Perhaps the most important indication for a CTA is a failed MR arthrogram. If iodinated contrast is mixed with the gadolinium contrast agent before injection, a single-contrast CTA can be performed immediately at the termination of the MR examination without an additional injection. Despite patient screening and the patient's and the technologist's best efforts, MR examinations do fail. Aside from equipment failure, causes of a failed MR examination include severe claustrophobia, inability of the patient to cooperate, unusually large patient size, and metal or postoperative artifacts.

Major reasons to perform a CTA in lieu of an MR arthrogram include severe obesity or extremely large body habitus, implanted MR-incompatible metal devices or hardware (Fig. 1), metallic hardware, metal fragments or shrapnel adjacent to the joint, claustrophobia, and a previously failed MR examination. In some instances, CTA is performed because access to MR equipment is limited.

MR examinations usually are performed to image joints because MR is non-invasive, has high soft-tissue contrast, and is able to image directly in the coronal and sagittal planes. Although in routine clinical practice CT has higher in-plane spatial resolution than MRI, soft-tissue contrast is limited on CT images and patients are exposed to ionizing radiation. The introduction of iodinated contrast media into the joint during an arthrogram outlines joint structures, increasing their visibility at CT imaging. CT arthrography is a slightly invasive examination with a nominal risk of contrast allergy and infection.

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