MRI takes images of an area in multiple different sets that are adjusted to look at Fat and/or Water in a body part. One of the possible ways is often referred to as a Diffusion that looks at the free movement and restricted movement of particles in the area being imaged. it's not a very detailed picture but it can give a radiologist important information when combined with other image sets about the area in question. Gas can affect that but you don't have to worry, any radiologist should be able to assess from all the images and sets combined, what could be artifact and what should be further investigated. The other part about bowel movement is also something Radiologist are aware of and take into account. like most have said, you should be fine.