Dynamic Contrast-Enhanced MRI to Differentiate Parotid Neoplasms Using Golden-Angle Radial Sparse Parallel Imaging

Discussion

Our results highlight the technical advantages of GRASP, a sequence that concurrently obtains high spatial resolution and fat-suppressed anatomic imaging of the entire neck as well as permeability information for noninvasive functional characterization of parotid neoplasia. We were able to document the independent processing and interpretation of DCE-MR imaging data in parotid neoplasms by multiple readers, which is a helpful step in attempting to generalize the application of DCE-MR imaging beyond the research setting. To our knowledge, only 1 prior study has evaluated interreader reliability (with 2 readers) for DCE-MR imaging analysis in parotid neoplasms.¹

The results of our semiquantitative analysis are consistent with previous studies, specifically the findings that pleomorphic adenomas are associated with a relatively long Tmax, while malignant lesions are associated with a shorter Tmax.^1,2,6,8 Multiple Tmax threshold values are reported in the literature for the prediction of pleomorphic adenoma or malignancy, possibly due to differences in the DCE technique, which limits direct comparison of values across studies. Similar to prior analyses, we found that relying on this 1 parameter for the prediction of specific histologic diagnoses was limited, primarily due to the overlapping TIC features of Warthin tumors and oncocytomas with those of malignant lesions. The most useful way to apply permeability data in clinical practice appears to relate to its high negative predictive value for malignancy. Namely, parotid neoplasms with a long Tmax are unlikely to represent malignant neoplasms. Moreover, our multivariate analysis showed that AUC, washout slope, and SImax were also unique predictors of parotid malignancy. Finally, all of our semiquantitative metrics were normalized to an anatomic reference (common carotid artery). Most prior studies did not normalize their data in this fashion. Reference tissue normalization is important because it provides a method to correct for the numerous sources of variability across different patients and scanners.^{1,2,6,8,19,20,22⇓–24}

Most interesting, our subset analysis incorporating ADC values showed that a multivariate model combining ADC values with Tmax showed a significantly improved performance for the prediction of pleomorphic adenoma compared with either parameter alone. With respect to malignancy characterization, previous studies have shown that the combined information of neovascularization (shorter Tmax) and higher cellularity (lower ADC) may help to identify parotid malignancies as distinct from benign neoplasms.^20,21 While our results did show that ADC combined with Tmax outperformed Tmax alone when predicting malignancy, our analysis here was limited because we were not able to combine ADC with the full logistic regression model that included Tmax, AUC, washout slope, and SImax.

While several of our results support the results of prior literature, the GRASP technique has several key features that represent an improvement over techniques used in prior studies of DCE-MR imaging in parotid neoplasia. Uniquely, GRASP simultaneously provides high-spatial-resolution anatomic T1-weighted images of large anatomic coverage (eg, entire neck) with dynamic high-temporal-resolution permeability imaging of the entire imaging volume. GRASP uses a continuous radial stack-of-stars acquisition of k-space, and an arbitrary number of consecutive radial spokes can be combined into a single image frame to create DCE imaging of a desired temporal resolution. Additionally, by combining all spokes into a single image frame, one can create high-resolution fat-suppressed anatomic images of the entire neck with an excellent signal-to-noise ratio. Thus, GRASP can be routinely integrated into a parotid gland imaging protocol as a diagnostic anatomic sequence without using examination time solely to obtain permeability information. Although some authors have performed DCE-MR imaging at a similar temporal resolution, the techniques used in those studies were often limited to either a single slice or a smaller portion of the volume of interest at a lower spatial resolution.^6,22,23 Because GRASP acquires high-temporal-resolution permeability imaging for entire neck coverage, it has an added benefit of ensuring that a useful reference standard (such as the common carotid artery) is included in the imaging volume for normalizing imaging metrics. It also removes the possibility of inadvertently excluding portions of parotid neoplasms when selecting the imaging subvolume for DCE acquisition. Finally, a distinct advantage to radial k-space sampling is its substantially reduced sensitivity to motion artifacts. In fact, none of the cases in our study required exclusion due to motion artifacts.^10⇓–12

One limitation of our study is our cohort size (n = 41), and we specifically note here the important 2016 study by Yuan et al,²⁵ which included DCE imaging in a large cohort of parotid neoplasms. However, despite our smaller patient cohort, we believe our study remains of interest, predominantly due to the application of the GRASP imaging technique as discussed above. Specifically, our GRASP technique enabled entire neck coverage with 2-mm slice thickness (compared with 5 mm in the Yuan study) despite a similar reported temporal resolution. Moreover, Yuan et al did not normalize their DCE metrics using an anatomic reference as we did. Aside from such technical differences, we performed a comprehensive assessment of interreader reliability for independently processing and categorizing the parotid lesion TIC using DCE-MR imaging, whereas Yuan et al used a consensus reading.

Additional limitations of our study include its retrospective design. We undertook qualitative and semiquantitative methods of analysis rather than “quantitative” analysis using baseline T1-mapping that allows determination of parameters such as the volume transfer constant between the extravascular extracellular space and plasma or extravascular extracellular space volume per tissue volume.²⁶ However, such quantitative techniques are themselves limited by the application of various physiologic assumptions across different patients. We included MR imaging examinations from both 1.5T and 3T scanners and acknowledge that different scanners might yield differing DCE data. Finally, the GRASP sequence is currently limited to a single vendor and thus not yet available to all MR imaging sites for routine clinical use.