It's not the quality of the scan (although that can certainly be a factor). It's that there is a lot of variation in human anatomy, and "these pixels are bright

It's not the quality of the scan (although that can certainly be a factor). It's that there is a lot of variation in human anatomy, and "these pixels are brighter than the rest" can mean any of many things. Relatedly, for many types of brain tumors, you simply don't know what exact kind of tumor it is until you actually do a biopsy (= you undergo surgery). This can be important information ("how aggressive is it?"), so in some hospitals it is common practice to send a tissue sample for identification and get back the result while the surgery is still going on.