Ultrasound Mirror Artifact of a Thyroid Nodule by Trachea Mimicking a Tracheal Mass

Dear Editor:

Neck ultrasound (US) is a safe, easy-to-perform, nonexpensive tool for screening the thyroid for malignancy (1,2) and is commonly used by endocrinologists as an extension of physical examination, thus providing better information than that contained in the radiological report (1).

The probe of an US medical equipment reproduces images of body structures by generating, detecting, and transducing USs after the latter have been propagated through and reflected by body tissues (1,2). Visual artifacts are common in US practice (3) as well as in US of the thyroid (1,2) and arise according with the physical properties of US beam and those of tissues through which the beam propagates (3). The term artifact refers to an image that does not correctly reproduce the appearance of the original anatomic structure by (a) missing an anatomically present structure, (b) generating an image of a nonexistent anatomical structure, or (c) displaying substantial differences between the image and the anatomical structure (1,3). Among the various mechanisms involved in the origin of different types of US artifacts (1 –3), mirror artifacts consist in the production of a false image by a reflective surface, which is specular to the image of the real anatomical structure (3).

A 45-year-old man sought endocrinological consultation because of slight neck enlargement. Medical and US examinations revealed a well-defined hypoechoic thyroid nodule between the right lobe and the isthmus (Supplementary Fig. S1; Supplementary Data are available online at www.liebertonline.com/thy), consistent with a benign lesion on cytological analysis. At US a suspected mass within the tracheal wall specular to the thyroid nodule with similar dimension was also detected (Supplementary Fig. S2). This US imaging was difficult to interpret, because usually the air present within the trachea prevents both US transmission and imaging formation (1). A computed tomography scan of the neck did not reveal any tracheal mass obstructing the lumen (Supplementary Fig. S3), thus suggesting the hypothesis of a possible mirror US visual artifact, consisting in a false mirror image of the thyroid nodule specular to the original anatomical structure. This kind of artifact generates when a highly reflective interface repeatedly reflects the US signal, thus producing multiple reverberations and bending of the beam (4) (Supplementary Fig. S4).

In this case, this unusual mirror artifact was probably produced because of the high reflecting properties of tracheal cartilage and the smoothness of the tracheal rings, both reinforced by the posterior air column within the trachea. Usually, highly reflective, concave body structures as the diaphragm plus the pleural–air interface and the distended bladder are involved in originating mirror artifacts, the latter being very usual in abdominal US (3,4). Conversely, no imaging is generally expected to originate from structures posterior to the anterior tracheal wall in neck US because of lack of beam transmission through the air–tissue interface (1,2). Moreover, mirror image artifacts have not been described and are probably rare in the neck because of the absence of concave structures. In this case, the body structure of this lean man and a pronounced neck hyperextension during US enhanced both tracheal surface reflectivity and concavity, facilitating the mirror artifact generation.

The artifact was unrecognized at first, implying that even skilled endocrinologists do not share with radiologists a good knowledge of technical and physical background of diagnostic US, which is required to recognize visual artifacts (3). Understanding the principles and mechanisms of artifacts is one of the most important aspects of learning to perform and interpret US (3,4). As a practical point, when the artifact was first seen, changes in positioning the probe or in patient's position, neck hyperextension reduction, and manipulation of the transducer may have disclosed the unreal nature of the image seen, resulting in correct US imaging interpretation.

Not recognizing the artifact leads to further time-consuming, unsafe, expensive, and unnecessary clinical examinations.

In conclusion, it is important to know and recognize mirror artifacts in thyroid US practice, because they may be a source of misdiagnosis for the interpreting physician and may lead to unnecessary examinations. We occasionally encounter thyroid US artifacts in clinical practice but there is little information about them in the literature (see Supplementary Data). They may be a source of confusion, as in the case of the mirror artifact described here. This case demonstrates that mirror artifacts may arise also in US of the neck, this information being of concern for the beginners performing US, some radiologists, as well as clinicians. The ability of endocrinologists involved in diagnostic US to recognize and, if necessary, to make any corrective maneuvers during US examination is important for improving image quality and achieving optimal patient care.