Fluid around the liver on CT has many different causes. Often times, the history and imaging appearance of the fluid can narrow down the possibilities. The fluid can be simple water, complex or bloody in appearance. The fluid can appear free flowing or appear like a pocket of fluid. There can be a trace of fluid or large amount distending the abdomen. All these details are important to determine the cause.
In everyday clinical practice, I see fluid around the liver most commonly because of ascites. This is simple fluid of varying quantity that is usually free flowing. The most common reason is liver cirrhosis. An end stage scaring and failure of the liver which can occur because of numerous diseases. A few common reasons are damage from years of alcohol abuse and hepatitis.
Another common reason is cancer. Advanced cancer which has spread to the abdominal peritoneal cavity can cause ascites. Some of the more common cancers to cause ascites include ovarian cancer, gastrointestinal and pancreatic. Often there will be other evidence of cancer on the CT including spread to other organs, peritoneal cavity and lymph nodes.
Other causes can include heart failure and kidney disease. Fluid around the liver can be caused by bowel abnormalities such as when it becomes obstructed or deprived of blood flow (ischemic bowel). Often there is ascites and fluid around the liver when patients undergo dialysis through the peritoneal cavity. Pancreatitis or inflamed pancreas will often lead to fluid around the pancreas and liver.
Bloody fluid around the liver can be caused by trauma to the liver. These can include lacerations to the liver from injuries. Patients who undergo biopsies of the liver can also have fluid around the liver which can be bloody. Some underlying liver abnormalities can also bleed. Bloody fluid is more urgent and can be life threatening. The distinction can often be made on CT by measuring the density of the fluid which will be higher than simple fluid.
Abscesses or pockets of pus will be localized and not free flowing like ascites or blood. Pockets of pus can form in the liver or around it. One of the most common reasons is infection elsewhere. Risk factors include elderly and immunocompromised patients. On CT these will look like pockets of fluid with a thick wall and gas internally in some cases. These can be treated with antibiotics if small or drainage by image guidance if larger.
Fluid around the liver has many causes as detailed above. Many will be known from the history and appearance of the fluid. Some causes are secondary to other disease processes like liver disease. Bloody fluid will often be a direct result of injury to the liver or from an abnormality which has bled. Abscess will be localized and have gas inside in many cases. The latter two causes need to be treated urgently and can be life threatening.