4D CT consists of a CT of the neck and upper chest performed during multiple time points before and after contrast injection. Typically, the scans are acquired with non-contrast, arterial and venous phases. Some centres also use a delayed phase. Adenomas will typically demonstrate avid contrast enhancement on the arterial phase and show washout on the more delayed phases. The non-contrast scans are important to distinguish an adenoma from adjacent thyroid tissues which will be hyperdense on unenhanced images. Figure 1 demonstrates the typical enhancement pattern of a parathyroid adenoma. Occasionally, an enlarged vessel can be traced into the superior or inferior aspect of the adenoma. This is known as the polar vessel sign (Figure 2). This is best demonstrated on multiplanar thin slice images.
When performing a 4D CT, the field of view needs to be from angle of the mandible to the carina, in order to cover potential locations of an ectopic adenoma. It is important that the arms are by the side, with the shoulders as low as possible to reduce beam hardening artefact in the region of interest. This can affect the measurement of contrast enhancement and potentially obscure an adenoma, as in Figure 3.
When reading a 4DCT it is important to be aware of the potential location of a parathyroid adenoma, as well as the enhancement characteristics of an adenoma. Adenomas can be positioned in the normal location of the parathyroid gland (eutopic) or in a location based on the glands embryological pathway (ectopic).
There are two pairs of parathyroid glands, the superior and inferior parathyroid gland on either side of the neck. The superior glands arise from the fourth branchial pouch together with the thyroid gland C cells. The inferior glands arise from the third branchial pouch together with the thymus.
The superior parathyroid glands can usually be located dorsal to the superior pole and midpole of the thyroid gland. Less common locations of the superior parathyroid glands include above the upper pole of the thyroid gland (3%), retropharyngeal (1%), retroesophageal (1%), carotid sheath (rare) and posterior mediastinum (rare). Figure 4 demonstrates a retropharyngeal adenoma in a patient who underwent a 4D CT following a failed surgical exploration. This patient also had a second adenoma in the carotid sheath (Figure 5).
The inferior parathyroid glands are more variable in their location. Two-thirds are located adjacent to the lower pole of the thyroid gland. Less common locations of the inferior parathyroid gland include along the thyrothymic ligament and in the superior mediastinum (26%), anterior mediastinum (2%), and above the thyroid gland (rare). Figure 6 demonstrates an adenoma within the anterior mediastinum that was “missed” on the 4D CT due to an inadequate field of view.
It is also important to be aware of potential mimics of an adenoma. These include lymph nodes and exophytic thyroid nodules (Figure 8). Occasionally, lymph nodes may be seen posterior to the thyroid or in level VI. However, these are usually easy to distinguish from a parathyroid adenoma as they will not sure the avid enhancement seen in the arterial phase and retain contrast in the venous phase. Figure 7 demonstrates the typical appearance of a node on multiphase imaging. Likewise, thyroid nodules will typically be hyperdense on the non-contrast scans, helping to differentiate between an exophytic nodule and an adenoma. Occasionally, thyroid nodules may appear completely separate from the gland on imaging (Figure 8).