 We are using dual-phase single photon emission tomography SPECT imaging with technetium 99m sestamibi and visual thyroid subtraction with technetium 99m as pertechnetate. This method can achieve sensitivities of 68-95 and specificities of 75-100 1. SPECT imaging is dependent on the differential washout of sestamibi between normal thyroid tissue and abnormal parathyroid tissue. Theoretically, sestamibi will washout of normal thyroid tissue much more quickly than it will from abnormal parathyroid tissue. Therefore, SPECT imaging is usually performed immediately and then at 90 and 180 minutes after sestamibi injection. SPECT imaging, however, has several limitations, not the least of which is its inability to provide discrete anatomic detail. 