Discussion
We evaluated the clinical utility of DWI for the pre-operative localization of PTAs. While the PTAs were faintly identified with intense distortion on SS-EPI, they were fairly identified with moderate distortion on RS-EPI. The inter-rater reliability was substantial or perfect, and the validity of DW-MRI for the localization of PTAs was demonstrated. In addition, the PTAs could be discriminated from cervical lymph nodes and thyroid glands by measuring the ADC values, and thus RS-EPI is expected to become the optimal diagnostic modality for the localization of PTAs.
With the introduction of 3T MRI, improved diagnostic performance for the detection of PTAs has been reported in relation to the higher spatial resolution and contrast-to-noise ratio.(1) PTAs are usually T2 hyperintense, and fat suppression is effective to achieve an accurate depiction.(1) However, fat suppression is not easy to use in the neck region, especially in the mediastinum, because of the magnetic field inhomogeneities due to the proximity of lungs and upper airways. It thus remains challenging to obtain good images of PTAs with MRI, and there is currently no consensus on the optimal MRI protocol for the evaluation of PTAs.
The diagnostic value of 4D MRI and of the IDEAL sequence for the detection of PTAs has been reported.(19, 20) Although the use of these imaging techniques improved the diagnostic sensitivity for the detection of PTAs, false-positive diagnoses are occasionally attributed to cervical lymph nodes or adjacent/ectopic thyroid tissues. To distinguish PTAs from lymph nodes and thyroid tissues, Yildiz et al. used conventional DWI provided by a 1.5T system, and they reported higher ADC values in parathyroid lesions compared to lymph nodes and thyroid tissues; however, the statistical significance of these differences was not reported.(9)
Although SS-EPI is a well-established conventional method for the acquisition of DWI data with short scan times, it suffers from geometric distortion, signal dropout, and image blurring. Alternatively, RS-EPI is a multi-shot sequence that reduces susceptibility artifact and blurring arising from T2* decay, and thus lower distortion can be achieved.(21) The clinical utility of RS-EPI has been well documented in some regions, including the head and neck.(13) In our present case series, the acquisition parameters were quite different between RS-EPI and SS-EPI, because they were independently set to optimize their image quality within clinically acceptable acquisition times (2–3 min). Since a sufficient signal-to-noise ratio (SNR) was not expected for SS-EPI, the number of excitations were increased to four. A sufficient SNR was expected for RS-EPI, so we increased the number of readout segments to five, to reduce the distortion. The results demonstrated that the PTAs were well identified with less distortion by RS-EPI compared to SS-EPI, and the conclusions of the two independent readers showed a good correlation.
RS-EPI is thought to be a useful and reliable imaging technique for the evaluation of PTAs. However, we observed herein that even with RS-EPI, some PTAs were poorly identified with intense distortion, which might mask the improvement in the lesion identification score. The distortion might come from (1) the relatively large field of view (FOV) needed to obtain whole neck images, (2) susceptibility artifact that remained even with RS-EPI, and/or (3) motion artifact from respiration. In addition, it is not still easy to obtain good-quality DW images at the upper mediastinum due to the complex stricture facing the air, and the establishment of a feasible imaging protocol of the cervical region to the upper mediastinum using DW-MRI is expected.
In this study, we used ADC values for discriminating the PTAs and other cervical organs, and the PTAs were well distinguished from the thyroid glands and cervical lymph nodes. Cervical lymph nodes exhibit low ADC values,(22) and benign neck pathologies including adenomas are reported to show relative high ADC values,(23, 24) which is consistent with our present observations. Higher ADC values were observed on RS-EPI compared to SS-EPI in our case series. Differences in ADC values between RS-EPI and SS-EPI are still controversial. Although higher ADC values on RS-EPI have been reported in the human mammary gland and in a phantom study,(21, 25) Koyasu et al. reported no significant differences in the ADCs of salivary gland lesions.(13) In our present cases, the accurate evaluation of lesions due to less distortion on 3T RS-EPI may have contributed to the precise selection of the area, while more distorted lesions on 1.5T SS-EPI might have been contaminated by surrounding fat tissue and consequently resulted in a decreased ADC, which we observed particularly in the thyroid glands. Adding DWI using RS-EPI to common sequences such as STIR and T2WI could thus be beneficial for the pre-operative evaluation of PTAs.
This is the first study to investigate the clinical utility of RS-EPI in the pre-operative localization of PTAs, to our knowledge. One of the limitations of our study was its retrospective design. In addition, the acquisition protocols for SS-EPI and RS-EPI were independently set to maximize their own features (rather than a comparison of the two protocols), which resulted in different acquisition schemes. The sample size was small, and there would be a selection bias because selected patients with surgically confirmed pathologies were included. Further prospective studies with larger sample sizes are warranted to clarify the diagnostic utility, indications, and limitations of RS-EPI for the localization of PTAs.