidly changing colors, mimicking turbulent flow, and presents deep to the strongly reflecting surface, such as a calcified nodule, on both color and power Doppler. It is most likely caused by narrow bandwidth noise induced by “phase jitter” in the Doppler circuitry of the US system.


Occurs at the margin of a strong, smooth reflector (such as a calcified mass or cortical bone), resulting in persistent color along the rim of a strong reflector. Spectral analysis in this case will show no vascular flow pattern. Edge artifact is seen more commonly with power Doppler than color Doppler imaging.


Even a small amount of motion will give rise to noise on color or power Doppler imaging. Noise appears as random dot-like flashes on color or power Doppler imaging. It is caused by low-frequency Doppler shifts and can be minimized by high-pass wall filters, though these filters will reduce sensitivity to detect slow-flowing blood. Very slow-flowing blood is best detected on grayscale imaging alone when moving red cell aggregates will be seen as slow-moving intravascular echoes.

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Apr 9, 2020 | Posted by in MUSCULOSKELETAL IMAGING | Comments Off on Musculoskeletal Ultrasound Artifacts

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