SPECT/CT may provide improvement in sensitivity in comparison to single-tracer planar imaging, especially for posterior adenomas. In our opinion, the use of subtraction method using ¹²³I and ^99mTc-MIBI with pinhole planar acquisition (neck) followed by SPECT/CT (neck and mediastinum) is the most sensitive approach to detect and localise hyperfunctioning glands. SPECT can be performed as single tracer imaging or as a dual tracer technique with simultaneous acquisition (Figs. 3.2, 3.3 and 3.4). The energy windows for acquisition are the same as for planar and pinhole images [13, 40].

SPECT/CT provides the advantage over planar techniques to localise adenomas in the anterior-posterior plane [43, 44], offering critical information for surgeons. SPECT may reclassify apparently inferior adenomas (on planar images) to superior, P4-derived, adenomas prolapsed behind the lower pole of the thyroid gland. These adenomas can be located very deeply in the neck, in paraesophageal or in retroesophageal locations and may be missed by neck ultrasound (Figs. 3.2 and 3.3) [45]. By contrast, inferior glands are mostly located at the tip of the inferior pole of the thyroid lobe, in the thyrothymic tract or in the upper thymic horn and remain anterior on SPECT imaging (Fig. 3.4) [44]. These 3D information may also influence the surgical strategy such as resection via an endoscopic lateral approach for posterior adenomas (most often P4-derived) or a small cervical incision (mini-open approach) cervicotomy for P3-derived adenomas [46]. SPECT also enables a better localisation of ectopic glands (Fig. 3.3). The precise anatomic localisation of mediastinal adenomas provided by SPECT/CT may also influence the surgical strategy (mediastinoscopy /anterior mediastinotomy vs left thoracotomy) (Fig. 3.5).

Using a single-isotope protocol, SPECT/CT acquisitions are usually performed from 10 to 60 min after injection of ^99mTc-MIBI [47–49]. Some teams acquire both early and late SPECT images. Iterative reconstruction methods are well suited to improve image quality. There is some evidence that early-phase SPECT or SPECT/CT imaging performs better than delayed imaging, presumably because of a rapid tracer wash-out in some hyperfunctioning glands [49–51] (Table 3.1). The use of SPECT/CT also provides an advantage over SPECT alone in patients with nodular goitre [53]. To minimise radiation exposure, the CT part of the SPECT/CT acquisition should probably be limited to the neck area in cases of absence of mediastinal foci on planar images [57].

SPECT/CT may increase sensitivity of single-tracer planar imaging [57] but probably have little impact on sensitivity in comparison to highly sensitive subtraction protocols using pinhole collimators [13] (Fig. 3.6). SPECT/CT is particularly helpful as a complement to planar imaging to pinpoint the position of mediastinal foci and other ectopic parathyroid glands [13].