"Taller-than-wide sign" of thyroid malignancy: comparison between ultrasound and CT.

OBJECTIVE The objective of our study was to investigate the mechanism of the "taller-than-wide sign"--that is, an anteroposterior dimension-to-transverse dimension ratio of >or= 1 on ultrasound. MATERIALS AND METHODS Ultrasound and CT images of 90 pathologically proven thyroid masses (57 malignant and 33 benign) smaller than 2 cm in 77 patients (mean age, 45 years) were retrospectively reviewed. Two readers assessed the anteroposterior and transverse dimensions of the mass, anteroposterior-transverse ratio of the mass, anteroposterior dimension of the ipsilateral thyroid lobe, and the position of the common carotid artery (CCA) relative to the thyroid lobe. In addition, the difference in the anteroposterior-transverse ratio of the mass between ultrasound and CT was correlated with the ultrasound characteristics of the thyroid mass (i.e., maximal diameter, location, location within lobe, and composition), histopathologic results, and ultrasound operator. RESULTS The mean (+/- SD) anteroposterior-transverse ratio of the thyroid masses on ultrasound was significantly lower than that on CT (0.97 +/- 0.34 vs 1.07 +/- 0.28, respectively; p < 0.001), and the differences were significantly greater in benign masses than malignant masses, in masses located at the anterior or mid third of the lobe than those located at the posterior third, and in cystic masses than mixed or solid masses. There were statistically significant differences between the two techniques with regard to the anteroposterior dimension of the ipsilateral thyroid lobe and the position of the CCA, suggesting the effect of probe compression. CONCLUSION The mechanism of the taller-than-wide sign is no or minimal compressibility of a thyroid mass by the ultrasound probe, which occurs more frequently in malignant masses than in benign masses.