Rectal Gas Removal Through Small Catheter Placement Prior to MRI of the Prostate

Overview

The presence of rectal gas can cause distortion at air-tissue interfaces on diffusion-weighted images (DWI) of prostate MRI and reduce image quality due to these susceptibility artifacts. Small catheter placement in the rectum before MRI is one of the ways that is advocated in PI-RADS v2 guidelines to reduce rectal gas. The goal of this study is to prospectively evaluate the effect of small catheter placement on artifacts on diffusion weighted images and to see whether it improves diagnostic accuracy.

The value of MRI in the detection of clinically significant prostate cancer has already been shown in many studies \[1\]. With MRI widely available and general accepted by urologists and radiologist for prostate imaging, a good execution and high quality of the images have become crucial. Recently Caglic et al published an article about optimizing prostate mpMRI \[2\]. One of the factors that influences image quality is rectal loading and bowel movement. The prostate is located anterior in the immediate vicinity of the rectum, so prostatic MRI is prone to artifacts caused by bowel movements and to susceptibility artifacts at the air-tissue interface \[3\]. Rectal movements are correlated to the degree of rectal distension and result in significant displacement of the prostate gland \[4\]. Rectal distension has a negative impact on the quality of both T2-weighted (T2W) and diffusion weighted images (DWI) \[5\], which are the two most important sequences in the detection of clinically significant prostate cancer. A study by Lim et al \[6\] showed a significant correlation between the amount of stool in the rectum and the severity of motion artifacts, however reducing the amount of stool with an enema did not improve the quality of T2W, DWI or dynamic contrast enhanced (DCE) images compared to the non-enema group. As the authors have declared, this could be due to the fact that only a minority of patients in the non-enema group had moderate or large amounts of stool (15.6%). This study by Lim et al also showed that the amount of rectal gas did not correlate with the severity of distortion artifacts on diffusion images, which could mean that even a small amount of gas could already cause substantial (susceptibility) artifacts on DWI. A larger study by Griethuysen et al \[7\] showed that a micro-enema shortly before the examination reduces both the incidence and the severity of gas-induced artifacts. So, moving gas appears to be the main concern and preparation is recommended. There is no evidence that one rectal emptying strategy is better than another \[8\]. In PI-RADS v2, different approaches are suggested, the aforementioned micro-enema, performing the MRI exam with patient in the prone position or to decompress the rectum using a small catheter \[9\]. This study aims to assess the efficacy of small catheter placement just before the MRI examination in reducing susceptibility on diffusion weighted images.

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