DWI and ADC value have been proven to be a useful imaging marker for tumor diagnosis, differentiation of histologic grade, prediction of disease survival and therapeutic monitoring in various tumors [19]. Clinical DWI is usually scanned using SS-EPI technique because of the fast scan speed, however it is prone to geometric distortion, image blurring, and image artifacts, which is more severe in head and neck region [3]. As a solution, RESOLVE technique showed remarkable advantages over SS-EPI reflected in reduced distortion and artifact [3]. However the improvement in image quality is achieved at the cost of longer acquisition time, which is a potential drawback limiting the wide application of RESOLVE DWI in clinical practice. In our study, we found that SMS-RESOLVE DWI could allow a substantial reduction of scan time while maintaining image quality with no significant difference, thereby improving the clinical applicability of SMS-RESOLVE DWI in assessing parotid gland tumors.
Two problems are usually taken into account in DWI of head and neck region. The first one is artifact, and another one is the display capability of lesions. Previous studies have demonstrated the absolute advantage of RESOLVE DWI on these two aspects, compared with SS-EPI [4, 6] .When SMS technique is combined with RESOLVE DWI, whether the reduced acquisition time would hamper such advantage is not clarified. In our study, no significant differences on the subjective scores of artifact and lesion displayed, and the objective measurements of SNR ratio and CNR ratio were observed between SMS-RESOLVE DWI and RESOLVE DWI, which was similar with the study of Filli et al [10]. Our study result indicated that SMS technique reduced the scan time without a compromise on image quality and lesion display capability, which can increase the clinical usability of RESOLVE DWI for assessing parotid gland tumors.
One more thing we must concern is that whether the advanced DWI technique would influence the ADC value. Previously, several studies compared the ADC value derived from SS-EPI and RESOLVE DWI, and paradoxical results were obtained. Zhao et al found that the ADC value of the sinonasal lesions on RESOVE DWI was lower than that on SS-EPI, while Bogner et al indicated that there was no significant difference on the ADC obtained from two DWI sequences [18, 20]. In this study, we compared the ADC derived from SMS-RESOLVE DWI and RESOLVE DWI respectively, and no significant difference was found on the ADC of both masseter and tumor, which was consistent with the findings of Filli et al [10]. Our study results indicated that SMS technique would not affect the measurement of ADC values. The derived diagnostic threshold value achieved from RESOLVE DWI based studies could be directly applied in SMS-RESOLVE DWI related study.
The scan time is 5 minutes and 46 seconds for RESOLVE DWI of parotid gland in our study, which seems too long in clinic. During our study design, we tried a highly-optimized imaging parameter. Thirty slices with a slice thickness of 3 mm were used in our study, while the slice thickness usually ranged from 4 to 6 mm in previous studies [1, 2, 21]. The voxel size was 1.1 × 1.1 × 3.0 mm3, and the average number of each b value was set as 2. In our opinion, DWI based on so highly-optimized parameters can provide more anatomical information on diffusion map, which is very crucial for clinical evaluation of tumor and its adjacent structures. SMS technique can reduce the scan time by nearly 2 minutes, increasing the applicability of so highly-optimized parameters in clinical practice.
There were several limitations should be noted. First, we did not compare our sequences with conventional SS-EPI based DWI, because the advantage of RESOLVE had been well demonstrated in previous studies [7, 8]. Second, we calculated the SNR ratio, CNR ratio and ADC values using multiple small and round ROIs those were manually placed. This method was prone to sampling bias. Third, most parotid gland tumors in our study were benign ones with clearly demarcated margin. Further large-scale studies enrolling more tumors with infiltrative margin and various histo-pathological subtypes could help us to confirm our findings, and evaluate the effect of SMS-RESOLVE DWI on diagnostic accuracy.