It is not a problem which is simple. It is a complicated problem of biomedical image processing. I would not comment over the solution, just give you some directions... Try to segment the tumor. This usually requires machine learning algorithms.
Here try to take the image of a healthy brain. Normalize the data and register them ( i understand that you know medical image registration since u r working on a medical imaging problem). And then detect the relevant edges and subtract the two images. Correspondingly find the points in the image and hence localize the tumor and compute its intensity
