Figure 1713

Plate pseudoarthrosis. (A) AP X-ray of the lumbosacral junction showing bilateral lateral bony fusion between the transverse processes of L5 and the sacral ala. (B) Axial CT through the same area as (A), showing a plate pseudoarthrosis.

For example, the T1- and T2-weighted images (T1WI and T2WI) reflect different aspects of proton relaxation, and therefore the examined substance will appear differently in the two sequences. This becomes even more intricate as a substance on T2WI will appear differently according to the pulse sequence used for acquisition of the image. Thus, fat will appear bright on fast-spin echo (FSE), whereas it will appear darker or with an intermediate signal when acquired with conventional spin echo.

MRI is an excellent diagnostic tool for providing detailed anatomical features of soft and neural tissues and also for depicting sagittal coronal reformation by showing the spinal canal and intervertebral foraminae. In addition, because MRI is capable of revealing not only morphological alterations but also biochemical changes, it is the imaging procedure of choice for patients following spine surgery. It also has the ability to differentiate between fat, scar tissue, and disc material by using various MR sequences with or without a magnetic contrast medium such as gadolinium diethylenetriamine penta-acetic acid (Gd-DPTA).

Early Normal Postoperative Changes in the Lumbar Spine

A previous laminectomy site is identified on T1WI as a loss of the normal low signal of the cortical bone and of the high signal of the bone marrow If the laminotomy is relatively small, the absence of ligamentum flavum may be the only sign of a laminectomy. The postoperative signal obliterates the typical paraspinal fat-muscle planes. The normal epidural pattern is replaced in the immediate postoperative period by a variable amount of heterogeneous intermediate signal on the T1WI and by a hyperintense signal on T2WI. These altered signals represent soft tissue edema. Discectomy produces an intermediate soft tissue signal anterior to the thecal sac at the site of the disc herniation. This soft tissue signal blends with the posterior disc on T1WI and may produce a mass effect on the dural tube. The amount of anterior extradural soft tissue decreases by 2-6 months after surgery. On sagittal T2WI, a tear in the anulus may be seen as a high-intensity signal, which tends to fade after a few months. To summarize, in the immediate postoperative period, an epidural soft tissue mass effect, which mimics the preoperative findings, is seen at the operative site. The soft tissue mass effect in the immediate postoperative period almost precludes the differentiation between this normal soft tissue response to surgery and a residual or recurrent disc herniation. Often, this postoperative appearance mimics the preopera-tive disc herniation. In some cases, this mass effect can persist, as long as one year after clinically successful discectomy.

Postoperative hematoma will ordinarily show increased signal intensity on both T1WI and T2WI. It may, however, have a variable MRI appearance according to the age of the hemorrhage. Postoperative hematoma can be confused with disc herniation on T1WI and T2WI. A better differentiation is possible with a T1-weighted gradient echo (GE) out-of-phase sequence, in which a hematoma will appear bright.

Bone marrow edema may sometimes be observed on both sides of the disc space following simple discectomy or after interbody fusion. This may be associated with intense back pain and should be distinguished from vertebral osteomyelitis. The absence of fever and normal erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) values usually rule out vertebral infection. Bone marrow edema is usually a self-limiting process.

MRI is the most useful imaging tool for distinguishing residual or recurrent disc herniation from normal postsurgical epidural scar tissue. Intravenous contrast will identify scar tissue because of its relative vascularity (compared with the avascular nature of discal material). It must be remembered that postsurgical changes and resolving hematoma or fluid collection following successful surgical intervention (discectomy or decompression) produce a mass effect on MRI, and this may preclude proper interpretation during the first six postoperative months. Nevertheless, it must be borne in mind that postoperative scarring is part of a normal process.

Although most authorities recommend the use of IV contrast (gadolinium) in the diagnosis of scar tissue, some have found that routine use of contrast material is not mandatory and obtaining FSE or FLAIR T2WI may suffice in the differentiation between scar and recurrent disc. Epidural scarring shows an intermediate signal intensity on both T1WI and T2WI and cannot be easily differentiated from disc material. Intravenously injected Gd-DPTA will enhance the fibrous tissue but not disc material. Thus, the lack of early central (as opposed to peripheral) contrast enhancement is a clear indication of recurrent disc herniation, whereas a more uniform enhancement pattern indicates epidural scarring (Figures 17-14 and 17-15). In most cases MRI

Marrow Edema Cervical Spine
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