Operative note excerpt: "We began our inspection of the gastrojejunostomy and the Roux limb which appeared to be slightly dilated but normal. We attempted to trace the Roux limb distally; however, there was a significant amount of tension on the Roux limb and we turned our attention to a Petersen defect. The majority of the small bowel including the jejunojenuostomy had herniated through the Petersen defect. This small bowel was reduced .... thus we turned our attention to the closure of Petersen's defect."

This is a very challenging case that could be considered normal at first glance, which makes it particularly alarming. The Petersen defect is a potential space located between the jejunal mesentery of the Roux limb and transverse mesocolon. There are a few imaging clues which help to direct you to the diagnosis of internal hernia: 

• mild mesenteric edema and swirling/clustering of the proximal jejunum 
• focal narrowing of the SMA
• high grade narrowing and swirling of the SMV, with diminished contrast filling the distal branches
• it is not possible to follow many of the distal SMV branches back to the central vein on coronal images indicating they are swirled and narrowed 
• the small bowel is midline or left of midline, indicating that it is confined within the abdomen
• the terminal ileum is stretched leftward across the midline and courses around the clustered small bowel in the left mid abdomen

This case was interpreted as either partial volvulus or internal hernia which appropriately directed surgery to consider a diagnostic laparoscopy. To reach this diagnosis, consider the following:

• always complete your search pattern including running proximal and distal bowel and vasculature
• be aware of an early presentation of internal hernia 
• if you are concerned but not sure, raise the possibility with the surgical team and recommend observation with careful clinical follow-up, with a low threshold for repeating the examination if the patient fails to improve