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MD Consult: Books: Goldman: Cecil Medicine: COMPUTED TOMOGRAPHY

Goldman: Cecil Medicine, 23rd ed.

Copyright © 2007 Saunders, An Imprint of Elsevier

COMPUTED TOMOGRAPHY

CT employs an x-ray source with multiple detectors housed in a circular gantry that rotates 360 degrees around the patient, who lies on a table surrounded by the gantry. The computer processes the data that is displayed tomographically and in multiple planes with thin slices. A three-dimensional reconstruction of the body part can be rendered. This allows for analysis of regions with complex anatomy, such as the pelvis, hip, and spine. These images can be rotated and sometimes made into a plastic model for surgical planning. Newer CT scanners have increased numbers of detectors and are able to image more quickly and with thinner slices. They employ spiral or helical scanning through continuous rotation of the x-ray source and the detectors while the patient moves on the table through the gantry.

CT is useful for evaluating vertebral alignment and fractures as well as abnormalities of the flat bones and pelvis. Complex fractures are best evaluated with CT. Joints that are difficult to assess on radiographs, including the sacroiliac, temporomandibular, wrist, and sternoclavicular joints, are well seen with CT. However, internal derangement of these joints is better appreciated on MRI, which provides better soft tissue contrast.

There are many different iodinated contrast agents that can be injected intravenously to enhance the CT study. The contrast has higher attenuation and spreads hematogenously to areas of increased vascularity and inflammation. MRI with gadolinium is the preferred method for demonstrating pannus and joint inflammation and for characterizing a soft tissue mass, but CT with contrast can be used to determine whether the mass is cystic or solid in patients in whom MRI is contraindicated, such as those subject to claustrophobia and those with aneurysm clips or cardiac pacemakers. Ultrasonography can also be used for this purpose. Fluid collections such as abscesses, seromas, bursae, and ganglia are identified on all three imaging modalities.

CT arthrography may also be performed. This is useful when MRI is contraindicated. Intra-articular injection of iodinated contrast material or room air, or both, coats the joint structures and can be useful for visualization of small and large cartilage defects, fibrocartilage, tendon and ligament tears, and ossified and cartilaginous bodies.

One must keep in mind that the radiation dose from CT is high, especially with the newer scanners. Repeated imaging with CT or a request for CT in a child or pregnant woman must be carefully considered in terms of risk versus benefit. Algorithms to reduce tissue dose are used for children and other vulnerable patients. The resulting images are of lower resolution but usually provide adequate information for diagnosis.

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