No imaging test is perfect. Each have their strengths and weaknesses. PET's strength is that it gives an idea of the metabolism of things that they are looking at. The downside is that they have the least resolution of all the scans, and they're the most expensive. CT scans offer some of the best resolution through-out the body although they struggle to view things in the abdomen (like the intestines and especially the spaces in between) since that stuff is movign all the time. They are also not good when someone for whateer reason can't have IV iodine contrast. MRI are the least invasive test, provide good resolution (somewhere in between PET and CT depending on the structures) and don't bombard the patient with radiation.
No test is "better" than another by it's very nature, and they all have their specific uses for specific individual patients. My initial reaction to an MRI of the liver-only (if that is what exactly is being proposed) following chemotherapy is that it's a little limited. Followup imaging is usually chest/abdomen/pelvis, regardless of the "mode" of scan chosen. Liver-only is a little limited as recurrences for CRC are possible in many areas. True it's most likely in liver/lungs, but lymph nodes, abdominal, and bone mets are all possible.
Why not bring up your concerns with your mom's onc, and find out why he/she prefers MRI vs the other scan modalities. I would hope he/she has a good answer
11/13/09 5cm Stage IV 9/25 lymph nodes w/2cm peritoneal met at 29 YoA
12/15/09 LA right hemi-colectomy
6/16/10 Folfox FINISHED
8/10/10 Prophylactic HIPEC
10/9/10 got Married
Still NED and living life to the fullest
"Can any one of you by worrying add a single hour to your life."