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Case report.Diagnosis of symptomatic extra-foraminal lumbosacral stenosis using diffusion tensor imaging (DTI).Conventional magnetic resonance imaging (MRI) has sometimes proved inadequate for evaluating symptomatic spinal nerve lesions. DTI has been developed to visualize anisotropy of nerve-fiber tracts to evaluate nerve degeneration. We report a case of nerve compression causing a far-out lesion diagnosed using DTI.A 68-year-old patient presented with an 8-month history of severe right-sided sciatica. Computed tomography and MRI showed right L5–S1 foraminal stenosis and contact of the L5 transverse process and S1 ala without canal stenosis at the L4–L5 level. We evaluated the fractional anisotropy (FA) of the right L5 spinal nerve and compared it with bilateral L3–S1 spinal nerves to determine the L5 spinal nerve compression site.DTI revealed narrowing of the right L5 spinal nerve between the L5 transverse process and S1 ala. FA was significantly decreased in the right L5 spinal nerve between the L5 transverse process and S1 ala. There was no significant difference in the FA of spinal nerves between the right and left sides at L3, L4, or S1. The right L5 spinal nerve from the central spinal canal to the extra-foraminal lumbosacral lesion was exposed during surgery and found to be severely compressed by the L5 transverse process and S1 ala. Postoperatively, the patient's symptoms disappeared immediately.We used DTI to diagnose a symptomatic lesion as an extra-foraminal lumbosacral lesion caused by compression of the L5 spinal nerve at the foramina. Because DTI can quantitatively measure damage to nerve fibers, it may be advantageous for the diagnosis of far-out syndrome.