Parathyroid Adenomas and Hyperplasia on Four-dimensional CT Scans: Three Patterns of Enhancement Relative to the Thyroid Gland Justify a Three-Phase Protocol.

To describe the prevalence of three relative enhancement patterns of parathyroid lesions on four-dimensional (4D) computed tomographic (CT) scans.The institutional review board approved this HIPAA-compliant study and waived the need for informed consent. The authors retrospectively reviewed preoperative 4D CT scans obtained from November 2012 to June 2014 in 94 patients with pathologically proven parathyroid adenomas or hyperplasia. Lesions were classified into one of three relative enhancement patterns. All patterns required lesions to be lower in attenuation than the thyroid on non-contrast material-enhanced images, but patterns differed in the two contrast-enhanced phases. Type A lesions were higher in attenuation than the thyroid in the arterial phase, type B lesions were not higher in attenuation than the thyroid in the arterial phase but were lower in attenuation than the thyroid in the delayed phase, and type C lesions were neither higher in attenuation than the thyroid in the arterial phase nor lower in attenuation than the thyroid in the delayed phase. The prevalence of the relative enhancement patterns was compared. The t test was used to compare mean attenuation differences in Hounsfield units between the relative enhancement patterns.Ninety-four patients had 110 parathyroid lesions, including 11 patients with multigland disease. The sensitivity for single-gland disease was 94% (78 of 83) and that for multigland disease was 59% (16 of 27). Type B enhancement was most common, with a prevalence of 57% (54 of 94), followed by type C (22% [21 of 94]) and type A (20% [19 of 94]). Five lesions were interpreted incorrectly as parathyroid adenoma (false-positive), and all lesions had the type C pattern. Relative to the thyroid, lesions categorized as type A by readers had mean attenuation difference (± standard deviation) of 39 HU ± 13 in the arterial phase, and type B lesions had a difference of -58 HU ± 26 in the delayed phase. These values differed from the mean attenuation difference of lesions not in these categories (P < .001).Parathyroid adenomas and hyperplasia can be grouped into three relative enhancement patterns based on a protocol with a non-contrast-enhanced and two contrast-enhanced phases. The type B pattern is most common and could be diagnosed with two contrast-enhanced phases. However, almost one quarter of lesions have the type C pattern and thus could be missed without the non-contrast-enhanced phase.