Thyroid scans with either ^99mTc-pertechnetate, ¹²³I, or ^99mTc-MIBI are obtained by a gamma-camera equipped with a parallel-hole collimator. Sometimes dedicated “pinhole” collimators are employed to increase focal resolution, in particular to increase the detection rate of little nodular lesions. Nevertheless, a significant geometric distortion should be taken into account. Planar images, acquired in the anterior view for some minutes, provide a reliable map of thyroid function and metabolism (Fig. 2.2). Additional views may be useful when searching for ectopic tissue. Tomographic techniques, such as SPECT and SPECT/CT, may be very useful to better localize in the body the pathological findings, particularly in case of ectopic tissue or intrathoracic goiter. To be noted, as the diagnostic specificity is decreased in lesions that are below the resolution threshold of gamma-cameras, thyroid scans are not indicated in patients with subcentimetric nodules [5]. For oncologic indications a planar whole body imaging in both anterior and posterior views is the technique of choice for the assessment of metastatic spread of the disease. The recent introduction in the current practice of the SPET/CT derived hybrid imaging improved the interpretation of abnormal uptakes, particularly in studies with radioiodine, providing integrated anatomical and metabolic images suitable also to plan the further therapeutic strategies [3]. Finally, the use of positron-emitting tracers, such as ¹⁸F-FDG and ¹²⁴I, requires dedicated PET scanners or, preferably, hybrid PET/CT scanners that provide whole body integrated morphologic-metabolic images [13].

The peculiarity of the parathyroid scintigraphy derives from the fact that a specific tracer able to characterize selectively the parathyroid tissue and to map the function of the parathyroid cells does not exist. Consequently, it is not possible to visualize normal parathyroid glands or use the uptake of tracers to grade/measure the parathyroid function.

In routine parathyroid nuclear medicine imaging, oncotropic radiotracers mapping cell density and cellular metabolism and viability are employed. These tracers are taken up not only by the hyperfunctioning parathyroid glands but also by other tissues. The detection of the hyperfunctioning lesions is, therefore, due to the contrast between the increased metabolism of the parathyroid glands and that of the surrounding normal tissues [14, 15].

^99mTc-MIBI is the most used tracer for parathyroid scintigraphy. The well-known high uptake of the thyroid parenchyma makes necessary, in particular to detect hyperfunctioning parathyroid glands localized within the thyroid parenchyma, a comparison with a second tracer, which is taken up by the thyroid gland only, such as ^99mTc-pertechnetate (^99mTcO4⁻) or ¹²³I. The distributions of the two tracers can be compared and, afterward, the thyroid scan can be digitally subtracted from the parathyroid scan to remove the thyroid activity and enhance the visualization of parathyroid tissue. This technique of images is defined as “dual-tracer scintigraphy” [14, 15].

^99mTc-MIBI usually washes out from normal and possibly abnormal thyroid tissue more rapidly than from abnormal parathyroid tissue. A “dual-phase” imaging with early and delayed images, generally obtained 20 min and 2 h after ^99mTc-MIBI injection, has been proposed to maximize this characteristic and it is used in many centers to increase the detection rate of parathyroid lesions [14, 15].

^99mTc-tetrofosmin can be used alternatively to ^99mTc-MIBI using the dual-tracer subtraction procedure. ^99mTc-tetrofosmin localizes in both parathyroid tissue and functioning thyroid tissue, but in contrast to ^99mTc-MIBI, there is no differential washout between thyroid and parathyroid tissues [16].