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Background
The peritoneal cavity if often involved by common disease process such as bowel perforation, infections, malignancy and trauma. It is essential to understand the peritoneal and retroperitoneal anatomy in order to localize disease and to formulate differential diagnosis during the radiological interpretation.

Educational Goals / Teaching Points
The aim of this presentation is to discuss anatomic principles of peritoneal ligaments and spaces and retroperitoneal spaces and planes, to review the basic embryological development of the peritoneum, to describe the mechanism by which diseases spread through the abdomen and pelvis, to correlate the radiological features with surgical findings and to provide key diagnostic findings.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
The embryonic development of the peritoneum starts during the gastrulation stage, at which point a three-layered disc is formed and a layer of endoderm, ectoderm and mesoderm emerge. At four weeks gestational age the lateral plate mesoderm becomes separated into two layers by an intraembryonic coelom, which becomes the parietal and visceral layers. Initially, the developing peritoneal cavity is divided into right and left by the ventral and dorsal mesenteries of the primitive gut, the remnants of which form many of the ligaments and mesenteries. The peritoneum consists of a layer of dense stroma covered on its inner surface by a single sheet of mesothelial cells and has the function of supporting organs within the abdominal cavity. The omentum is a mesentery or double layer of peritoneum that extends from the stomach and duodenal bulb to adjacent organs. The first pair of ligaments in the upper abdomen are the gastrohepatic and the hepatoduodenal ligaments also known as lesser omentum. The mesentery is a double fold of peritoneal tissue that suspends the small intestine and large intestine from the posterior abdominal wall. The portions of the mesentery are the sigmoid mesocolon, the transverse mesocolon and the small bowel mesentery. Peritoneal spaces are potential spaces between visceral and parietal peritoneum defined by ligaments and mesenteries. The transverse mesocolon divides the peritoneum into the supramesocolic and inframesocolic spaces. The retroperitoneum is delimitated by the parietal peritoneum and the transversalis fascia. It is divided into three distinct compartments: the posterior pararenal space, the anterior pararenal space and the prirenal space. Diseases can spread through the abdominal and pelvis by different mechanisms such as subperitoneal spread, intraperitoneal spread, interfascial spread and direct invasion.

Conclusion
Understanding the mechanisms by which diseases can spread through the abdominal and pelvis is essential to provide an accurate interpretation of the radiological findings. Radiologist must be familiar with the anatomy of the peritoneal and retroperitoneal to avoid potential diagnostic misinterpretation.