A 48-year-old with primary hyperparathyroidism is scheduled for a two-phase neck study this afternoon. The protocol calls for a lipophilic cation that diffuses into mitochondria-rich cells and clears from normal thyroid faster than it clears from an adenoma. Which radiopharmaceutical matches that localization mechanism?

Tc-99m sestamibi is the matching radiopharmaceutical: its lipophilic cation crosses cell membranes and accumulates in mitochondria-rich tissue. In a two-phase study, normal thyroid tissue typically clears it more rapidly than many parathyroid adenomas, so persistent delayed activity can help localize an adenoma. Tetrofosmin is a legitimate lipophilic cation used in nuclear imaging and has been investigated for parathyroid localization, but it does not provide the specified, dependable thyroid-to-adenoma washout contrast; its general cationic character alone is not enough to meet the protocol. Fluorocholine PET is also a useful parathyroid localization method, but its signal reflects choline uptake and incorporation into cell membranes, rather than the stated mitochondrial cation mechanism and differential washout. Pertechnetate subtraction is a recognized nuclear medicine technique for separating thyroid activity from suspected parathyroid activity, but pertechnetate maps thyroid trapping; it does not localize adenomas through mitochondrial cation retention. Exam caveat: more than one modality can localize parathyroid adenomas, so distinguish the requested tracer mechanism from other valid localization methods. Practical check: confirm the protocol’s radiopharmaceutical and early-versus-delayed acquisition schedule before interpreting persistent focal activity.