Introduction
Primary hyperparathyroidism (PHPT) is a clinical condition characterized by elevated serum parathyroid hormone (PTH), followed by increased serum calcium (Ca). Patients with PHPT frequently manifest symptoms such as hypercalcemia, anorexia, nausea, and nephrocalcinosis. The treatment for PHPT used to be a total parathyroidectomy, but PHPT is usually caused by a single parathyroid adenoma (PTA)(1, 2), and selective parathyroidectomy has been recently preferred for the treatment of single adenomas as a less invasive option(3, 4). For a successful selective parathyroidectomy, accurate imaging techniques are required for the preoperative localization of PTAs. Ultrasonography and technetium 99mTc-sestamibi scintigraphy have often been accepted as first-line imaging to localize PTAs. When first-line imaging fails to localize the parathyroid lesion, or PHPT is recurrent or persistent after surgery, computed tomography (CT) and magnetic resonance imaging (MRI) are considered as second-line imaging techniques (1, 5). Recently, the multiphasic CT (4D CT) has made it possible to identify PTAs through their hypervascular perfusion pattern compared with lymph nodes and thyroid gland(5), however CT has the disadvantage of ionizing radiation.
MRI has the advantage of lack of ionizing radiation, and with the introduction of high field magnetic systems, the use of MRI for the localization of parathyroid adenomas has been re-assessed as a supplemental imaging modality. The sensitivity of 1.5T MRI for the detection of PTAs was reported to be 43%–71%(6-8), but since 3T scanners were introduced into clinical practice in the early 2000s, the higher signal-to-noise and contrast-to-noise ratios have improved the sensitivity of MRI to 97.8%.(1) However, some limitations remain because the MR appearance of PTAs overlaps considerably with those of cervical lymph nodes and an ectopic thyroid gland, which leads to difficulties in distinguishing PTAs from other structures in the neck.
The measurement of the apparent diffusion coefficient (ADC) value in diffusion-weighted imaging (DWI) has been reported to be helpful in distinguishing parathyroid tissues from other soft-tissue structures of the head and neck region.(9) Single-shot echo-planar imaging (SS-EPI) has been the most widely used DWI sequence, but SS-EPI suffers from susceptibility artifacts which manifest as geometric distortion and image blurring,(10) and the detection of PTAs by DWI thus remains challenging. To overcome this weakness, readout-segmented echo-planar imaging (RS-EPI) was developed; with this imaging modality, there is a substantial reduction in the distortion and blurring caused by T2* decay during the readout(11, 12). RS-EPI is expected to improve the diagnostic accuracy of detecting PTAs on DWI as it provides less distortion, and its advantages are reported in some organs including the head and neck(13-15).
However, to our knowledge, there is no literature focusing on the usefulness of RS-EPI for the localization and distinction of PTAs. We conducted the present retrospective study to determine whether RS-EPI can reduce image distortion and improve the lesion identification in PTAs compared to SS-EPI, and to determine whether PTAs can be differentiated from other soft-tissue structures of the head and neck region by using the ADC value.