Type:
Educational Exhibit
Keywords:
Cerebrospinal fluid, Myelography, MR, CT, Neuroradiology spine, CNS
Authors:
G. Harisis, M. Lukies, A. Jarema, M. Scicchitano; VIC/AU
DOI:
10.26044/ranzcr2019/R-0008
Imaging findings OR Procedure details
MRI
MRI is the imaging modality of choice, with routine axial and sagittal T1, T2 and T2 fat saturation sequences usually sufficient. Post contrast imaging can also be performed to exclude the presence of an enhancing mass.
Several imaging features can help differentiate arachnoid webs from ventral cord herniation and spinal arachnoid cysts2,3.
- The classical imaging finding of a dorsal arachnoid web is that of a focal deformity of the dorsal spinal cord with associated prominence of the dorsal CSF and obliteration of the CSF space ventral to the cord2,3. It has been suggested that the shape of the expanded dorsal CSF bares resemblance to a surgical scalpel, and hence this has been termed the ‘scalpel sign’ (Figure 2).
- While this sign is an important imaging feature, a similar finding can be seen with arachnoid cysts, which makes differentiation of these entities challenging3. One clue that favours an arachnoid cyst as the underlying aetiology is the presence of wide scalloping of the dorsal cord surface3. The thin wall of an arachnoid cyst may also be perceptible on T2 sequences whereas arachnoid webs are not routinely discernable on MRI3 (Figures 1 and 3).
- Differentiating arachnoid webs from ventral cord herniation can also be problematic. In contradistinction to the abrupt calibre change caused by the indentation of the chord by an arachnoid web, the deformity seen in ventral herniation has been described as a smooth 'C-shape'2. Importantly, no fluid signal should be observed at the ventral aspect of the cord (Figures 4 and 5).
CT Myelography
- Due to its higher spatial resolution, CT myelography can be a useful adjunct when findings on MRI are equivocal.
- Myelography is performed by administering intra-thecal contrast prior to CT acquisitions. At our institution, a lumbar puncture is performed under fluoroscopic guidance and approximately 12 mL of Omnipaque 240 contrast is infiltrated into the intra-thecal space. The patient is kept supine and a CT of the spine is then performed at 30 min with images reviewed while the patient remains on the table. Depending on the findings, repeat imaging is considerd after a further short time interval.
- Arachnoid webs can be differentiated from arachnoid cysts as the latter will demonstrate slow filling relative to the remainder of the CSF space. This is best appreciated with short interval scanning (i.e 30 min and 90 min after contrast injection), Figure 6.
- The key to differentiating arachnoid webs from ventral cord herniation is the identification of CSF constrast anterior to the cord at the level of the cord deformity3. If this is present, then cord herniation can be excluded (Figure 7).
Intra-operative Ultrasound
- Although not routine, intra-opertive ultrasound is effective in visualisation of the thin arachnoid tissue that constitutes a web.
- Figure 8 demonstrates an ultrasound of the cord immediately prior to surgical lysis of the arachnoid web in the patient from our case report.
- Following adhesiolysis, the cord can be seen pulsing freely with the cardiac cycle (Figure 9).