The spectrum of positive scan patterns in parathyroid scintigraphy

Parathyroid scintigraphy provides the clinician treating primary hyperparathyroidism with valuable information regarding the presence and location of parathyroid adenomas. In dual-phase imaging of the parathyroid glands, a widely employed technique that exploits the radiotracer washout characteristics of parathyroid adenomas, images are typically obtained at 20 minutes after administration of the radiotracer (Tc-99m sestamibi or Tc-99m tetrofosmin) and again at 2 hours after injection. Additional imaging of the thyroid is frequently performed to localize thyroid tissue, using Tc-99m pertechnetate or iodine-123.A positive examination can display one of several different patterns; a focus of increased radiotracer activity representing a parathyroid adenoma may be detected on the initial images, on the delayed images, or both. On the thyroid scan, the parathyroid adenoma (if it is discernible) may appear as a cold defect or a persistently hot focus. In our retrospective review of 148 consecutive patients over a 2-year period, 74 examinations were positive and had pathologic confirmation. These examinations were divided into 4 patterns: I (hot focus seen on initial and delayed images, and not on thyroid scan), II (hot focus seen only in initial images), III (hot focus seen only on delayed images), and IV (hot focus seen on initial, delayed and thyroid scan images). Results were as follows: pattern I, 88% (65/74); pattern II, 7% (5/74); pattern III, 3% (2/74); and pattern IV, 3% (2/74). Parathyroid adenomas produce several different patterns on dual-phase scintigraphy. To interpret the examination correctly, it is important for the radiologist to be aware of these patterns of positivity.