A comet-tail artifact is a form of reverberation artifact between two reflective interfaces resulting in closely spaced reflective echoes. The returning sequential echoes may be so close together that individual signals are difficult to perceive, with the later echoes decreasing in amplitude secondary to attenuation. On ultrasound, this is displayed as an artifact with decreased width and signal strength and manifests as a triangular, tapered shape [1, 11, 12] [see Fig. 12.4 and Chap. 5].

Traditionally, all echogenic foci with a comet-tail artifact were thought to be a strong sign of benignity. This is because the artifact was thought to be arising from the inspissated crystals of benign colloid nodules [1, 2, 12]. A recent study however has shown that the rate of malignancy of nodules containing echogenic foci with small (<1 mm) comet-tail artifact was similar to those without acoustic artifact (around 15 %) [2]. This finding suggests that some malignant microcalcifications may also demonstrate small comet-tail artifact. This is supported by the literature that shows several forms of calcifications in a variety of organs and of various sizes demonstrate comet-tail artifact [13]. Why some microcalcifications presumably demonstrate small comet-tail artifacts while others do not is unclear but may in part be related to factors such as ultrasound machine settings (e.g., spatial compounding, frame averaging, speckle reduction), transducer frequency, scanning operator variability, and other technical factors. Again, based on these findings, the additional parenchymal characteristics of the nodule need to be closely evaluated in helping to determine if biopsy of the nodule is warranted.

A recent study demonstrated a very low rate of malignancy (3.9 %) in nodules possessing echogenic foci with large (>1 mm) comet-tail artifact as compared to other echogenic foci. This suggests that these artifacts are most likely secondary to those seen with inspissated colloid crystals in benign colloid nodules . An older study described the comet-tail artifact as a benign feature in avascular cystic nodules likely related to colloid and determined four patterns of the artifact in colloid cysts. In their series, nodules with one of these features showed 100 % sensitivity and specificity for benignity [12]. Thus, a thyroid nodule containing echogenic foci with large comet-tail artifacts in a primarily cystic nodule suggests benignity and may preclude the need for biopsy [2] [see Fig. 12.5].

Echogenic foci associated with posterior acoustic shadowing are true thyroid calcifications . In order to cause the shadowing, the calcification is typically larger than 1 mm in size and often much larger and can be quite variable in shape [see Fig. 12.6]. Pathologically they represent fragmented plates, spicules, or granular deposits of calcium within fibrous septa in the thyroid gland parenchyma and/or nodules. In the literature, these foci are commonly referred to as “macro-” or “coarse” calcifications and are thought to be dystrophic calcifications secondary to tissue necrosis. The ACR committee on thyroid ultrasound in their recent publication of a Thyroid Imaging Reporting and Data System (TIRADS) chose the term macrocalcification for this type of echogenic focus [14, 15]. They can be seen in multinodular goiter but also in solitary thyroid nodules. Approximately 15 % of nodules with these calcifications harbor malignancy. Of note, these are the most common type of echogenic foci in medullary type of thyroid cancer [2, 3, 8, 10].

These echogenic foci also are true calcifications and are curvilinear in shape, occurring along the periphery of nodules and are associated with posterior shadowing [see Fig. 12.7a, b]. In the literature, these peripheral calcifications are frequently referred to as “rim” calcifications or “eggshell” calcifications when smooth and regular and extend around the entire nodule. Frequently, the extent of posterior shadowing precludes further evaluation of the internal parenchymal features of the nodule. In fact, in some instances, attempting a biopsy of these nodules is nearly impossible if the periphery is completely circumscribed by calcification. As seen with large internal macrocalcifications, the presence of these foci is associated with an increased risk of malignancy, as compared to nodules without echogenic foci [2]. Nodules that have peripheral calcifications that are irregular in thickness or an interrupted peripheral calcification with soft tissue extending through the rim are highly likely to be malignant [16] [Figs. 12.8 and 12.9].