Fluid collections in the pelvis can occur for a variety of reasons. Some of the reasons are benign or normal and others indicate the presence of disease. A fluid collection in the pelvis on CT can be free flowing or organized. Free flowing means that the fluid settles into the most dependent part of the pelvis and looks like it’s free and it organized. Fluid collections can be composed of simple fluid, blood, pus in addition to other bodily fluids.
A small amount of free flowing fluid is usually a normal finding in a female who is undergoing menstrual cycles associated with ovulation. This can also be normal in a male but less common. In the absence of any other identifiable abnormalities, a small amount in men can be ok as well. Larger collections are abnormal and raise concern for cancer and liver disease amongst other possibilities.
A fluid collection usually implies more of an organized loculated collection. These are common after surgery like hysterectomy or removal of the prostate. These are called post operative collections. Collections after surgery can occur in the pelvis around the surgical site or under the skin where the incision was made. Fluid collections that are not infected will often go away on their own with time.
A fluid collection in the pelvis on CT can also be blood. A small bloody collection in the pelvis can occur with ovulation. A bloody collection can also occur when an ovarian cyst ruptures. This can be a small amount or a large amount necessitating surgery. Bloody collections can also occur from trauma. Often blood will be distributed throughout the abdomen. Often, an injury will be identified on CT to an organ.
An abscess or pocket of pus is another collection that can be identified on CT in the pelvis. A collection of pus or abscess, will look walled off and sometimes contain gas. This is commonly seen after diverticulitis of the colon. This happens when a small out pouching of the colon becomes inflamed and an abscess forms. Another common cause is an abscess related to pelvic inflammatory disease. This will occur around the ovary. Abscesses can also occur after surgery about the incision or site of surgery in the pelvis.
There are various causes and appearances of fluid collections in the pelvis on CT. Some fluid collections are normal, like that in women around ovulation. Others like abscesses need to be drained urgently. Some collections need to be sampled with a needle to tell whether it is infected or not. Some collections need to be sampled to see if there are cancerous cells. Both the imaging appearance and clinical history are therefore very important to identify the cause of the collection.