3-phase dual-energy CT scan as a feasible salvage imaging modality for the identification of non-localizing parathyroid adenomas: a prospective study

Abstract

Objectives: Accurate pre-operative imaging of parathyroid adenomas (PAs) is essential for successful minimally invasive surgery; however, rates of non-localizing PAs can be as high as 18 %. Multiphasic dual-energy CT (DECT) has the potential to increase accuracy of PA detection by enabling creation of paired material maps and spectral tissue characterization. This study prospectively evaluated the utility of 3-phase DECT for PA identification in patients with failed localizatio n via standard imaging.

Methods: Patients with primary hyperparathyroidism and non-localizing PAs underwent a 3 phase post-contrast DECT scan acquired at 25, 55, and 85 s. The scans were prospectively evaluated by two head and neck radiologists. Pre-operative localization was compared to intraoperative localization and final histopathology. A post-hoc DECT spectral density characterization was performed on pathologically-proven PAs.

Results: Out of 29 patients with primary hyperparathyroidism and non-localized PAs, DECT identified candidates in 26. Of the 23 patients who underwent parathyroidectomy, DECT provided precise anatomic localization in 20 patients (PPV = 87.0%), one with multi-gland disease. The virtual unenhanced images were not found to be useful for diagnosis but successful diagnosis was made without an unenhanced phase regardless. Spectral analysis demonstrated a distinct spectral Hounsfield attenuation curve for PAs compared to lymph nodes on arterial phase images.

Conclusion: 3-phase DECT without an unenhanced phase is a feasible salvage imaging modality for previously non-localizing parathyroid adenomas. Optimal interpretation is achieved based on a combination of perfusion characteristics and other morphologic features. Advanced spectral DECT analysis has the potential for further increasing accuracy of PA identification in the future.

Fig. 1

Typical perfusion characteristics in an intrathyroidal parathyroid adenoma. 70 keV VMIs at (a, b) 25 s and (c, d) 55 s are shown of a surgically and pathologically proven intrathyroidal parathyroid adenoma (large arrow). The feeding vessel supplying the adenoma is also seen (small arrow) and is helpful for diagnosis, sometimes referred to as the polar artery. There is the typical rapid and robust enhancement on arterial phase images (a, b). On the more delayed images, there is contrast washout from the adenoma but increased attenuation of the thyroid gland and the adenoma cannot be clearly distinguished from the thyroid gland (c, d)

Fig. 2

Atypical perfusion characteristics in bilateral parathyroid adenomas. (a) 25 s 70 keV VMI, (b) 55 s 70 keV VMI, (c) 25 s 50 keV VMI, and (d) 25 s iodine overlay map are shown demonstrating surgically and pathologically proven bilateral parathyroid adenomas (arrows). In this case, a typical robust arterial phase enhancement with rapid washout is not shown (adenoma attenuation on the 25 s images was less than 100 HU). However, the presence of a fat plane separating the adenomas from the thyroid gland, location, and different appearance from normal lymph nodes enabled a confident pre-operative diagnosis in this case. The 50 keV VMI (c) is shown as an example of how DECT low energy reconstructions can accentuate the density of enhancing/iodine containing structures (compare c to a). DECT also enables creation of iodine overlay maps (d), highlighting the iodine content of tissues and enabling a quantitative estimation of tissue iodine content

Fig. 3

Parathyroid adenoma with cystic internal change. Example of surgically and pathologically proven parathyroid adenoma (arrow) extending to the right tracheo-esophageal groove

Fig. 4

Virtual unenhanced DECT images. a 70 keV VMI and (b) virtual unenhanced image of the intrathyroidal parathyroid adenoma in Fig. 1 are shown. The parathyroid adenoma seen on the 70 keV VMI (arrow) cannot be seen on the virtual unenhanced image (b) because of suppression of iodinated contrast on that image. The iodinated contrast in the vessels as well as iodine within the thyroid gland are also suppressed

Fig. 5

Spectral Hounsfield unit (HU) curve analysis of parathyroid adenomas compared to lymph nodes. Spectral HU analysis of 13 normal appearing lymph nodes (LN) and 14 surgical and pathologically proven adenomas (PA) from 13 patients are shown from the (a) 25 s and (b) 55 s DECT acquisitions. PA have different spectral HU characteristics compared to LN on the 25 s but not the 55 s acquisition with density separation on the left (lower energy) side of the curve. **P < 0.01, ***P < 0.001, ****P < 0.0001