Imaging of peritoneal and mesenteric disease: key concepts for the clinical radiologist.

The abdominal cavity extends from the diaphragm to the pelvic floor, and is lined by a serous membrane known as the peritoneum. The peritoneum is the largest serous membrane in the body, and its surface area is comparable to the surface area of the skin (approximately 2 m2, in adults). Infoldings of the peritoneum suspend the various organs contained in the abdominal cavity. The peritoneum is, therefore, comprised of two layers, a parietal layer lining the abdominal cavity, and a visceral layer enveloping the contained organs. The peritoneal cavity is enclosed between these two layers, and is normally empty, apart from a small amount of physiological fluid. As a result, the normal peritoneal cavity is only a potential space. In men, the space is closed. In women, the fimbriated ends of the Fallopian tubes open into the peritoneal cavity, and provide a potential route of communication with the outside. The peritoneal cavity and the specialized peritoneal infoldings, known as mesenteries, are important disease sites in the abdomen. However, plain radiographs, barium studies, and ultrasound are of limited utility in imaging peritoneal and mesenteric disease. Direct and consistent imaging of such diseases only became possible with the development of computed tomography (CT). Initially, magnetic resonance imaging (MRI) was of limited utility, because of image degradation by motion artifact. With modern fast MRI sequences, it is now possible to depict many of these peritoneal and mesenteric conditions with an accuracy similar to CT. This review will discuss the normal anatomy and physiology of the peritoneal space and mesenteries, and the related disease processes, with particular emphasis on CT and MRI findings. The review is structured along anatomical lines, because many disease processes in the abdomen are site specific, or spread along anatomical pathways. The radiology of peritoneal malignancy and peritoneal adhesions are discussed separately, because these two conditions are not site specific, and because they are conditions whose imaging features are not always well appreciated. Our aim is to present key anatomical and pathological concepts in the imaging of peritoneal and mesenteric disease. We hope this will facilitate a clearer understanding of the CT and MRI appearance of these diseases, and so enhance the ability of the clinical radiologist to formulate rational differential diagnoses, to understand pathways of disease spread, and to apply greater critical analysis in radiological interpretation. The review is not intended as an encyclopaedic description of peritoneal anatomy and pathology.