Low back pain is one of the most common health problems among population all over the world . About 75–84% of the general population suffers from low back pain and 5–10% of them suffering severe morbidity . Men and women are equally affected, and 50% of adults and 30% of adolescents are affected at least once . Lumbar disc prolapse is considered one of the most common causes for low back pain . Disc prolapse mechanically compresses adjacent nerve roots causing inflammatory changes, leading to sensory manifestations, motor abnormalities or sphincteric troubles .
The degree of nerve root compression observed in MRI is often inconsistent with the patient’s symptoms. Tractography is to illustrate the orientational architecture of tissues by combining pathways of maximum diffusion coherence . High ADC values are consistent with increase of the extracellular space, usually representing edema regardless of its origin. FA is an index of fiber organization, which reflects the integrity of axonal bundles and the directionality of water movement delimited by physiological barriers such as myelin sheaths, endoneurium, perineurium, or epineurium .
Our results were consistent with previous articles from the literature such as Balbi et al. , Shi et al. , and Zhang et al.  and revealed that the mean FA value for the compressed nerve root was significantly lower than that of contralateral healthy nerve root, and the mean ADC value of the affected nerve root was significantly higher than that of contralateral healthy nerve root.
These changes in diffusion parameters might be related to histological changes occurring in the compressed nerve root. As described in experimental studies, chronic compression and chemical irritation of lumbar nerve root lead to variable histological changes, including the acceleration of vascular permeability with disrupted nerve root barrier and development of intraneural edema and hyperemia in and around the nerves. Also, compression induces reduced blood flow and ischemia leading to demyelinated nerve fibers, Wallerian degeneration, and endoneural cracking . These microstructural changes occurring in compressed nerve roots lead to increase diffusion in perpendicular plane to the largest eigenvalue resulting in a decrease in FA, indicating a more isotropic diffusion in the tissue. As for the increase in ADC, we may assume that the intraneural edema and the increased distance in axon fascicle occurring in a compressed nerve root increases the water diffusivity along the nerve bundles. These hypotheses may be corroborated by two previous experimental studies reporting a decreased FA in rat and frog segments of sciatic nerves undergoing Wallerian degeneration . However, due to various factors (i.e., magnetic intensity, scanning parameters, motion and chemical shift artifacts, and ROI differences), the differences of the results in DTI metrics were frequently reported in previous studies as Wu et al. .
Our study also found that patients with disc extrusion had significantly lower mean FA values and higher mean ADC values than those with disc protrusion and those with disc bulge. Patients with disc protrusion had significantly lower mean FA and higher mean ADC values than those with disc bulge, which is consistent with other study by Zhang et al. .
Our study revealed that there was a statistically positive correlation between FA values of affected nerve and JOA scores. There was a statistically negative correlation between ADC values of affected nerve and the JOA scores, which is consistent with other study by Eguchi et al. .
In accordance with other study by Zhang et al. , our study revealed that there was a strong negative correlation between FA values of affected nerve and the VAS scores. There was also a strong positive correlation between ADC values of affected nerve and the VAS scores Contrary to our findings, another study by Takashima et al.  failed to detect any significant correlation between ADC values and VAS scores. They attributed their findings to the difference in the way of measuring of ADC, using short TI recovery as the DWI fat suppression method or the water-selective excitation technique as the DWI fat suppression method, which yields a higher signal-to-noise ratio than the short TI recovery method.
DTI has a few limitations. Firstly, tractography is numerical modeling of the diffusion tensor data using probability theories to model the most likely course of diffusion, and the number of tracts visualized by DTI did not present the real volume of nerve fiber trajectories. Second, the magnetic susceptibility effect and increases in motion artifacts lead to signal irregularities and image distortion so that nerve fiber follow-up is limited in areas with artifacts. Third, the evidence is insufficient to support DTI as a diagnostic tool or predictor of clinical outcomes. Fourth, automatic analysis methods such as tract-specific automatic ROI placement are needed .
When patients’ symptoms are inconsistent with MRI findings, or when MRI reveals multi-stage disc prolapse, accurate diagnosis can reduce the degree of decompression, limit surgical trauma, and diminish the cost of hospitalization. Recent studies have shown the application of MRI combined with DTI or paraspinal mapping (PM) can reduce false positives on MRI and also replace invasive methods such as discography and selective nerve root block to further enhance diagnosis .