The difference between the scans is not level of detail, but rather function. I believe you have confused your PET and CT results. A CT is a still photo of internal structures and cannot show activity of any kind. A PET highlights metabolic activity (increased metatbolic activity indicates an infection or inflammation). An MRI uses a magnet that is rotated to take multiple images of the same area from different angles to produce a 3D composite that shows the imaged area from a 360 viewpoint. This in turn provides date that allows estimating the mass and circumference, rather than just length and width.
I have had all 3, with multiple CTs and MRIs. The purpose isn't just to find out what is going on, but how and to what extent it is going on.