Recognizing the Normal Abdomen and Pelvis on Computed Tomography

It is estimated that around 80 million computed tomography (CT) scans of all kinds are performed in the United States each year. Almost 10% of all visits to the emergency department are for abdominal pain of a nontraumatic nature. Many of those patients undergo a CT scan of the abdomen and pelvis performed to detect or clarify their clinical findings. Since the advent of CT, exploratory surgery has become rare and the need for emergency surgery has dramatically decreased.

As with all imaging studies, one of the guiding principles in CT scanning of the abdomen and pelvis is to maximize the differences in density between tissues to best demonstrate their unique anatomy. Toward that end, CT studies make extensive use of intravenous and oral contrast agents.

CT scans can be performed with and/or without the intravenous administration of iodinated contrast material but, in general, they yield more diagnostic information that is more easily detectable when intravenous contrast can be used.

CT scans done with intravenous contrast are called contrast-enhanced or simply enhanced . Typically, the radiologist will choose the scanning parameters to optimize the CT study for the patient's particular clinical issues. For example, different rates of contrast administration and timing of the scan will allow diagnostic enhancement of hepatic vessels versus the liver parenchyma.

Although it might sound like a great idea to give everyone contrast, keep in mind that iodinated contrast can have adverse effects and produce serious reactions in susceptible individuals ( Box 14.1 ).

For abdominal and pelvic CT imaging, oral contrast may also be administered to define the bowel, although its use has diminished as the quality of CT images has improved. Oral contrast is usually not employed in chest CT scanning unless there is a particular question concerning the esophagus.

Orally administered contrast, frequently given in doses divided over time to allow earlier contrast to reach the colon while later contrast opacifies the stomach, is utilized for many abdominal CT scans except those performed for trauma , the stone search study , and studies specifically directed toward evaluating vascular structures such as the aorta.

One of two different types of oral contrast may be used. The most widely used is a dilute solution of barium sulfate , the same contrast agent employed in upper gastrointestinal studies and barium enemas. If there is concern for bowel perforation and the possibility that contrast may exit from the lumen of the bowel, an iodine-based, water-soluble contrast is sometimes used (e.g., Gastrografin ). Contrast may also be introduced rectally to opacify the distal colon more quickly than it would take for orally administered contrast to reach the large bowel or through a Foley catheter to quickly opacify the urinary bladder.

You will probably not be required to make the decision of when or if to use contrast as the radiologist will usually tailor the examination to best answer the clinical question being asked . That means it is always important to provide as much clinical information as possible when requesting a study.

Table 14.1 summarizes, in general, when intravenous and oral contrast are utilized for particular problems.

Table 14.2 outlines some of the common patient preparations that are generally suggested for a variety of imaging studies. Preparation instructions may vary depending on the facility and individual patient needs. In general, a patient is permitted to take medications with a small sip of water, even when instructions are nothing to eat or drink before the study.