Pre-processing on MRI images
It depends on how did you collect these MRI images from hospital and how do you want to use these MRI images. 


If you want to use these data in your deep learning model for segmentation or detection, it is possible to skip preprocessing.
If you want to apply a classical data science approach, you can do some processing like feature extraction.
Lastly, from another perspective, if you want to use also patient history with images, preprocessing can be necessary.