Figure 1: Axial STIR image demonstrates multiple nodules (all < 1 cm) in the mesentery with increased signal intensity. There is also some increased signal intensity in the surrounding mesenteric tissue.

Figures 2 - 3: VIBE images again demonstrate the multiple enhancing nodules in the mesentery with minimal stranding in the mesenteric fat.

Mesenteric panniculitis.

Mesenteric panniculitis is a non-neoplastic inflammatory disorder characterized by diffuse, localized or multinodular fibrofatty thickening of the mesentery of the small bowel and occasionally the mesocolon, sigmoid mesentery, omentum, and retroperitoneum. This disorder is notable for a spectrum of pathologic changes including inflammation, fat necrosis, and fibrosis. Other terms used to describe this process include retractile mesenteritis (when the predominant component is fibrosis), lipogranuloma of the mesentery, mesenteric lipodystrophy (when the prominent component is inflammation), and retroperitoneal xanthogranuloma. Mesenteric panniculitis has also been described as part of one disease that progresses through three pathological manifestations: degeneration of the mesenteric fat (mesenteric lipodystrophy), inflammatory reaction (mesenteric panniculitis), and fibrosis of the adipose tissue (retractile mesenteritis).

Most cases are idiopathic, however possible predisposing entities include infection, abdominal trauma, ischemia, autoimmune disorders, a history of previous abdominal surgery (mainly cholecystectomy and appendectomy), and concurrent disease (including cholelithiasis, cirrhosis, abdominal aortic aneurysm, peptic ulceration, and gastric carcinoma). The disease has a male predilection with a male:female ratio of 2-3: 1. The age range at presentation is wide, with peak incidence in the sixth and seventh decade. Symptoms at presentation are variable and include progressive or intermittent abdominal pain, fever, nausea, vomiting, and weight loss. These symptoms often persist for a year or more. Physical exam may be unremarkable or may reveal abdominal tenderness or a palpable mass. The diagnosis of mesenteric panniculitis is one of exclusion. Other diagnoses that must be considered include pancreatitis, inflammatory bowel disease, metastatic disease, and extra-abdominal fat necrosis (Weber-Christian disease).

Radiographically, upper gastrointestinal and barium enema examinations may demonstrate displacement of bowel loops by a mesenteric mass. Bowel loops may be dilated, fixed or narrowed, but complete obstruction is rare. Fibrosis can cause retraction of the mesentery as well as bowel loop narrowing and speculation which can mimic neoplastic processes. Although the exact MR imaging appearance of mesenteric panniculitis and its associated entities has not been clearly defined, MRI can be used in conjunction with CT scanning to help characterize these mesenteric masses. MRI provides similar information to CT scanning but can also define tissue characteristics (i.e. fatty versus fibrous) with greater sensitivity and provide a noninvasive technique to assess the surrounding vasculature. For example, fibrous tissue has low signal on all sequences secondary to its high proportion of non-mobile protons with very short T2 values. Although masses of fibrous origin can exhibit a variety of signal characteristics on MRI imaging depending on their fatty and vascular content, low signal intensity should suggest a mature fibrotic reaction. Therefore, T2-weighted or fat-suppressed image sequences may help distinguish benign end-stage fibrofatty proliferation from malignant tumors such as lymphoma or metastases.

In mesenteric panniculitis the changes in the mesentery can be diffuse or local and can vary depending on the stage of the disease and whether the inflammatory or fibrosis component dominates. When diffuse, the mesenteric fat is traversed by low-signal intensity strands on T1-weighted MRI images. When diffuse inflammation of mesenteric fat is the predominant component, the inflammatory lesions appear as well-defined fatty tumors through-out the small bowel mesentery. When fibrosis is the predominant component, the mesenteric lesions appear as predominantly soft-tissue densities. In the focal form, heterogeneous nodular masses are seen and these lesions can demonstrate a varying amount of fat, fluid, necrosis, calcification, and soft tissue. Focal nodules can also appear as high-intensity nodules with intensity identical to that of peritoneal fat. MRI can also demonstrate the presence of vessel invasion, an imaging finding that would exclude the diagnosis of mesenteric panniculitis and suggest malignancy.

A variety of malignant, inflammatory or infectious etiologies may result in inflammation of the mesentery which may produce an imaging appearance indistinguishable from that of mesenteric panniculitis. This differential includes mesenteric fibromatosis, inflammatory pseudotumors, idiopathic retroperitoneal fibrosis, various granulomatous lesions, metastatic disease, sarcoma, lymphoma, desmoid tumor, carcinomatosis, and carcinoid tumor. Although mesenteric panniculitis is frequently concurrent with malignancy, it is a benign condition. It typically resolves without treatment. Steroids have been shown to have a possible benefit toward decreasing inflammation. Surgery is not indicated.

References:

1. Semelka RC. Abdominal-Pelvic MRI. New York: Wiley-Liss Inc, c2002. pp. 683-685.
2. Patel N, Saleeb SF, and Teplick SK. Cases of the Day: General Case of the Day. RadioGraphics. 1999; 19: 1083-1085.
3. Hemaidan A, Vanegas F, Alvarez OA, et al. Mesenteric Lipodystrophy With Fever of Unknown Origin and Mesenteric Calcifications. Southern Medical Journal. 1999; 92(5): 513-516.
4. Badiola-Verela CM, Sussman SK, and Glickstein MF. Mesenteric Panniculitis: Findings on CT, MRI and Angiography. Clinical Imaging. 1991; 15: 265-267.