Seeing is believing: The subtleties of peritoneal detail

One of the subtleties of interpreting abdominal radiographs is peritoneal detail. It's a difficult region to evaluate, since it is the potential space between all the organs. It is normally filled with fat, including the falciform fat ventral to the liver and mesenteric fat in the omentum. Peritoneal detail means that the contrast between fat and the soft tissue of the organs in the abdomen makes the serosal surfaces of the organs visible. Poor peritoneal detail is the loss of this fat/soft-tissue interface.

Uniform poor peritoneal detail

Causes of poor peritoneal detail are loss, or apparent loss, of fat in the peritoneal space. Thin animals have very little fat to provide contrast, so their detail is poor and the organs are not well seen. Animals with peritoneal effusion (hemoperitoneum, transudate, exudate, urine) have more soft-tissue (fluid) opacity than fat because of all the fluid. In this case, the serosal surfaces are not visible.

Image 1

How do you tell the difference between lack of fat and effusion? The abdominal contour and skeleton will help you differentiate between them. Young animals have open vertebral physes, and typically have poor peritoneal detail (lack of fat, small amount of peritoneal fluid) until 2 to 3 months of age (Image 1). If the animal is thin, the soft tissues over the vertebral column have an undulating or tented appearance, and the abdomen is concave. Conversely, if there is fluid in the peritoneal space, the abdomen will be rounded (Image 2). If you consider the animal's age, body condition and abdominal contour, you should be able to determine if poor detail is due to lack of fat or excess fluid.

Image 2

Masses can cause focal, uniform poor detail by compressing the fat normally present between the serosal surfaces of organs. Look for evidence of a mass effect, such as displacement of structures away from the mass, and a distended abdomen (depending on the size of the mass). Peritoneal detail distant from the mass should be normal if there is no associated effusion or hemorrhage.

Image 3

Mottled poor peritoneal detail

There is another pattern of poor peritoneal detail that looks like a mottled or streaky increase in soft-tissue opacity within the fat (Image 3). This presentation has unique causes, and alternate differential diagnoses. The three differential diagnoses that I use are a small amount of peritoneal effusion, peritonitis and carcinomatosis. Generally the abdominal contour is normal with mottled peritoneal detail. That's because a small amount of fluid will just dissect between some of the fat folds and planes in the mesentery, and is not enough to distend it. Carcinomatosis is a diffuse neoplastic infiltration of the mesentery with multiple nodules, such as the cat in Images 3 and 4. This results in streaks or stripes of soft tissue opacity in the normally homogeneous peritoneal space. A focal peritonitis such as pancreatitis will also cause localized, mottled poor detail.

Image 4

Poor peritoneal detail

Use areas of normal fat opacity to compare to the peritoneum. The retroperitoneal space and the falciform fat ventral to the liver may remain unaffected by the peritoneal disease and give you a normal reference. Interpreting peritoneal detail involves critically evaluating the radiograph and correlating it with the signalment and body condition of the patient.

Differential diagnoses for poor peritoneal detail

Dr. Zwingenberger is a veterinary radiologist at the University of California-Davis.