Arachnoid isolation sign: A predictive imaging feature of spinal meningioma on CT-myelogram

Numerous studies have demonstrated the imaging features to differentiate spinal meningioma and schwannoma. However, diagnosis can be controversial among clinicians. The purpose of our study was to evaluate the diagnostic value of a novel imaging finding of meningioma on CT-myelogram (CTM).Imaging findings of patients who underwent surgeries for spinal cord tumors between April 2005 to May 2015 were retrospectively reviewed. Our inclusion criteria were as follows: 1) intradural extramedullary tumor, 2) both magnetic resonance imaging (MRI) and CTM were available. The exclusion criterion was a tumor that showed invasion to extradural space.Thirty spinal meningiomas and schwannomas were identified (eight meningiomas and 22 schwannomas). Among these, two meningiomas showed a characteristic imaging phenomenon on CTM: they were diagnosed as intradural tumors on MR imaging while on CTM, they were isolated by the contrast agent in subarachnoidal space from the intact spinal cord. We named this imaging feature, the arachnoid isolation sign (AIS). None of the schwannomas showed this imaging feature.We identified a unique imaging feature of spinal meningioma on CTM. The sensitivity and specificity were 25% and 100%, respectively. In the current case, its diagnostic value was the same as the calcification on CT. Although the number is still small, the AIS may be useful to discern a spinal meningioma from schwannoma.