Tips for improving consistency of thyroid nodule interpretation with ACR TI-RADS

Unlike macro-calcifications, neither of these sonographic observations are associated with posterior acoustic shadowing. Punctate echogenic foci (PEF) are round and ≤1 mm, while echogenic interfaces have a linear or tram like appearance (Fig. 1). Large echogenic foci >1 mm associated with comet-tail artifacts (+0 points) denote colloids, and are benign. In contrast, echogenic foci ≤1 mm with comet-tail artifacts should be treated as PEF (+3 points)⁽⁴⁾. Pertinently, if echogenic foci are found both in the nodule and in the surrounding thyroid parenchyma, these should not be classified as PEF⁽⁴⁾. Benign observations that can mimic PEF include the back wall of tiny cysts and speckle pattern in normal thyroid parenchyma⁽⁴⁾.

Fig. 1

The background thyroid parenchyma and adjacent neck musculature are used as the reference for determining the echogenicity of thyroid nodules. Compared to the thyroid parenchyma, hyperechoic nodules have higher echogenicity, isoechoic nodules have similar echogenicity, while hypoechoic nodules have lower echogenicity (Fig. 2) ⁽³⁾. Conversely, very hypoechoic nodules have lower echogenicity than the neck musculature⁽³⁾. To improve the accuracy and consistency of interpretation, these findings should be evaluated on several tissue planes on the still and cine images.

Fig. 2

The echogenicity of a mixed cystic and solid nodule should be evaluated based on the appearance of its solid component alone (Fig. 3). An anechoic appearance (+0 points) is therefore not an option, as this feature is synonymous with a cystic consistency. It is also important not to misinterpret debris/blood clot/necrotic tissue in a cyst for viable solid material⁽⁴⁾. Doppler US, careful interrogation of the cine clips, and appropriate adjustment of technical parameters (e.g. gain, depth, focal zone, frequency and dynamic range) may help in making this distinction⁽⁴⁾.

Fig. 3

In this scenario, given the absence of information available in the current literature, we recommend going with the finding that represents >50% volume of the nodule, as this finding is representative of the majority of the nodule (Fig. 4). When it is 50–50, the more conservative option with the lower point score should be selected (i.e. hyper/ isoechoic, +1 point).

Fig. 4

This is a judgment call, as this observation represents a continuum and is thus problematic. Tessler et al. in their article (TI-RADS: A User’s Guide) suggested that in general, predominantly solid nodules that contain small cystic components that contain <5% of the overall volume should be considered as solid⁽⁴⁾. However, we contend that a lower threshold of ≥80% solid volume may be a more practical cut-off for scoring a nodule as solid, as the four-fifths rule of thumb is an easy concept to understand and apply for visual assessment (Fig. 5). This also aligns better with other risk classification systems such as ACR O-RADS, where an adnexal lesion is considered solid if ≥80% of its volume is of solid consistency⁽¹²⁾.

Fig. 5

In this scenario, choose solid (+2 points) for composition and iso-or-hyperechoic (+1 point) for echogenicity (Fig. 6)⁽³⁾.

Fig. 6

By definition, a spongiform nodule is one that is composed of >50% of small cystic spaces that are evenly distributed throughout the nodule – this appearance resembles a sponge (Fig. 7)⁽³⁾. Intermediate nodules that do not meet this criterion but show a <80% solid volume should be described as being of mixed cystic and solid composition. When echogenic foci are found in a spongi-form nodule, these are not considered as PEF and should be ignored⁽⁴⁾.

Fig. 7

The ACR TI-RADS committee recommends a more conservative selection of wider-than-tall, or alternatively, not taller-than-wide (+0 points) (Fig. 8)⁽⁴⁾. It is important to note that the shape of the nodule should usually be assessed on the transverse plane of the thyroid gland⁽³⁾. The ACR TI-RADS committee notes that it may rarely be appropriate to assess this characteristic in the sagittal plane if the nodule is obliquely oriented in that plane⁽⁴⁾.

Fig. 8

The nodule size should be measured in three dimensions (a × b × c) (Fig. 9). On a transverse sonogram of the thyroid, obtain the dimension of the nodule along its longest axis (a) followed by its largest dimension on a perpendicular plane to this (b). Next, on a sagittal sonogram of the thyroid, obtain the largest cranio-caudal dimension (c)^{(3, 4)}. Calculating the volume of a nodule (formula = 0.52 × a × b × c) should also be performed, as this can provide a better indication of nodule enlargement compared to assessing measurements in three dimensions separately⁽¹³⁾.

Fig. 9

The margins of a nodule represent its interface/border with the adjacent intra- or extra-thyroidal tissue.

Optimal visualization can be achieved by ensuring that the border of the nodule that is closest to the skin surface is perpendicular to the US beam, and that the appropriate gain and focal zones are used on a high-frequency linear transducer⁽⁴⁾. A smooth margin (+0 points) is where the entire circumference of the nodule’s outline is regular and sharply marginated⁽⁴⁾. If a nodule’s border is not depicted clearly in spite of optimizing technique and US parameters, then it should be categorized as ill-defined (+0 points). If there are protrusions, angulations or lobulations of parts of the nodule into the surrounding tissues, this should classified as lobulated or irregular (+2 points). Extra-thyroidal extension (+3 points) is a feature reserved for cases where there is an obvious and unequivocal invasion into the surrounding tissues such as the neck muscles, trachea, larynx, vasculature or esophagus⁽⁴⁾. Lee et al. found that the combination of two sonographic findings including 1) more than 50% tumor contact with the thyroid capsule and 2) tumor disruption of the thyroid capsule as denoted by loss of the capsule’s outline, had a 23.7% sensitivity, 95.4% specificity, and area under the curve of 0.64 for predicting extra-thyroidal extension⁽¹⁴⁾.