The planar scintigraphy technique is the simplest and easiest protocol for the detection of parathyroid disease. Planar images may be obtained using a standard low energy parallel hole collimator, or preferably via pinhole collimation which provides magnification of small structures [30]. When optimized with pinhole imaging, the dual-phase planar technique may be up to 82–91 % sensitive for the detection of adenoma and hyperplasia [23]. When compared to tomographic (SPECT) imaging, the planar protocol is logistically more robust as it does not require patients to remain still for long periods without a break, and it can be performed with even the most basic Anger camera. In addition, planar images can be obtained in patients for whom SPECT may be challenging, such as very obese patients and those with severe claustrophobia. For these reasons, it is unlikely that planar imaging will disappear from the standard parathyroid scintigraphy protocol.

However, data obtained over the past decade suggests that dual-phase planar imaging by itself provides inadequate sensitivity for the detection of parathyroid adenomas, even in single-gland disease (see discussion below). Even the maximal sensitivity of planar imaging is compromised in patients with ectopic disease. Thus, imaging with SPECT has become a mainstay of evaluation.

The availability of multi-head Anger cameras has allowed for the proliferation of SPECT in scintigraphic imaging protocols. By allowing for a tomographic three-dimensional reconstruction of tracer distribution in the head and neck regions, SPECT overcomes the interpretation difficulties related to the superimposition of tracer activity on planar images (see Fig. 12.1b). Thomas et al. [47] demonstrated a sensitivity of dual-phase SPECT of 67 % as compared to 42 % for dual-phase planar imaging, somewhat similar to sensitivities of 62 % and 57 %, respectively, as reported by Lavely and colleagues [31]. Additionally, the use of SPECT increases reader confidence when compared to planar imaging [44].

In that it does not add to patient radiation dose, the primary disadvantage of SPECT as compared to planar scintigraphy is the prolongation of the imaging protocol. In addition, though few, there remain centers with cameras that are not capable of routine tomographic imaging, and in these locations SPECT is simply out of reach.

Finally, SPECT-CT is considered by some to be an expensive add-on to SPECT—while the value of tomographic imaging is recognized [10, 31, 39], the additional cost and radiation exposure imposed by hybrid CT or post-acquisition fusion of diagnostic CT images to the functional SPECT information are not uniformly appreciated [4, 22, 36]. Practically, the addition of anatomic information (CT) to functional information (SPECT) can be achieved by using either a hybrid SPECT-CT capable of the consecutive acquisition of SPECT and multi-slice CT in one unit or the software fusion of separately acquired diagnostic CT and SPECT images. In the former, the CT obtained is often done in the absence of intravenous contrast, and kVp and mAs are minimized. Though this serves to limit radiation dose to the patient, the acquisition of diagnostic-quality CT images, even in the absence of intravenous contrast, increases the utility of the combination image for the purposes of surgical planning and intraoperative correlation. In most cases, the current literature examines the diagnostic capabilities of SPECT-CT obtained utilizing a hybrid SPECT-CT camera, and it is generally assumed that post-acquisition fusion SPECT + CT images are similarly valuable though perhaps technically challenging due to the need for high-accuracy software fusion of SPECT and CT images. In many cases, therefore, hybrid SPECT-CT images are obtained and are correlated with diagnostic CT images preoperatively.

There seems to be little doubt that SPECT-CT is superior to planar-only imaging in the diagnosis of parathyroid adenomas [31, 48]. The exact nature of the benefits provided by SPECT-CT over SPECT remains a source of some debate, though it seems clear that SPECT-CT offers improved accuracy in ectopic glands . A recent meta-analysis of 24 articles on scintigraphy for hyperparathyroidism suggests that SPECT-CT has overall better sensitivity and specificity when compared to planar or SPECT-only protocols [4]. In their study comparing single-photon imaging protocols, Lavely et al. [31] identified early SPECT-CT imaging in concert with any (three-view planar, SPECT, or SPECT-CT) delayed imaging as the most sensitive protocol for the accurate identification of parathyroid adenomas. Since then, numerous studies have sought to identify the incremental value of the addition of CT to SPECT, with somewhat mixed results. In their subset of patients with hyperparathyroidism, Bural et al. [14] suggested that SPECT-CT allowed for more accurate parathyroid adenoma localization on a per-patient basis than did SPECT (sensitivities of 97 % for SPECT-CT and 61 % for SPECT-only). Furthermore, the addition of CT to SPECT has been shown to increase reader confidence, thereby decreasing the number of false-positive SPECT findings (see Fig. 12.2). In a retrospective analysis of SPECT-CT images from 50 patients undergoing preoperative parathyroid localization, Mandal et al. [33] suggest that early-only MIBI SPECT-CT is sufficiently sensitive and specific, and that delayed-phase images may be unnecessary. In large part, the gains seen with SPECT-CT over SPECT relate partially to improvements in the localization of ectopic versus eutopic glands and partially to increased reader confidence [20, 42] (see Fig. 12.3). A recent study [2] has suggested that the method of imaging reconstruction utilized in SPECT-CT versus that performed with SPECT-only scanning may also contribute to the superior performance of the former; further study in this area may be of benefit, as it may have important clinical implications for centers with the capability to perform SPECT but not SPECT-CT.

It is worth noting at the outset that dual-tracer protocols impart a higher radiation dose to patients than do protocols that utilize MIBI only. Members of the medical imaging community consider this incremental increase in radiation dose to have a high risk-benefit profile, and therefore, it is not typically considered to be prohibitive. For further discussions of radiation doses related to scintigraphic imaging, please see Chap. 41.

In an unselected representative patient population, the addition of planar thyroid imaging with subtraction to a dual-phase MIBI imaging protocol has been shown to increase specificity for parathyroid adenoma detection to over 94 % [12]. In a small study comparing dual-tracer techniques to four different single-tracer (MIBI) protocols, Tunninen et al. [49] concluded that the dual-tracer technique utilizing I-123 sodium iodide was superior to MIBI protocols employing either pinhole planar or SPECT-CT methodologies. Interestingly, recent work by Heiba and colleagues [27] compared dual-tracer dual-phase pinhole planar imaging to various protocols (including single-tracer MIBI imaging) with and without SPECT-CT. Their findings suggest that dual-tracer pinhole planar imaging is equivalent to dual-phase single-tracer pinhole imaging, but that the addition of SPECT-CT to dual-tracer pinhole planar protocol yields the highest performance with an accuracy of over 95 %. Neumann and colleagues [36] performed thyroid subtraction SPECT, and evaluated its performance with and without concomitant CT, concluding that the addition of CT to the SPECT protocol significantly improved specificity on a per-lesion basis (96 % for SPECT-CT as compared to 48 % for SPECT-only), with both having sensitivities of about 70 %. These studies suggest that, while dual-tracer methodologies are valuable particularly in patients with thyroid disease, they cannot supplant the benefit of CT correlation with SPECT. Further evaluation of these options may be valuable, as even low-dose CT adds, on average, 1–2 mSv to the approximately 8 mSv effective radiation dose of a planar + SPECT sestamibi protocol [as a comparison, a dose of pertechnetate for thyroid imaging adds up to 2 mSv to the dose of the same protocol, and a typical dose of I-123 NaI would add ~4 mSv to a sestamibi parathyroid imaging study] [43, 57].

The literature clearly documents that the accuracy of scintigraphic imaging suffers in the setting of multi-glandular disease. Katz et al. [29] demonstrated that study sensitivity decreased to 23 % in patients with pathologically proven multi-gland disease as compared to a sensitivity of 70 % in solitary adenomas; in their study, none of the bilaterally diseased glands among 15 patients in their cohort were correctly localized on preoperative planar MIBI (an undisclosed number of patients also underwent SPECT imaging). In an analysis of just over 400 patients, Chiu et al. [16] found that patients with multi-gland disease were more likely to have non-localizing MIBI scans, frequently revealing no focal uptake at all. Similarly, a multicenter study examining primary hyperparathyroid patients with failed surgery revealed that over half of patients with persistent postoperative hyperparathyroidism suffered from multi-gland disease not localized on the combination of ultrasound and MIBI scintigraphy [11]. Notably, using a dual-tracer dual-phase protocol, Guerin et al. [24] showed preoperative parathyroid scintigraphy to be localizing in as many as 61 % of patients with multi-glandular disease.