Liver transplant recipients are, like anybody, susceptable to colitis. In the CT scan above, notice the thickened colonic walls (aarows) involving the right and transverse colon. CMV colitis should be in your differential diagnosis, particularly if the patient is early post-transplant (first 6 months) or recently discontinued prophylaxis. Patients are particularly susceptable to CMV in the weeks to months following discontinuation of prophylaxis, and if they are donor (+) and recipient (-) in serology.
If blood is negative for a CMV viral load, but suspicion remains high, colonoscopy with biopsy is still indicated; the virus can invade the colonic tissue and still be undetectable in serum. For some reason, right side involvement is very common with CMV, so it is not satisfactory to do a simple flexible sigmoidoscopy and obtain random rectal and sigmoid biopsies.
A differential diagnosis should also include ischemic colitis, other infection (bacterial and viral), edema from ascites (if present), and inflammatory bowel disease.