ª The Author(s) 2015. This article is published with Abdom Imaging (2015) 40:2710–2722 Abdominal open access at Springerlink.com DOI: 10.1007/s00261-015-0429-5 Published online: 26 May 2015 Imaging The subperitoneal space and peritoneal cavity: basic concepts Harpreet K. Pannu,1 Michael Oliphant2 1Department of Radiology, Memorial Sloan Kettering Cancer Center, 1275 York Avenue, New York, NY 10065, USA 2Department of Radiology, Wake Forest University School of Medicine, Winston-Salem, NC, USA Abstract The peritoneum is analogous to the pleura which has a visceral layer covering lung and a parietal layer lining the The subperitoneal space and peritoneal cavity are two thoracic cavity. Similar to the pleural cavity, the peri- mutually exclusive spaces that are separated by the toneal cavity is visualized on imaging if it is abnormally peritoneum. Each is a single continuous space with in- distended by fluid, gas, or masses. terconnected regions. Disease can spread either within the subperitoneal space or within the peritoneal cavity to Location of the abdominal and pelvic organs distant sites in the abdomen and pelvis via these inter- connecting pathways. Disease can also cross the peri- There are two spaces in the abdomen and pelvis, the toneum to spread from the subperitoneal space to the peritoneal cavity (a potential space) and the subperi- peritoneal cavity or vice versa. toneal space, and these are separated by the peritoneum (Fig. 1). Regardless of the complexity of development in Key words: Subperitoneal space—Peritoneal the embryo, the subperitoneal space and the peritoneal cavity—Anatomy cavity remain separated from each other, and each re- mains a single continuous space (Figs. 2A, 3A). Distin- guishing the subperitoneal space from the potential space The article is based on the comprehensive and of the peritoneal cavity is important for understanding authoritative book by Drs Meyers, Charnsangavej, and the distinct patterns of disease spread in each. Oliphant titled Meyers’ Dynamic Radiology of the The term subperitoneal refers to tissue that is deep to Abdomen [1]. Key concepts from the book are high- the peritoneum and includes the extraperitoneal space, lighted in the following text and in Table 1. The text and the ligaments and the mesenteries and their suspended accompanying illustrative images are divided into 3 sec- organs (Fig. 2A). Organs whose surfaces are covered by tions on anatomy, an overview of the spread of disease, peritoneum are therefore subperitoneal. Subperitoneal and disease spread for selected organs. organs that are deep to the posterior peritoneum are called extraperitoneal. Since there are only 2 spaces in the Anatomy abdomen and there are no organs in the peritoneal cav- Lack of organs in the peritoneal cavity ity, all the abdominal pelvic organs, and their associated vessels, lymphatics, and nerves are in the subperitoneal The peritoneum is a serous membrane made up of vis- space. In other words, all the structures seen in the ceral and parietal layers. The visceral layer of the peri- abdomen and pelvis on cross-sectional imaging are in the toneum lines the surface of organs and the parietal subperitoneal space. The organs lie in the abdominal peritoneum lines the coelomic cavity. The peritoneal cavity, not the peritoneal cavity (Figs. 1, 2A). The peri- cavity is a potential space between the visceral and toneal cavity is a potential space devoid of organs. parietal layers of the peritoneum. There are no organs in the peritoneal cavity. The potential space of the peri- toneal cavity is normally not visible on imaging as it Importance of visualizing the subperitoneal space contains only a small amount of fluid (about 100 mL). as a single space The subperitoneal space is a large continuous space that is formed by regions interconnected by ligaments and Correspondence to: Harpreet K. Pannu; email: [email protected] mesenteries. Ligaments and mesenteries refer to the H. K. Pannu, M. Oliphant: The subperitoneal space and peritoneal cavity 2711 Table 1. Key concepts regarding the peritoneal cavity and subperitoneal space (SPS) Concept Discussion Spaces of the abdomen and pelvis There are 2 mutually exclusive spaces—the peritoneal cavity and the SPS The structure that defines the SPS & separates The peritoneum it from the peritoneal cavity The peritoneal cavity The peritoneal cavity is a potential space and is inconspicuous on normal imaging studies. There are no organs in the peritoneal cavity. The SPS The SPS is a single extraperitoneal space encompassing the entire abdomen and pelvis. It is divided into regions but remains one interconnected space. The SPS contains all the organs, vessels, lymphatics, and nerves of the abdomen and pelvis. The importance of distinguishing the SPS from The routes of disease spread differ for the SPS and the peritoneal cavity the peritoneal cavity The distinct patterns of disease spread in the SPS Disease spread can occur via mesenteries, ligaments, and lymphatics, and by periarterial, perineural, transvenous, and intratubular routes The distinct patterns of disease spread within Disease spreads along the pathways of fluid flow in the single continuous the peritoneal cavity space of the peritoneal cavity Transperitoneal disease spread Transperitoneal spread occurs when disease crosses the peritoneum which separates the SPS from the peritoneal cavity. Transperitoneal spread is bidirectional Fig. 1. The peritoneal cavity vs the subperitoneal space. all the abdominal pelvic organs which are interconnected via This is a schematic diagram showing the peritoneal cavity ligaments and mesenteries. Dotted lines show some of these (PC) on the left and the subperitoneal space (SPS) on the interconnections which allow for disease spread. Abbre- right. The diagram illustrates that these are 2 completely se- viations for the peritoneal cavity: IC inframesocolic compart- parate spaces within the abdominal cavity. The peritoneal ment, LP left paracolic recess, LS lesser sac, M Morison’s cavity and the subperitoneal space are separated by the pouch, RP right paracolic recess, RS right subphrenic space, peritoneum. The peritoneal cavity is one continuous space and RV rectovesical space. Abbreviations for the subperi- with interconnecting recesses, some of which are shown. The toneal space: B bladder, C colon, K kidney, L liver, P pan- subperitoneal space is also one continuous space containing creas, S spleen, SB small bowel, and ST stomach. subperitoneal tissue between suspended organs and the The relationship between the bowel mesentery extraperitoneal space. Visualizing the subperitoneal and the peritoneum space as a single space explains the spread of disease between different regions of the abdomen pelvis and The mesenteries of the abdomen and pelvis are composed between the organs covered by peritoneum and the ex- of subperitoneal tissue between 2 layers of visceral peri- traperitoneum. toneum. Comparing an axial CT image with a cross- 2712 H. K. Pannu, M. Oliphant: The subperitoneal space and peritoneal cavity Fig. 2. Relationship between the mesentery and the peri- area). Spine is shown on the diagram only for orientation pur- toneum. A Diagram of a 4-week-old embryo shows the coe- poses. A aorta, K kidney, S spine, and VM ventral mesentery. B lomic cavity which will form the peritoneal cavity (PC) Axial CT image shows the dorsal mesentery of the small bowel surrounding the primitive gut (G). The medial visceral layer as subperitoneal tissue (arrowhead) between 2 layers of vis- (arrows) of the peritoneum is apposed on the gut and the me- ceral peritoneum (black arrows). The parietal peritoneal reflec- sentery, while the parietal layer is lateral. The dorsal mesentery tion (white arrows) is also seen anterior to the colon (C), kidney, (DM) conveys vessels from the aorta to the gut. Other than the and in the anterior and lateral abdomen. There is fluid (asterisk) peritoneal cavity, all the tissue in the abdominal cavity portion of in the peritoneal cavity between the visceral (black arrows)and the diagram is the subperitoneal space (light gray shaded parietal (white arrows) layers of the peritoneum. sectional diagram of an embryo, the dorsal mesentery tissues anterior to the kidney leaving a connecting carries vessels from the aorta to the gut (Fig. 2). The splenorenal ligament that contains the distal splenic ar- visceral peritoneum surrounds the mesentery, forms the tery and vein. The midgut and hindgut form within the serosal layer of the gut, and is in continuity with the dorsal mesentery creating the small intestine mesentery parietal peritoneum which covers the extraperitoneal and the mesocolon. space. As the ligaments, mesenteries, and suspended or- Anteriorly, the liver forms in the subperitoneal tissues gans develop, the peritoneal cavity forms recesses that anterior to the stomach, or the ventral mesogastrium. remain interconnected as the peritoneal cavity and se- This divides the subperitoneal tissues into the gastro- parate from the subperitoneal space (Fig. 3A). hepatic ligament between the stomach and liver, and the falciform ligament between the liver and abdominal wall. Correlation between embryonic development and The free edge of the gastrohepatic ligament is the the abdominal ligaments hepatoduodenal ligament. The visceral peritoneum con- tinues over the stomach forming the serosal layer and The spleen, pancreas, liver, and gut form within the over the liver and spleen forming their capsules. mesentery that surrounds and suspends the primitive gut in the embryo. The development of these organs results Identifying the ligaments and bowel mesenteries in the creation of the abdominal ligaments that can be on imaging identified on CT. Posteriorly, the spleen and pancreas form in the The ligaments and mesenteries are named according to dorsal mesogastrium, or the part of the dorsal mesentery the viscera they connect and are identified by the vessels suspending the stomach (Fig. 3). The splenic artery runs that run in them. For example, the hepatoduodenal from the aorta through the dorsal mesogastrium to the ligament is identified by the portal vein, hepatic artery, spleen with branches continuing to the stomach.