The wide canal sign is a radiographic sign resulting from an increase in the anteroposterior diameter of the spinal canal, on midline sagittal MR images 1. It is considered a reliable indicator to differentiate between degenerative and isthmic spondylolisthesis 2.
Measurement
The sagittal canal ratio is calculated by measuring the maximum anteroposterior diameter of the canal at the suspected level and dividing it by the maximum anteroposterior canal diameter at the level of L1.
Interpretation
A ratio of 1.25 or greater is considered a reliable predictor of the presence of defects of the pars interarticularis at the level of spondylolisthesis 1,2.
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