Complicated sigmoid diverticulitis
This is a very challenging case (and great for learning) because it was first imaged as a non-contrast CT, but presented with several complications of diverticulitis. This patient has perforated sigmoid diverticulitis, probable small bowel venous ischemia, numerous hepatic abscesses, portal vein thrombosis, and pelvic abscess.
The presence of free air is the most important findings on the original non-contrast CT, and working through that case will be the focus of the discussion (rather than the contrast exam performed after colectomy). Free air means that hollow viscus is perforated. Careful inspection of the bowel and this the history of LLQ pain shows inflammation and small locules of air centered around the sigmoid colon. Therefore, this is perforated sigmoid diverticulitis.
Focusing on the liver lesions, it is apparent that there are many throughout the liver of varying sizes and imaging features. The largest lesion in segment 2/3 is quite impressive. As this patient does not have a history of primary malignancy or signs of tumor elsewhere (no adenopathy, no bone metastases, etc), abscess must be considered most likely, though other indeterminate lesions may also be present, and should be further characterized with a contrast exam.
Focusing on the abnormal small bowel: there is a short segment of abnormally thickened jejunum in the mid abdomen and several loops in the pelvis. This does not look like infection. This is a good look for venous ischemia in the setting of SMV thrombosis and is most likely considering the portal venous clot on the follow-up study (the SMV clot may have migrated or resolved). The other differential would be reactive inflammation from peritonitis in the setting of bowel perforation.