Parathyroid Lesions: Characterization with Dual-Phase Arterial and Venous Enhanced CT of the Neck

Discussion

Although some studies showed 4D-CT to be more efficacious than sonography and sestamibi imaging in localizing and lateralizing hyperfunctioning parathyroid tissue, 4D-CT has not been widely adopted by radiologists.^5,9,10 The high radiation dose delivered with multiphase imaging, equivalent to 3–4 neck CT scans, is of concern. There are also variations in the 4D-CT imaging protocols currently performed at different centers, with some institutions including precontrast and single postcontrast phases,¹⁰ other centers using postcontrast arterial and venous phases only,¹¹ and others obtaining precontrast and 3 postcontrast phases of neck imaging.^6⇓⇓–9 In this clinical report, we describe the enhancement characteristics of PTH-secreting lesions on dual-phase arterial and venous imaging, which would differentiate them from other soft tissue structures in the neck.

The rationale behind multiphase imaging is that parathyroid lesions have enhancement characteristics of “rapid uptake and washout.”^7,9,12 Beland et al⁷ were the first to quantify the enhancement by measuring the attenuation values of adenomas. They reported higher mean densities of parathyroid adenomas at 30 and 60 seconds after contrast injection in comparison with cervical lymph nodes. In their study, which used 4 phases of scanning, the noncontrast and delayed phase scans did not discriminate adenomas from normal tissue. Our results show that on dual-phase imaging, there are characteristic enhancement features of parathyroid lesions distinct from other soft tissue structures. All PTH-secreting lesions in our study had early arterial enhancement and rapid venous phase washout of contrast, resulting in a significantly higher relative enhancement washout percentage and higher arterial phase tissue-vascular ratio compared with lymph nodes, thyroid glands, and submandibular glands. The clinical significance of these findings is that the 4D-CT protocol could potentially be reduced to 2 phases, eliminating the noncontrast and delayed postcontrast venous phases of imaging.

Single-phase CT imaging has been performed for parathyroid lesions in the venous phase with a reported sensitivity of 70%.³ Our study shows that using the venous phase alone is the least helpful, because the enhancement characteristics of PTH-secreting lesions overlap with thyroid or submandibular glandular tissue and lymph nodes. Single-phase arterial imaging has been advocated in 1 study as an adjunctive test to sonography.¹ We found that imaging in the arterial phase alone could be helpful, based on the tissue-vascular ratios, but there would be foreseeable circumstances in which high-attenuation thyroid nodules may be difficult to differentiate from PTH-secreting lesions. Kutler et al¹⁰ address this potential pitfall by combining an early postcontrast phase (50 seconds delay) with a noncontrast phase CT. We did not study the role of the noncontrast phase and compare it with our protocol, but an advantage of our protocol, with 2 contrasted phases, is that it provides contrast washout information, which allows potential lesions to be assessed visually or quantitatively in the form of calculated relative enhancement washout percentages. In addition, the change in enhancement on the 2 contrast phases would indicate a hyperenhancing lesion was not a high-attenuation thyroid nodule.

We recognize that our method of relying on attenuation values alone could give rise to false-negative and false-positive results. False-negative results may be seen in parathyroid lesions that do not avidly enhance, such as cystic or fat-containing adenomas. Fortunately, in the literature, these atypical adenomas are generally larger lesions.¹³ False-positive results may arise from pathologic hypervascular lymph nodes or thyroid nodules that may mimic hyperenhancing parathyroid tissue.

There are several limitations to our study. First, this was retrospective with a small study population. The small number of patients in our study underscores the intended use of 4D-CT as a second-line imaging for preoperative localization of PTH secreting-lesions in patients with primary hyperparathyroidism. At our institution, this study is only performed if the sonography and sestamibi scan failed to localize the parathyroid pathology or if the patient has a history of unsuccessful surgical neck exploration. Future validation of this method will require study of more patients. Second, our 5 parathyroid lesions had heterogeneous pathology, including a rare case of PTH-secreting paraganglial hyperplasia. None of the cases were typical parathyroid adenomas on pathology.