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Artifacts occur frequently in gray-scale ultrasound due to the physical and technical properties of ultrasound.
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Some artifacts may be avoided by modifying equipment settings and by the appropriate performance of the ultrasound examination.
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Some artifacts aid in distinguishing tissues and lesions; most, however, hinder the acquisition of images.
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Certain artifacts occur predominantly or exclusively in musculoskeletal ultrasound; awareness of them and regular maintenance of the equipment are of paramount importance.
Artifacts are alterations in the original characteristics of an object, image, sound, or waveform. Artifacts commonly occur in almost every medical imaging technique, including conventional radiography, computed tomography (CT), scintigraphy, magnetic resonance imaging (MRI), and ultrasonography. Each modality is characterized by different artifacts that occur at various rates; artifacts seem to occur more often in ultrasound and are uncommon in MRI.
In all imaging modalities, the quality of the acquired image is strongly determined by the signal-to-noise ratio (SNR). Adequate images require a high SNR. As with other imaging modalities, SNR in ultrasonography is separated into two parts: acquisition and image processing. In sonography, the acquisition phase requires a high SNR in the acoustic signal, whereas in the processing phase, SNR plays an integral part in the digitalization of the signal; this phase is analogous to processing in other imaging modalities.
Humans are familiar with the optical artifacts that occur in nature. Fata Morgana, commonly known as a mirage, is an optical phenomenon that results from a temperature inversion (i.e., increase in temperature with advancing height). Fata Morgana, which is usually seen in the morning after a cold night, typically is an object such as an island, ship, or building on or beyond the horizon that appears elongated and elevated, such as castles in fairy tales. The undisturbed interface between cold, dense air and overlying warm air near the surface can act as a refracting lens, producing an inverted image, over which the distant direct image appears to hover.
Another common optical artifact may be seen when a pencil placed in a glass of water ( Fig. 3-1 ) or a guardrail reflected in a swimming pool appears to be broken. These artifacts are caused by the refraction of light under water. The velocity of light (or any electromagnetic wave) is reduced when traveling through a denser medium (its highest speed is attained in a vacuum). When light reaches an interface between two media at an angle (leading to refraction), the speed of the wave is reduced, usually causing a change of direction. Refraction is an optical artifact, but it also occurs in acoustics.
Gray-Scale Artifacts
Ultrasound artifacts are echoes appearing on the sonographic image that do not correspond in location or intensity to actual interfaces in the tissue. Understanding artifacts is important to avoid misinterpretation or false results. Artifacts are predictable phenomena that in some cases may be minimized or avoided. The term artifact is often used incorrectly as a synonym for pitfall, bias, or error. A pitfall is a hidden or unsuspected danger or a difficulty occurring during acquisition or interpretation of an image.
The technical design of ultrasound machines is based on a set of core principles that apply only under selected conditions:
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The ultrasound beam is narrow and has a uniform width.
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The speed of sound travels 1540 m/s in soft tissue.
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The attenuation of ultrasound is uniform.
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Ultrasound energy travels in a straight line directly to the reflecting object and back to the transducer.
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Echoes from all depths are allowed to reach the transducer before the next ultrasound pulse is emitted.
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The amplitude of the echo is directly related to the reflectivity of the scanned object.
Typically, one or more of these principles is impaired, leading to the development of artifacts. Several textbooks have classified artifacts based on the impaired principle.
Artifacts can be classified according to the difference between the original object and artificial object or the acoustic process during which the artifact is generated. The observed artifact may differ from the original with respect to a number of attributes: dimension (i.e., smaller or bigger than the evaluated object), form, position, number, brightness, or edge clarity. Objects may appear missing or partly missing, and new “ghostly” objects may appear. Artifacts may also be classified according to avoidability and utility value (i.e., helpfulness of the artifact). Another system distinguishes among operator-dependent, machine-dependent, and patient-dependent artifacts. A practical classification of artifacts is based on the site of origin of the artifact within a system composed the sonographic equipment and the examined patient ( Table 3-1 ).
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Artifacts Caused by Equipment Properties or Errors
Electronic and Acoustic Noise
If the insulation or grounding of the ultrasonographic equipment is inadequate, electromagnetic interference from other electronic devices may distort the image. The two main sources of noise are acoustic noise produced by the transducer, which arises from reflections of the sound beam from other sources, and electronic noise, which is an innate characteristic of all electronic devices. Typically, acoustic noise appears larger and is harder to avoid than electronic noise. Electronic and acoustic noise is more common with portable equipment. These properties are related to the equipment, and users’ options are limited to selecting the placement of the equipment within the examination room, because location may affect the appearance of noise. In case of unavoidable noise, the company’s representative should be contacted.
Electronic Circuit Damage, Piezoelectric Crystal Damage, and Cable Break
Electronic circuits within the sonographic equipment and piezoelectric crystals within the transducer may become damaged, and mechanical breaks may damage the transducer cable, usually resulting in the appearance of vertical anechoic lines seen on the display. The artifactual image cannot be confused with other artifacts, but the site of the damage may be hard to identify. Small calcifications in the skin (e.g., juvenile dermatomyositis) can mimic this artifact, but in such cases, the line disappears if the transducer is moved ( Fig. 3-2 ). When mechanical damage to the equipment is suspected, contacting the seller’s representative is advisable.
Transducer Reverberation
Transducer reverberations usually appear as multiple, hyperechoic, parallel lines in the upper part of the image. The intensity of the lines decreases with distance from the surface. These artifacts are routinely visible before the start of the examination, and they disappear after the transducer comes into contact with the skin. The artifact appears because the soft tissue–air interface is a strong reflector and echoes are reflected back to the transducer ( Fig. 3-3 ).
Rubber Surface
The rubber surface artifact may be encountered during routine examinations. In the top part of the display, above the hypoechoic, inhomogeneous area corresponding to the gel, the first parallel echoic line that is not the skin but the rubber coating of the transducer may be seen ( Fig. 3-4 ).
Artifacts Originating between the Transducer and the Skin
Metallic Objects
Metallic objects interposed between the transducer and the skin of the patient can produce an echoic line through the image. These lines often can be seen when metal clippers are used for marking a location for local injection ( Fig. 3-5 ). Artifacts produced under the skin will be discussed later in the section on artifacts occurring inside tissue.
Contact Artifacts and Skin Irregularities
Contact (coupling) artifacts or artifacts due skin irregularities may arise in case of insufficient contact or intermittent contact between the transducer and the skin, resulting in a dropout artifact ( Fig. 3-6 ). Different skin lesions, including psoriatic plaques, wounds, skin tumors, and hirsutism, can lead to the formation of artifactual images. These artifacts can be avoided by using copious amounts of gel and applying less pressure with the transducer.
Patient Motion
Patient motion may degrade image quality for all imaging modalities (e.g., conventional radiography, MRI, CT) and can result in misinterpretation of the findings.
Patient movement produces blurred, grainy images. This artifact can be prevented by the patient remaining as still as possible during the examination. Two types of patients may be unable to follow clear commands to stay still: young children and patients with certain neurodegenerative conditions (e.g., Parkinson’s disease, Huntington’s disease) characterized by tremor and erratic movement. In these cases, the careful use of restraints may be required to complete the examination.
Artifacts Originating within Tissue
Axial and Lateral Resolution
Resolution refers to the ability of the ultrasound beam to differentiate individual interfaces from one another. The two main types of resolution are lateral and axial resolution. Axial resolution denotes the minimal distance between two objects located in a vertical line in the path of the ultrasound beam that allows their identification as two separate interfaces. Axial resolution depends on the wavelength and the pulse length. Lateral resolution denotes the similar separation of two objects as separate interfaces that are situated on a horizontal line in the path of the ultrasound beam. Lateral resolution depends on beam width and is the major limiting factor in the quality of diagnostic ultrasound images. Lateral and axial resolution limitations are artifactual in nature because a failure to resolve means a loss of detail, and two adjacent structures may be visualized as one.
Apparent resolution close to the transducer (i.e., speckle) is not directly related to tissue texture but is a result of interference effects from the distribution of scatterers in the tissue. Better lateral resolution at a given axial range can be achieved by acoustic focusing. A weakly focused transducer (5 to 7.5 MHz) shows reduced resolution in near and far fields. To reduce the artifacts near the field, the distance between the transducer and the skin must be increased. This can be achieved using an external standoff pad, which enhances the focal zone of the beam; however, its use is inconvenient. Using a higher-frequency transducer can increase resolution. Because of axial or lateral resolution artifacts, two or more small tendons may appear as one structure.
Slice Thickness or Elevational Resolution
Slice-thickness artifact is also known as a partial-volume artifact or volume-averaging artifact. An ultrasound beam has a width that varies according to the design characteristics of the transducer. The image displayed in two dimensions on the screen is the depiction of a three-dimensional (3D) volume that is scanned by the beam. Compression from 3D to two dimensions (2D) produces slice-thickness artifacts.
Slice-thickness artifacts are produced when echoes from outside the assumed plane of origin appear on the display. This type of artifact is frequently seen in fluid-filled structures (e.g., blood vessels). The characteristic appearance consists of loss of signal and distal acoustic shadowing at the edge of the tendons that can mimic or obscure fluid or inflammation in the paratenon ( Fig. 3-7 ). Slice-thickness artifacts can be reduced by the use of multiple focal zones. If a single focal zone is used, it must be set to the depth of the item of main interest.
All types of spatial resolution depend on the physical properties of the transducer. As a general rule, axial resolution is better than lateral resolution, which is better than elevational resolution. Compared with results from 2D imaging, the use of 3D/4D ultrasound imaging is still controversial for judging gray-scale artifacts.
Secondary Beam Artifacts: Side Lobes and Grating Lobes
Side lobe beams are generated from the edges of the transducer and project radially in directions different from that of the main beam. Although much weaker than the echoes generated by the main beam, they may be powerful enough to travel back to the transducer when the echo backscatters from a very strong reflector without a significant angle. This may lead to the formation of an artifact, because the transducer assumes that all the echoes causing reflections have originated from the main beam; in this case, their positions on the display are incorrect.
Grating lobes are caused by an array of piezoelectric elements generated at the edges of the transducer. Manufacturers try to prevent this artifact by apodization , which decreases the relative excitation of the elements or decreases the relative sensitivity of the elements near to the edges of the radiating surface of the transducer. Crystal element isolation is another option to block cross-talk between elements producing grating lobes. Near-field clutter is a related phenomenon caused by acoustic noise generated in the vicinity of the transducer due to high-amplitude oscillations of the piezoelectric elements. This artifact may considerably hinder differentiation between structures within 1 cm of the transducer.
Overgain and Undergain and Incorrect Use of Time Gain Compensation
Gain, which is the artificial increase of signal strength, refers to the amplification of the received signal. All parts of the image on the screen are equally affected. Adjustments of gain should be done with respect to tissues of known echogenicity. Inappropriately low gain settings may result in the apparent absence of an existing structure (i.e., missing structure artifact), whereas inappropriately high gain settings can easily obscure existing structures ( Fig. 3-8 ).
