By far the best scan for this patient would be the MRI. MRI scans show much more detail concerning the vasculature than CT scans do. As a matter of fact, sometimes the vessels on CT can't be visualized at all. CT scans are more useful with hard, boney structures. If the pancreas is thought to be necrotic (dead), the patient is a candidate for an angiogram, ERCP, and MRI (magnetic resonance imaging). Soft tissues, like the pancreas, and associated blood vessels are better visualized with the MRI scan. Further, if CT scan was employed, it would have to be a scan with contrast. If the pancreas is necrotic, the patient wouldn't be able to tolerate the contrast media anyway.
As a general rule, CT scan is more sensitive and threfore preferable for bone or calcific related anatomical changes. MRI better differentiates conditions involving soft tissue structures. Therefore, I would be inclined to think that since the pancreas is a soft tissue/organ structure, MRI would be the most helpful in imaging the organ and best reveal any pathological changes.
Of course, only a radiologist would know for sure.