 Hello everyone, my name is Vivek Pai, I'm currently a Neuro Radiology Fellow working in Singapore. It's an absolute honour and privilege to be on this educational platform. This is a compilation of my experiences with the Peritonium. I hope you enjoy. This presentation is a brief review of the Peritonal Anatomy on CT. The images are obtained from patients with acitis to mimic a real-life reporting scenario. I will also discuss relevant pathologies to help consolidate understanding. We begin with identifying the basic extent of the Peritonium on this sagittal diagram. The Peritonium, marked in red, outlines the inner aspect of the anterior abdominal wall. This portion is called the Peritonium. It extends inferiorly and reflects over the bladder, the uterine fundus and the rectum to form small intervening pouches. Further, the Peritonium lines the posterior aspect of the abdominal cavity and suspends multiple bowel structures which we will discuss shortly. On an actual section, Figure B, the Peritonium is seen as a thin membrane that outlines the abdominal cavity. The healthy Peritonium, as in Figure C, is rarely seen but can be made more conspicuous with the help of narrow window settings. However, it may be involved in different diseases that cause it to get thickened or nodular. Figure D is a contrast enhanced image of a patient with tuberculosis peritonitis. In this image, the involved parietal peritonium is thickened, enhances and hence easily visualized. As the Peritonium lines the posterior aspect of the abdominal cavity, it wraps around the gastrointestinal tract via the visceral peritonium. In a codo-cranial direction, we first encounter the sigmoid mesocolon marked in orange. It suspends the sigmoid colon and contains the hemorrhoidal and sigmoid vessels. Figure B illustrates the normal appearance of the sigmoid mesocolon on an actual CT section. This, like most other peritonial ligaments, demonstrates fat attenuation and signal intensity on CT and MR respectively. Figure C, a coronal reconstruction of the same patient demonstrates the normal appearance of the sigmoid mesocolon. Swirling or spiraling of the vessels within the sigmoid mesocolon indicates twisting of the ligament. This causes a closed loop obstruction of the sigmoid colon, also known as sigmoid volvulus. Figure C, though a CT scout, shows the classic coffee bean-shaped appearance of the over-distended and obstructed sigmoid, the end result of such twisting. Superiorly, the peritonium suspends the small bowel loops via the mesentry marked in yellow. On sagittal CT images, the mesentry can be easily identified as the fan-shaped fat density membrane that contains the superior mesentric artery shown by the hollow arrow. On an actual section, the mesentry can be identified posterior to the small bowel with a sinus accumulating along the anti-mesentric border. The mesentry is involved in multiple diseases, ranging from inflammatory to neoplastic etiologies. One such example is seen in this patient who presented with abdominal pain. Increased density of the mesentry, also known as misty mesentry, was noted, compatible with mesentric paniculitis. A few imaging signs help identify this condition, like the pseudo-capsule sign indicated by the hollow arrow, referring to the curvilinear band of soft tissue attenuation around the stranding. The fat halo sign, indicated by the dotted arrow, is another useful sign and refers to the preservation of normal fat density around the mesentric vessels. Nodal manifestation of abdominal lymphoma is another common disease process involving the mesentry. This patient is a known case of lymphoma with multiple enlarged lymph nodes on either side of the mesentric vessels. This appearance has been described as the sandwich sign. Parsonoid tumors of the mesentry demonstrate a soft tissue density with speculated margins. This is also known as the spoke wheel appearance and is due to the desmoplastic reaction secondary to the release of serotonin and other hormones. The net result is retraction of the mesentry with displacement and angulation of the bowel loops. The fold of peritoneum highlighted in blue that suspends the transverse colon is called the transverse mesocolon. It contains the middle colic vessels and divides the peritoneal cavity into the supramesocholic and infamesocholic compartments. On a sagittal CT section, it can be traced back from the transverse colon, especially in the presence of acytis. In the absence of acytis, the extent of the transverse mesocolon can be estimated by following the marginal vessels as they drain into the middle colic vein. Owing to its proximity with the pancreas, the transverse mesocolon is involved in multiple pancreatic diseases. This image is one such example that shows inflammatory stranding involving the transverse mesocolon in a patient with acute pancreatitis. The integrity of this ligament is also important when evaluating pancreatic cancers. If involved, pancreatic cancer can be rendered inoperable. Coming back to the illustration, the peritoneal lining extends posterior to the liver, forming the anterior boundary of the retroperitoneal space. Just along the inferior surface of the diaphragm, the peritoneum reflects to cover most of the liver. Further, it continues as a short ligament to the stomach and the proximal deodenum. This portion is called the lesser momentum and is marked in purple. Essentially, the lesser momentum is composed of two components. The larger is known as the gastro-hypatic ligament shown in figure B. It lies between the liver and the lesser curvature of the stomach. The anatomic marker of this ligament is the left gastric artery shown in figure C. The gastro-hypatic ligament also contains the coronary vein. The other, smaller component of the lesser momentum is called the hipato-deodenal ligament and lies between the liver and the first part of the deodenum. The anatomic marker of this ligament shown in figure C is the common hepatic artery. The ligament also contains the portal vein, common hepatic ducts and part of the cystic duct. The lesser momentum is an important conduit for spread of neoplasms and hence must always be scrutinized closely. Figures A and B demonstrate invasion of the gastro-hypatic ligament in a patient with carcinoma of the stomach. Similarly, in another patient with cholangio-carcinoma, images labeled C and D show spread of the disease along the hipato-deodenal ligament. From the greater curvature of the stomach, a peritoneal fold extends as a double layered sheet known as the greater momentum. It stretches in front of the small bowel loops only to eventually fold upon itself and attach to the transverse colon. On a sagittal CT section, the greater momentum can be identified as the fatty apron anterior to the stomach, transverse colon and the bowel loops. On axial sections, figures B and C, it can be distinguished due to its anterior location relative to the bowel loops and the rest of the peritoneal folds that we have just discussed. Metastatic deposits involving the greater momentum are common, exemplified in figure A and its corresponding pet CT image, figure B. Such involvement is primarily because the greater momentum is involved in resorption of the peritoneal fluid, making it an important hot spot. Metastatic involvement of the greater momentum varies in size ranging from subtle nodularity to frankamental caking when large as seen in figures C and D. Don't miss the metastatic disease involving the transverse mesocolon as well shown by the hollow arrows. Apart from soft tissue masses, the greater momentum may undergo infarction as shown on the CT image. These lesions are usually seen as an area of encapsulated stranding larger than 5 centimeters. They are frequently seen along the right edge of the momentum due to the poor blood supply to this region. An important differential diagnosis of this is inflammation and infarction of the epiploic appendages. Normally, these structures are seen in patients with acytus. They are fatty visceral outpouchings that contain small blood vessels. They arise from the serosal surface of the colon. Seldom, they may undergo torsion resulting in inflammation and acute ischemic infarction. This pathology, also known as epiploic appendicitis, generally manifests as a small oval fat attenuation lesion surrounded by a ring of soft tissue anterior to the colon. A hyperdense dot may be seen within signifying a thrombosed vein. In addition to the large peritonal reflections that we have reviewed, multiple smaller suspensory ligaments are seen in the supramezocolic compartment. Of the more easily seen suspenders is the gastrophrenic ligament. As the name suggests, it suspends the stomach from the left dome of the diaphragm. Another ligament in relation to the stomach is the gastro splenic ligament. It connects the greater curvature and the splenic hylum. This ligament contains the left gastroepiploic and short gastric vessels. The splenorenal ligament inserts in the splenic hylum and extends to the anterior pararenal space. It contains the tail of the pancreas, the distal splenic artery, and the proximal splenic vein. This coronal T2 weighted image demonstrates enlarged and tortuous venous collaterals in the splenorenal ligament. Remember, this is an important site of photosystemic collateral shunting. In this CT image, the spleen is located in the lower abdominal cavity. This condition is known as wandering spleen. The ectopic location of the spleen occurs due to incomplete development or laxity of its suspensory ligaments, including the gastro splenic and the splenorenal ligaments. Overall, this condition is associated with torsion of the splenic ligaments and the spleen at large, a rare surgical emergency, as seen on the actual sections of the same patient. The potential space between the peritoneal ligaments are divided into two. The smaller of these is the lesser sac posterior to the stomach and the omenta, highlighted in white. Figure B is a sagittal CT image which demonstrates the lesser sac distended with fluid within. The larger anterior space is known as the greater sac, highlighted by the brown area. A sciatic fluid, as in Figure B, is the most common pathological content seen within this space. Having said this, fluid collections of different densities are not uncommon within the greater sac. Take for example these images. Contrast material due to rupture of the dome of bladder, highlighted by the dotted area, is seen filling up the peritoneal cavity. The close relation of the peritoneum with the bladder helps understanding why the bladder dome ruptures interperitoneally while the rest of it ruptures in the extroperitoneal pelvis. The space around the liver is compartmentalized by a few perihepatic ligaments. The larger of these is the falsiform ligament. It is a vestigial peritoneal reflection that contains the obliterated umbilical vein. It suspends the liver from the ventral abdominal wall. The triangular ligaments are peritoneal reflections that also anchor the liver. Of these, the right triangular ligament marked as RTL is seen along the right lobe of the liver. The space between the falsiform and the right triangular ligament is called the right subphrenic space marked as RSS. The region inferior to the right triangular ligament is called the right subhypatic space labeled as RSH. It is also known as the hepatorenal recess or the Morrison's pouch. The left triangular ligament is short and seldom visualized. Unlike its counterpart on the right, the left triangular ligament does not compartmentalize the space to the left of the liver. Thus, the left subphrenic space labeled as LSS is identified as the region to the left of the falsiform ligament between the diaphragm and the gastric fundus. The space extends around the spleen as well. Inferiorly, the left subhypatic space labeled as LSH is bound by the lateral segments of the liver and the stomach. All these spaces generally accumulate acetic fluid but every once in a while unusual contents may be seen within. This ultrasound image is obtained from a patient presenting with abdominal distention and vague pain. Fluid was seen in the right subphrenic space. However, the presence of ecogenic foci within and the scalloping of the liver suggested a diagnosis of pseudo-mixoma peritonei. CT was later performed which confirmed the diagnosis with scalloping of the liver margins. The cause of the disease was found to be a ruptured musinus cyst adenocarcinoma of the ovary. This case is specifically discussed since this condition, though a diffuse peritoneal process, may mimic acitis both of which tend to collect in the perihepatic spaces. Scalloping of the liver is the most important differentiating feature. Free air is another common pathological content of the subphrenic spaces in patients with bowel ruptures. The image highlights the value of evaluating the abdomen in a long window setting to increase contrast between the viscera and free air. Of the more challenging lesions encountered in this region was in this patient presenting with prolonged fever and weight loss. A peripherally enhancing lesion was seen in the right subphrenic space. It demonstrates heterogeneous T2 signal intensity shown in the inset. The lesion was later confirmed to be an extra hepatic hydrated cyst. The paracolic spaces or gutters shown by the arrows in figures A and B are located lateral to the ascending and descending colons. The right paracolic gutter is larger than the left and communicates freely with the right subphrenic space. On the left, the phrenicocolic ligament indicated by the hollow arrow in figure C partially limits the communication between the left paracolic gutter and the left subphrenic space. However, the paracolic gutters communicate freely with the pelvic spaces. As we discussed in the initial slides, the paratonium reflects over the pelvic organs to create spaces between them, named after their location. Therefore, in women, the space between the bladder and the uterus is called the vesica uterine space. Similarly, the space between the uterus and the rectum is known as the recto uterine space, also called the pouch of Douglas. The paratonial pelvic spaces in women also contains the adnexa and the uterine ligaments which are seen best on MR imaging. In men, the pelvic paratonial space is simply called the vesicorectal space. Figure B is a sagittal CT section of the pelvis which depicts the pouch of Douglas filled with acetic fluid within. Figures C and D demonstrate an unusual finding. This patient was investigated for pelvic flow weakness. The images were a part of the defecogram protocol and were obtained at rest and straining respectively. Upon straining, figure D, note the descent of all the pelvic organs with a loop of small bowel herniating into the pouch of Douglas, hopefully more obvious now with the understanding of the anatomy. Beyond this being a rare finding, the importance to identify such small bowel herniations, also known as entro seals, lies in the fact that some consider this to be a contraindication to star surgery due to the risk of injury. Since we have now delineated all the paratonial spaces and ligaments, it would be worthwhile to understand a key concept. Fluid within the paratonial compartment is constantly circulated with the help of diaphragmatic and peristaltic movements. Fluid tends to stagnate in certain portions, especially the gravity dependent areas like the perihepatic, paracolic, and recto uterine regions. Hence, these are extremely important preview areas when evaluating for paratonial metastasis. These are just examples to further emphasize the value of routinely evaluating these gravity dependent regions. Figures A and B show right sub-phrenic and left paracolic metastatic deposits, while figures C and D show deposits along the pouch of Douglas. The identification of deposits in these locations are extremely crucial in pre-operative planning as well as prognostication. Events following a gallbladder rupture are an extension of the paratonial fluid circulation and gravity dependence stagnation. Extra polluting from what we just discussed, it would now be clear to understand why in patients with gallbladder perforations, gallstones shown in figures B and C are commonly found in the pelvic pouches. In summary, the paratonial ligaments help limit disease spread but may also act as conduits facilitating spread of neoplasms. Identification of these ligaments in abdominal diseases not only helps in prognostication but may also influence surgical approach. I hope this presentation has helped in understanding and clarifying the anatomy of the paratonium. Before I conclude, I would like to express my gratitude to the Indian radiologist platform for giving me this opportunity to share some of my work. I also acknowledge the case contributions of Dr. Nikhil Muda and would like to thank Mrs. Shraddha Surana for helping me with the illustrations. Thank you for watching.