Pre-processing on MRI images I have MRI images of brain tumors collected from a hospital (not a benchmark dataset). And I am planning to use them to predict/classify tumour types using a typical machine learning approach: texture analysis for feature extraction to build a classification module.
My question is, how to decide whether these images needs pre-processing or not, such as noise removal by using median filter for example? In other words, how to determine if there is noise in this data?
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.