## 1 Introduction  
Colorectal cancer is a leading cause of high morbidity and mortality worldwide, where 30-35% of colorectal cancers occur in the rectum [1; 2]. Magnetic Resonance Imaging (MRI) is the preferred diagnostic tool for the local staging of rectal cancer, which provides excellent soft-tissue contrast and may be used to assess treatment response and detect postoperative local recurrence [3]. Diffusion Weighted Imaging (DWI) is a fast, non-enhanced MRI technique that does not require a contrast agent for assessing the diffusive movement of water molecules in tissues. Employing DWI with conventional MRI improves diagnostic confidence and may be used to predict rectal cancer [4; 5]. Additionally, the Apparent Diffusion Coefficient (ADC) from traditional DWI provides biological implications for cell density and can identify tumor progression [6].  
Due to its high speed, single-shot echo-planar imaging (ss-EPI) DWI is widely used in radiology practice, with K-space filling after a single excitation. However, due to the slow filling along the phase-encoding direction, the low bandwidth in this direction results in blurred and distorted images. Additionally, ss-EPI is highly susceptible to magnetic field inhomogeneities and may result in geometric distortions [7]. Parallel acquisition techniques and multi-shot EPI sequences, such as readout-segmented echo-planar imaging (rs-EPI), can reduce this limitation [8]. rs-EPI divides K-space into segments along the readout direction to shorten the echo interval [8]. Previous studies have also shown that rs-EPI is superior to ss-EPI in assessing lesions in patients with rectal cancer and improving image quality; lesion conspicuity, overall image quality, geometric distortion, and distinction of anatomic structures were significantly better in rs-EPI [9].  
rs-EPI, however, is limited by its long scan time, which can cause motion artifacts due to bowel peristalsis and patient discomfort. Based on the principle of simultaneously exciting multiple slices, a new method using simultaneous multislice (SMS) technology has been proposed to reduce DWI scanning time [10]. This technique involves simultaneous excitation and acquisition of multiple slices, enabling a reduction in scan time based on a greater number of simultaneously excited slices as determined by the acceleration factor (AF) [11]. In this technique, the number of slices acquired over the same pulse repetition time can be increased by setting a higher AF without the need for an increase in the gradient demand [12]. The SMS combined with DWI has been successfully applied to the kidney, liver, pancreas, breast, and other organs [13; 14; 15] with promising results. According to [PERSON] et al. [11], SMS combined with ss-EPI with an AF of 2 yields an image quality and stable ADC values comparative to those of conventional ss-EPI. In theory, image acquisition in sms-DWI can be accelerated to a large extent by increasing the applied AFs. However, an arbitrary increase of the AF is expected to come along with a deterioration of image quality [11; 13]. To ascertain the optimal ratio between scan time minimization and diagnostic image quality, the use of higher AFs and the resulting image quality need to be investigated systematically. What is more, there are no studies using SMS in combination with rs-EPI in rectal cancer as far as we know, and the impact of rs-EPI combined with different SMS AF\({}_{\text{s}}\) on the image quality and ADC values of rectal cancer is also unknown.  
Thus, the purpose of this study was to investigate the feasibility of SMS rs-EPI of rectal cancer with different acceleration factors (AF\({}_{\text{s}}\)) and its influence on image quality, acquisition time and apparent diffusion coefficients (ADC\({}_{\text{s}}\)) in comparison with conventional sequences.