Discussion

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 [1]. Ultrasound imaging artifacts of acoustic origin occur as displays that are duplicated (not real), are missing, or are of improper location, brightness, shape, or size [2].

Ultrasonography uses the physical properties of ultrasound waves to construct images of different tissues and structures. According to Kremkau [3], the premises for the design of ultrasound equipment are the following: (1) sound travels in straight lines, (2) echoes originate from objects located on the beam axis, (3) the amplitudes of returning echoes are related directly to the echogenicity of the objects that produced them, and (4) the distance to echogenic objects is proportional to the round-trip travel time. When any of these assumptions are violated, an artifact appears.

Ultrasound waves traveling through biologic tissues typically obey the laws of reflection and refraction. The most common image artifacts encountered in clinical practice can be grouped based on the following mechanisms:

• The physics of reflection and refraction: reverberation, acoustic shadowing, mirror artifact, and refraction artifact
• The properties of the ultrasound beam: side-lobe artifact and beam-width artifact
• The equipment: near-field clutter [4].

A mirror image artifact (Image 1) is created when the primary beam encounters a highly reflective surface (known as a “specular reflector”) that acts like a mirror [4]. Ultrasound waves reaching a strong reflector are reflected toward objects closer to the transducer than the reflector. These intervening objects reflect the waves back to the strong reflector, which in turn sends them back to the transducer. Because of the assumption of wave propagation (that all the returning sound waves comes from objects in the initial direction of the sound beam), the scanner displays these objects below the strong reflector, at a distance equal to the distance between strong reflector and the true intervening objects (Image 1). The false image acts as a mirrored object: inverted and moving in the opposite direction as the true structure. The existence of several reflectors could explain the appearance of multiple mirror images [5].

Mirror image artifact is commonly seen in the liver leading to duplication of hepatic structures caused when the diaphragm acts as a strong reflector [6]. The typical display of the mirror artifact consists of two similar structures separated and at similar distances from the reflective interface. The mirror image is hypoechoic, blurred, and distorted compared to the image of the real structure. The image found in the “phantom” uterus may be somewhat different from the intrauterine image. This is because the mirror is not exactly perpendicular to the plane of the probe, so the uterus is being imaged in a slightly different cross-section [5].

It has been reported in vascular, abdominal, cardiac, and musculoskeletal ultrasound, but is uncommon in obstetric ultrasound, having been published on seven occasions to the best of our knowledge, and practically always in the first trimester (with transabdominal and transvaginal probes) [5, 7-10]. There is only one case published in the second trimester of pregnancy [11]. In the first trimester, the psoas muscle acts as a reflective surface when the bladder is over distended and the uterus is pushed up into the abdomen [8]. At more advanced gestational age, gas and fluid in the bowel are the main contributors to the ultrasound artifact, not only creating a strong interface with the posterior uterine wall, but also serving as a space of representation of the mirrored image in the ultrasound screen [11].

A refraction artifact (Image 2) is produced when the probe is placed in the abdominal midline including the lateral margins of the rectus muscles [12,13]. Refraction can result in improper positioning of an object on the display screen or it may cause a single real structure to appear as two objects due to the existence of a refracting structure close to the transducer, which behaves like a lens [3]. Ultrasound waves directed through the ‘‘lens’’ are refracted toward the object and then re-refracted back to the original direction of transmission on the return acoustic path, resulting in a duplicate image of this object but in the original direction of the beam (Image 2) [4]. The rectus muscle interposed between the transducer and the area of interest acts like a lens and refracts the ultrasound beam, producing the duplication (or even triplication) of a single object [13]. When a part of the image of a relatively spherical structure such as the pregnant uterus is displaced by the refraction caused by the edge of the rectus muscle, the overlapping image produces a discontinuity (“nodal point”) in the uterine outline, and the shadow of the edge effect is evident [12]. Buttery and Davison call it the “ghost artifact” [12], although Müller et al prefer to call it "double image artifact" [13], which clearly describes the finding and avoids introducing confusing terms. Ahn et al speak of a ghost twin as a consequence of a mirror artifact, leading to further confusion in using this "ghost" terminology [11].

Although the duplication artifact is more commonly observed in the pelvis, it may also be seen in the upper abdomen. At first glance, it might look like a mirror artifact. However, we can differentiate them if we take into account that in the refraction artifact the repeated image appears next to instead of underneath the true image, and is not reversed as in the mirror artifact [3]. Also, when fetal movement is seen on real-time ultrasound examination, the two images move in unison [12].

Ultrasound artifacts are created by the machine’s interpretation of returning echoes but do not correspond to the actual anatomy of the patient. Physicians have to be aware of mirror or refraction artifact to avoid a misdiagnosis of heterotopic or twin pregnancy. Once identified, these artifacts can be decreased or eliminated by moving the transducer to center it on the region of interest or by placing the focal zone at the level of interest in the real structure [6, 14]. Maneuvers (transabdominal-transvaginal probe change, filling or emptying the bladder, maternal position changes) to verify correct diagnosis and resolve potential mirror or double image artifacts should be used before proceeding with surgical management of a false diagnosis of heterotopic pregnancy [9]. In the same way, more detailed imaging techniques such as magnetic resonance imaging (MRI) may be considered [10].

References

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