Figure 1717

Postsurgical vertebral osteomyelitis and epidural abscess. (A) Sagittal T1WI showing endplate irregularities with bone edema suggesting spondylodiscitis (same patient as in Figure 17-8). (B) The best sequence for evaluating an epidural abscess is the postcontrast T1WI because the signals of an abscess and of CSF are both hyperintense, making it difficult to distinguish between them on T2WI. Following contrast administration enhancement at the periphery of the epidural abscess is noted. (C) Sagittal T1WI with IV contrast showing enhancement in both L4 and L5 vertebral bodies and a small epidural abscess at the L4-L5 disc level.

tomy was accompanied by endplate curettage, the MRI picture may mimic postoperative spondylodiscitis.

The best sequence for evaluating an epidural abscess is the contrast T1 study because the signals of an abscess and of CSF are both hyperintense on T2WI, making it difficult to distinguish between them on those images (Figures 17-17B and 17-17C). On the other hand, the two different enhancing T1 patterns contribute to the diagnosis of epi-dural abscesses in two ways: homogenous or heterogeneous enhancement in the granulomatous type of abscess, and the rim enhancement pattern typical of the liquefied type.

Pseudomeningoceles appear as well-circumscribed areas of abnormal fluid collection, similar to the appearance of CSF, which is located posterior to the dural sac (Figure 17-18). There are some variations in the signal intensity according to the composition of the fluid in the meningocele, including an increased signal on T1WI if the fluid is xanthochromatous. The normal posterior bulging of the thecal sac after a laminectomy should be taken into consideration in the differential diagnosis of a pseudomeningocele.

Pseudomeningocele

Postlaminectomy pseudomeningocele.

Sagittal T2WI showing a hyperintense FIGURE 17-19

fluid collection. Postdiscectomy epidural fat graft. Sagit

Postlaminectomy pseudomeningocele.

Sagittal T2WI showing a hyperintense FIGURE 17-19

fluid collection. Postdiscectomy epidural fat graft. Sagit tal T1WI of the lumbar spine showing a fat graft compressing the thecal sac.

Surgeons sometimes use autologous fat grafts to reduce postoperative epidural scarring. Occasionally, these grafts may compress the thecal sac (Figure 17-19). The characteristic appearance of fat grafts is on T1WI, in which they display as a high-signal-intensity mass.

Residual bony stenosis, in particular lateral recess stenosis, is the most frequent finding in symptomatic postsurgical patients. Bony stenosis may have a variable appearance on MRI due to the variability of the marrow content of the bone. Osteophytes or sclerotic bone will have a low signal intensity on both T1WI and T2WI. Bony spurs, on the other hand, may produce a bright signal on T1WI due to their rather fatty marrow.

MRI is also useful after fusion surgery with instrumentation. Placement of spinal implants degrades MRI quality by producing streak artifacts that may make the evaluation of the tissues adjacent to the implants challenging. Artifacts are especially prominent with stainless steel implants. In contrast, titanium implants cause minimal distortion in homogeneity, and spine-echo and FSE sequences show the least interference with metal implants. The use of drills in spine surgery, a technique that is associated with local shedding of metallic debris, may also cause artifacts on MRI.

Cervical Spine

The ability of MRI to evaluate the spinal cord is unsurpassed. On gradient echo images CSF demonstrates greater signal intensity than the cord, producing a pseudomyelographic effect. On the other hand gradient echo images tend to exaggerate the severity of bony canal stenosis. A better evaluation of the latter condition can be obtained by axial T1WI.

MRI images obtained shortly after ACDF surgery will display the graft material within the disc space or cage. The signal of the bone graft compared with that of the adjacent marrow will be varied. This varied appearance depends on the composition of the graft marrow (i.e., cellular, fatty). The endplates adjacent to the operated disc may display features of bone edema. Between 6 and 12 months following surgery, however, the MRI will usually display a bony union (i.e., a continuous marrow signal without evidence of the disc space). It will show either an isotense or a patchy increased signal on T1WI.

Gradient echo sequences are more sensitive to metal artifacts, and therefore are a poor choice for imaging the postoperative cervical spine. It is better to obtain the T2 FSE sequence, which is less susceptible to metal artifacts, thus facilitating the evaluation of residual stenosis or disc herniation (Figure 17-20). Gradient echo sequences also show artifacts within the spinal cord and so they, too, are not suitable for evaluating postoperative spinal cord pathology. Intravenous contrast adds no useful information to the evaluation of the postoperative degenerative cervical spine. Unlike the difficulty encountered in the lumbar spine, it is less problematic to distinguish between scar and recurrent disc herniation in the cervical area. Gadolinium does not cross an intact blood-brain barrier. Gadolinium concentrates in areas of inflammation and infection and in tumors, making these areas more conspicuous on T1WI.

Some shortcomings of the FSE sequence are the possibility to overlook a small hemorrhagic focus in the cord and the difficulty in differentiating between osteophytes and disc fragments. Three-dimensional imaging with a fast imaging sequence (gradient refocused echo imaging, GRE) is preferred to better delineate residual foraminal herniation.

Finally, MRI is useful in evaluating postoperative epidural hematoma (Figure 17-21), and in evaluating cord pathology, such as myelo-malacia or cord atrophy.

Contraindications for Performing MRI

In addition to the classic contraindications to MRI (i.e., cardiac pacemakers, intracardiac wires, various types of artificial heart valves, intracranial aneurysm clips, ferromagnetic intraocular foreign bodies), the only absolute contraindication to MRI after surgery is an implanted epidural spinal cord stimulator.

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