## 4 Discussion  
Our study found that, using an AF of 2, 2 \(\times\) SMS rs-EPI allowed for a considerable reduction of acquisition time while maintaining high diagnostic image quality and lesion conspicuity. When increasing the AF to 3, a significant deterioration in image quality was observed in patients with reduced visibility of all organ parts in the diffusion weighted images and reduced image quality of the corresponding ADC maps. Moreover, artefacts and signal inhomogeneity in 3 \(\times\) SMS rs-EPI were significantly increased, which led to a poorer overall image quality and to a limited lesion conspicuity, thus discarding the use of higher AFs for clinical routine applications in its current implementation. Based on these results, further evaluation of AFs higher than 3 was waived. The scan acquisition time can be accelerated by increasing the acceleration factor of SMS, while an excessive acceleration factor is expected to be accompanied by a deterioration in image quality. Therefore, the most important thing is to find the balance between shortening the scanning time by SMS AFs and obtaining the best image quality. In our study, we found that 2 \(\times\) SMS rs-EPI can reduce the scanning time while obtaining better image quality, since there was no difference in scores of overall image quality, lesion conspicuity, and margin sharpness of the DWI images and ADC map compared with rs-EPI. These results are the same as those previously reported [11; 18; 19]. Neither the parotid gland tumors nor nasopharyngeal carcinoma were affected by the significant motion of the organ, and a better image quality can be obtained using 2 \(\times\) SMS rs-EPI, but the rectum is more affected by intestinal peristalsis; 2 \(\times\) SMS rs-EPI can still obtain stable, high-quality images. However, a significantly worse overall image quality was observed, for lesion conspicuity and margin sharpness, between 3 \(\times\) SMS rs-EPI and the other two sequences. SMS acceleration includes acceleration factor phase encoding (PE) and SMS factor. Excessive SMS acceleration factor reduced the intra-slice SNR, making it impossible for the GRAPPA algorithm used in acceleration factor PE to decode the parallel acquired data and create artifacts. Therefore, although an acceleration factor of 3 can significantly reduce the scan time, it is not feasible in rectal applications. [PERSON], C et al. [18] compared the CNR and lesion distortion between rs-EPI, 2 \(\times\) SMS rs-EPI and 3 \(\times\) SMS rs-EPI in 105 nasopharyngeal carcinomas and reported that no significant differences were found in the CNR and lesion distortion between rs-EPI and 2 \(\times\) SMS rs-EPI; however, the 3 \(\times\) SMS rs-EPI shows the lowest CNR and lesion distortion. In this study, we found similar CNR but less geometric distortion in 2 \(\times\) SMS rs-EPI vs. rs-EPI, similar to [PERSON] et al. 's study [19]. It is important to reduce distortion in rectal cancer MR imaging, and a true rectal display helps us to observe the circumferential resection margin (CRM) status, epidural vascular invasion (EMVI), depth of epidural invasion (EVI), discontinuous epidural vascular spread/deposition, and mucus presence; besides the T and N stages, these high-risk features also facilitate clinical decision making and can be  
Figure 4: Box plot of the differences between ADC values (10\({}^{-3}\) mm\({}^{2}\)/s) in tumors and healthy tissue measured using the three sequences. used to classify rectal cancer patients according to their prognosis [23; 24]. Our study result indicated that the SMS technique with an acceleration factor of 2 reduces the scan time without compromising image quality and with less distortion in the rectum, which can increase the clinical usability of rs-EPI for assessing rectal cancers. The ADC values of rectal cancer may reflect cancer aggressiveness, which can predict the histologic T stage, differentiation, and therapeutic response after chemoradiotherapy therapy [25].