Figure 1712

Misplaced pedicle screw. Axial CT cut through a lumbar pedicular level showing bilateral pedicle screw placement. The right-side screw is violating the lateral recess.

CT is an excellent diagnostic tool in the immediate postoperative period for evaluating pedicle screw placement, especially if the CT cuts are done parallel to the pedicular screws and sagittal and coronal reformations are obtained (Figures 17-10 and 17-12).

When evaluating non-union, it is important to obtain thin slices through the intended fusion area by means of coronal and sagittal reformations. For example, plain films may show evidence of bony union, but a CT with coronal reconstructions may indicate that a plate pseudoarthrosis has developed (Figure 17-13). The CT demonstration of bridging bone trabeculae between the instrumented vertebrae may indicate that the fusion is solid. On the other hand, gas in a disc within the fusion area or lucency around pedicle screws may indicate a pseudoarthrosis. The presence of pseudoarthrosis per se is not proof for its being the source of a patient's symptoms.

Stainless steel pedicle screws, and to a lesser extent titanium implants, may cause artifacts on CT scans. It is better in the latter case to evaluate the spine with bone windows or, even better, with myelog-raphy and post-myelography CT. Another advantage of CT-myelogra-phy is that it is a true dynamic examination. Although CT-myelography can also detect arachnoiditis and pseudomeningocele, both entities are better delineated on MRI.

Cervical Spine

There are many pitfalls with CT evaluation of the cervical spine. The cervical cord is poorly visualized, disc pathology is not clearly delineated, and beam hardening artifacts from the shoulders at the cervicothoracic junction (C6 to T2) blur most of the anatomical details. In addition, the use of metal spinal implants blurs the obtained images even further. CT-myelography may help to better visualize the anatomical details, especially in the cervicothoracic junction and in the presence of metal artifacts. Cord pathology, however, will still be missed on CT-myelography.

MAGNETIC RESONANCE IMAGING (MRI)

Although clinical imaging with conventional X-rays or CT is reflective of tissue density, imaging with MR is much more complex.

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