Its part of all MRI as I understand they just don't use it much for our purposes (prostate), though they use it a real lot in other places in the body. It is basically liquid diffuses though tissue and the MRI can pickup the diffusing action, but cancer tends to not diffuse liquids so they can see on the MRI there is an area of low diffusion and that is a tumor.

At least my main original lesion in 2021 did work on contrast MRI, it got a Pirads 5, then next year or 2022 a Pirads 2, then my final MRI before biopsy in 2023 got no Pirads score. So my lesion seemed to not work with contrast slowly over the years. But it is real obvious on the diffusion weighted images, so that part of MRI was working just fine. Not sure I got any good explanation from Mayo or anywhere else why the contrast wasn't working well slowly for me over a few years, but I am glad some doctors also work with diffusion weighted MRI. It was real important in my case. I have no idea how often that is the case, but I am told diffusion weighted images are one of the main tools used most places MRI are used to look for tumors/cancer like in the brain and so on.