Mesenteric Cyst Etiology: mesothelial-lined peritoneal surfaces failing to coalesce, arises from root of mesenteryUS: thin walled simple cyst with posterior acoustic enhancement Cases of Mesenteric Cyst Axial CT with contrast of the abdomen shows a large simple cystic mass in the left side of the abdomen. Two contiguous axial images from a CT with contrast of the abdomen show a whirlpool sign of swirling of the mesenteric vessels (above left) around a round low density structure just to the left of the spine (above right). Another axial image from the same exam (below left) shows a normal relationship of the superior mesenteric vein to the right of the superior mesenteric artery although both appear to be shifted to the left. AP image from a subsequent upper GI (below right) shows normal position of the ligament of Treitz in the left upper quadrant. Surgical image shows a large thin-walled cystic structure arising from the mesentery of the small bowel.