appendicular skeleton but not the spine. This modality has a lower radiation dose than a traditional DEXA scan and can achieve spatial resolutions in the order of 130 to 160μm (higher near the center of the eld of view) [117–119]. Various morpho­metric measurements can be generated, including cortical thickness, trabecular num­ber, trabecular separation, and cortical porosity [120]. These have been validated in several studies and highly correlated with metrics derived from μCT, a technique that can achieve higher spatial resolution but generally applied to exvivo tissues [121–

Due to their small size, normal parathyroid glands cannot usually be seen with current ultrasound equipment [14]. However, parathyroid adenomas may appear as a homogenous, hypoechoic, and hypervascular mass separate from and posterior to the thyroid on ultrasound [129]. Most parathyroid adenomas are located outside the posterior capsule of the thyroid gland [16, 17, 130]. The lesion lacks the echogenic fatty hilum that is typically seen in lymph nodes. A lesion along the posterior aspect of the thyroid gland in the context of hypercalcemia is likely a parathyroid adenoma. It is common to see the “the indentation sign”– an indentation made by the parathy­roid adenoma on the posterior side of the thyroid gland [14]. There is usually an echogenic line of separation between the thyroid and the parathyroid gland, repre­senting the bro-fatty capsule. When seen on ultrasound, the parathyroid glands typically have variable shapes, allowing them to conform to the available space. In about 2–5% of cases, the parathyroid adenoma may be embedded in the thyroid gland, where it has the same appearance as a thyroid nodule [131]. The incidence of a parathyroid gland embedded in the thyroid may be as high as 15% in the setting of four gland hyperplasia [132]. The presence of a polar artery feeding a parathyroid adenoma can aid in detection and has been found in more than 80% of adenomas [133]. A “vascular arc” leading to the adenoma and diffuse blood ow within the adenoma have also been described [134].

In addition to presurgical imaging, the patient may be injected 2–4h before sur­gery, and a small gamma probe is used intraoperatively to nd the adenoma by the surgeon. This procedure is usually used if there is a solitary parathyroid adenoma and no thyroid adenoma; after removal, the parathyroid adenoma should be 20% higher than background thyroid activity [2].

Magnetic resonance imaging (MRI) does not play a major role in the workup of parathyroid adenomas in most clinical situations. The main advantage of MRI over CT and ultrasound is its higher soft tissue contrast, which generally allows better discrimination between tissues. Additionally, MRI does not require ionizing radia­tion. The major drawbacks are longer acquisition times and lower spatial resolution relative to CT and ultrasound. Parathyroid adenomas are typically hypointense on T1- and hyperintense on T2-weighted sequences [147, 148]. The addition of fat sup- pression techniques is crucial for both T2- and post-contrast T1-weighted sequences in order to discriminate the lesion from surrounding tissues because fat is

intrinsically hyperintense. Similar to CT, parathyroid adenomas show avid enhance­ment on MRI following the intravenous administration of contrast material. In one study of 98 patients with surgical and pathological correlation, MRI had a sensitiv­ity of 82% and a positive predictive value of 89% for detection parathyroid adenoma compared with a sensitivity of 85% and positive predictive value of 89% for techne­tium 99m sestamibi scintigraphy [147]. One retrospective study using a dynamic contrast MRI technique in 30 patients with surgically proven parathyroid adenomas found a sensitivity/specicity of 88/90% and 91/95% for differentiating adenomas from lymph nodes and thyroid tissue, respectively [145].