Dual-phase 99mTc-sestamibi imaging: its utility in parathyroid hyperplasia and use of immediate/delayed image ratios to improve diagnosis of hyperparathyroidism.

OBJECTIVE: Dual-phase (99m)Tc-sestamibi (methoxyisobutylisonitrile [MIBI]) imaging is the technique of choice for hyperparathyroidism (HPT), especially for localizing parathyroid adenomas. Prior studies have shown its utility for detecting hyperplasia is equivocal, but we believe this is not true. We attempted to quantitate the region-of-interest counts per pixel between immediate images and delayed images (I/D ratio) and use this ratio to distinguish normal parathyroid versus hyperplasia versus adenoma.

METHOD: Anterior pinhole and upper thorax images with a low-energy, high-resolution collimator at 20 min and 2 h after (99m)Tc-MIBI injection were obtained on 54 subjects. The results were analyzed retrospectively as hyperplasia, adenoma, or normal parathyroid by the persistence of activity in 2 or more foci, a solitary focus, or no activity on the delayed images. These interpretations were compared with pathology when available. I/D ratios were computed for all scans, and mean ratios were calculated for each type of pathology (normal parathyroid, hyperplasia, and adenoma). The resulting ratios were analyzed with a t test to determine significant differences between the ratios.

RESULTS: Sensitivity and specificity were 96% and 88%, respectively, for parathyroid hyperplasia. Mean I/D ratios were 2.26 +/- 0.68, 2.80 +/- 0.95, and 3.10 +/- 0.77 for subjects with hyperplasia, adenoma, and normal parathyroid, respectively (hyperplasia vs. normal, P = 0.020; adenoma vs. normal, P = 0.381; hyperplasia vs. adenoma, P = 0.033).

CONCLUSION: Dual-phase (99m)Tc-MIBI imaging is more sensitive and specific for parathyroid hyperplasia than reported previously, supporting its use to localize hyperplastic glands preoperatively and to help guide resection. A thyroid ratio between immediate and delayed images will aid in distinguishing hyperplasia from normal parathyroid in uncertain cases.