Lumbar nerve entrapment causes low back pain and radiculopathy, but discrepancies between clinical symptoms and the range of compression of the nerve root on conventional MR images are often found. Asymptomatic intervertebral disc degeneration and herniation are often seen in clinical practice, and these discrepancies can confuse spine surgeons.

Lumbar foraminal stenosis is a condition in which a nerve root or spinal nerve is entrapped in a narrowed lumbar foramen in degenerative lumbar spinal disorders. There is a dorsal root ganglion that functions as a pain receptor at this site, so this condition is refractory and can cause severe lower limb pain. However, lumbar foraminal stenosis is often overlooked, accounts for approximately 60% of failed back surgery syndromes, and plays a major role in lowering surgical success rates. Conventional MR imaging has been reported to produce false-positives in 30–40% of lumbar foraminal stenosis cases; therefore, this is a difficult condition to diagnose. New diagnostic imaging techniques to detect lumbar nerve root entrapment are urgently required.

We have shown that through the use of DTI in lumbar nerve lesions such as lumbar foraminal stenosis and lumbar disc herniation, it is possible to image interruptions of tractography at stenotic sites, permitting the diagnosis of stenosis. In compression lesions, DTI can reveal significant decreases in fractional anisotropy (FA) values with significant increases in ADC values.^1–4 Furthermore, there were strong correlations between FA value and indications of neurologic severity in patients with radiculopathy caused by lumbar disc herniation.² DTI parameters of the lumber nerve can be used for diagnosis, quantitative evaluation, and follow-up of lumbar nerve entrapment.

Recent advances allow the use of higher magnetic fields and pulse sequencing improvements that provide better neuroimaging, such as DTI in the lumbar nerve.

DTI of spinal nerves is more likely to be affected by magnetic susceptibility than the brain, so from a technical viewpoint, it can hardly be considered a widely employed test in clinical practice. We attempted to image high-resolution lumbar nerves using a reduced field of view (rFOV) method. Compared with traditional methods, rFOV allows for clear imaging of the lumbar nerve with good SNR and enables accurate measurements of the DTI parameters.

In the future, the introduction of high-resolution DTI will allow a more detailed visualization of lumbar nerve lesions that may be quantified into numeric data, and we look forward to further expansion in the functional diagnosis of lumbar nerve disorders.

References

1. Kanamoto H, Eguchi Y, Suzuki M, et al. The diagnosis of double-crush lesion in the L5 lumbar nerve using diffusion tensor imaging. Spine J 2016;16:315–21, 10.1016/j.spinee.2015.11.003
2. Eguchi Y, Oikawa Y, Suzuki M, et al. Diffusion tensor imaging parameters of radiculopathy in patients with lumbar disc herniation: preliminary results. Bone Joint J 2016;98-B(3):387–94, 10.1302/0301-620X.98B3.36036
3. Oikawa Y, Eguchi Y, Inoue G, et al. Diffusion tensor imaging of lumbar spinal nerve in subjects with degenerative lumbar disorders. Magn Reson Imaging 2015;33:956–61, 10.1016/j.mri.2015.05.002
4. Eguchi Y, Ohtori S, Orita S, et al. Quantitative evaluation and visualization of lumbar foraminal nerve root entrapment using diffusion tensor imaging: preliminary results. AJNR Am J Neuroradiol 2011;32:1824–29, 10.3174/ajnr.A2681

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