When is contrast needed for abdominal and pelvic CT?

INTRAVENOUS CONTRAST

All modern IV contrast agents are iodine-based. The iodine causes increased absorption and scattering of the incoming radiation, which serves to increase the attenuation or “brightness” of the tissue or organ.¹ Importantly, the IV contrast used in CT is distinct from the gadolinium-based IV contrast used in magnetic resonance imaging, meaning that there is no cross-reactivity between the two, which is important if the patient is allergic to one of them.

IV contrast is necessary for the evaluation of any kind of vascular disease, since it allows for easy identification of the blood vessel lumen.¹ In abdominal imaging, IV contrast is recommended in most cases.

ORAL CONTRAST

The primary benefit of oral contrast is its ability to distend the bowels to help distinguish them from adjacent abdominal structures.

Oral contrast comes in two forms: neutral or positive. Neutral oral contrast consists of water or a dilute, low-attenuation solution that mirrors water. Positive oral contrast is an iodinated (ie, gastrografin) or barium-based solution with high attenuation that further demarcates bowel by opacification.

The advent of multidetector CT, which offers improved resolution, has made it easier to differentiate abdominal structures without the need for the opacification with positive oral contrast. As a result, some have argued that neutral oral contrast may be preferable to positive oral contrast due to similar efficacy, cost-effectiveness, and easier patient tolerability.²

GENERAL INDICATIONS FOR CONTRAST USE IN ABDOMINOPELVIC CT

The decision to use contrast in abdominopelvic CT depends on the diagnosis suspected.

IV contrast is recommended in most cases (Table 1).³ It is useful in the evaluation of infection (appendicitis, colitis, diverticulitis, pyelonephritis) (Figure 1); inflammation (pancreatitis, inflammatory bowel disease), masses and malignancies; and vascular abnormalities (gastrointestinal bleeding, aortic dissection, abdominal aortic aneurysm). However, IV contrast is not necessary to diagnose bowel perforations, nephrolithiasis, or hematomas.

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Figure 1

Computed tomography with intravenous contrast enhancement in a patient with right-lower-quadrant abdominal pain. Axial (A) and coronal (B) views reveal diffuse mural and periappendiceal edema, with thickening of the appendix (red arrows). Coarse calcification within the appendix (yellow arrow) likely represents an appendicolith.

In vascular imaging, the study of choice is CT angiography, which is timed so that the image is taken when the IV contrast reaches the arterial system, making it easier to identify active bleeding.

The use of oral contrast is more controversial.^4–6 No clear consensus exists on the need for oral contrast, and expert opinion often drives current practices with regard to oral contrast use at academic medical centers.⁶ In general, when the primary reason for CT is to evaluate the liver, gallbladder, pancreas, spleen, adrenal glands, or urinary tract, oral contrast is unnecessary.

Alternatively, when evaluating the gastrointestinal lumen or bowel wall, oral contrast may be beneficial (Figure 2). However, oral contrast is not needed in the diagnosis of appendicitis or diverticulitis, even though both are luminal disorders.⁵

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Figure 2

Computed tomography with a neutral oral contrast agent. Axial (A) and coronal (B) views reveal multiple loops of dilated bowel (red arrows) with a transition point—ie, site of sudden luminal narrowing (yellow arrows)—in the left lower quadrant, findings consistent with a small-bowel obstruction.

Bowel “illumination” achieved with positive oral contrast is useful when searching for “breaks” in the bowel wall, such as what would be seen with fistulas and perforations, or for identifying fluid collections or abscesses between loops of opacified bowel.

Conversely, neutral oral contrast is preferred when evaluating for mural abnormalities or a suspected gastrointestinal bleed, as positive contrast will opacify and mask the luminal surface, making it challenging to identify the bleeding source.⁴

In patients presenting with nonspecific abdominal complaints, some would argue that the addition of the use of oral contrast optimizes the diagnostic yield of abdominal CT.⁶ Woolen et al found that 89% of patients would prefer oral contrast if it had any diagnostic benefit.⁷ Thus, in patients undergoing abdominal CT for vague or nonspecific complaints, the addition of oral contrast to the study appears reasonable.

ADVERSE EFFECTS OF CONTRAST

IV contrast carries a risk of an allergic reaction (incidence 0.6%), which can manifest as urticaria, pruritus, bronchospasm with wheezing, or anaphylactic shock.⁸ Several premedication regimens consisting of a steroid with or without an antihistamine are available for at-risk patients (Table 2).⁸