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- Grade A: no or minor stenosis
- A1: rootlets lying dorsally and occupying less than half of the dural sac area.
- A2: rootlets lying dorsally with a horseshoe configuration
- A3: rootlets lying dorsally and occupying more than half of the dural sac area
- A4: rootlets lying centrally and occupying the majority of the dural sac area
- Grade B: moderate stenosis: rootlets occupying the entire dural sac, but rootlets can still be individualized
- Grade C: severe stenosis: no recognizable rootlets with complete effacement of cerebrospinal fluid space, but epidural fat is present posteriorly
- Grade D: extreme stenosis: no recognizable rootlets and no epidural fat posteriorly