Diffusion tensor imaging (DTI) is a noninvasive MRI technique that measures the random motion of water molecules and provides information about the cellular integrity and pathology of anisotropic tissues. DTI can provide unique quantitative information on the microstructural features of white matter in the central nervous system .
This study was carried out on 30 patients presenting with spinal trauma and complaining from neurological symptoms and a control group of 10 subjects.
Fiber tractography (FT) is a valuable parameter of DTI measurement; it is a technique that uses specialized tracing algorithms to get a three-dimensional reconstruction of white matter tracts in the central nervous system. It is commonly used for evaluating fiber directions and defects in the brain and spinal cord. It can show the macroscopic orientation of fibers with dramatic representation of the disruption of tracts, which can be hardly seen on conventional MRI, allowing better delineation of damaged fiber tracts in the injured spinal cord [11, 12].
In this study, diffusion tensor tractography (DTT) in the injury site was reconstructed in all patients and controls. Normal orientation of white matter tracts in the cord was visualized in all 10 controls in the form of one bundle of homogenous orange color suggestive of preservation of the integrity of white matter tracts. In the 30 patients with spinal trauma, routine MRI scanning could detect spinal cord signal changes in 23 cases (sensitivity 76.67%), while on tractography, changes in cord integrity were observed in 27 cases in the form of distortion of homogenous orange color with or without interruption. In the remaining 3 patients, the spinal cord appeared grossly normal on qualitative tractographic analysis (Sensitivity 90%).
D’souza, et al.  also found normal orientation of white matter tracts in the cord in all 30 controls. In 20 patients, only 10 patients routine had changes in cord signal intensity (Sensitivity 50%), while tractography detected changes in cord integrity in 12 cases in the form of distortion of the homogenous color denoting disruption in cord integrity (Sensitivity 60%). However, Wang, et al.  found that all patients with abnormalities in DTT had apparent cord abnormalities in cMRI.
In other studies, 3.0-T MR machines were used to measure the fiber density of selected fiber bundles depending on the number of fiber projections per voxel. The study of Wen, et al.  was conducted on 15 healthy adults and 7 myelopathic spinal cord patients with b values of 0 and 600 s/mm2, they measured the tract numbers from C1-7. They found a significant difference in track numbers between the healthy and myelopathic groups; 3064.40 ± 482.38 and 2282.71 ± 293.80 in healthy and myelopathic adults, respectively.
Fractional anisotropy (FA) is the value which signifies the anisotropic part of diffusion as it measures the tendency of water to spread in a preferred direction within a group of axons. It is a function of the axonal density and integrity of white matter fibers, as well as of their degree of myelination .
We found that FA values were the most sensitive parameter of DTI for assessment of TSCI (93.33%). The mean FA values at injury level among patients (0.326 ± 0.135) were significantly lower than in controls (0.532 ± 0.074). This decrease could be attributed to anisotropic diffusion restriction in traumatized spinal cord. FA values indirectly measure the extent of myelination, so, higher FA values indicate the integrity of spinal nerves . No significant difference was observed in the mean FA values above or below the injury level in our patients as compared with controls.
Hassen and El-Kholy  demonstrated a decrease in FA values in TSCI patients similar to our study. Regional measurements of FA values at 5 cord levels showed a significant decrease at the level of cord injury.
Czyz, et al.  also found that FA values in cases with TSCI were significantly lower as compared with the controls (0.48 and 0.55, respectively). Similarly, Rao, et al.  found decreased FA values in TSCI patients compared with neurologically intact volunteers (0.220 ± 0.121 and 0.545 to 0.601 respectively).
Our study was also in accordance with the findings of Shanmuganathan, et al.  and Cheran, et al.  who showed reduced FA values in patients with TSCI as compared with controls.
This study did not reveal any significant difference in mean FA values above or below the injury level. This matches with the same findings reported by D'souza, et al. . However, Kamble, et al.  in their study showed that the FA values in the cord above and below injury level were considerably reduced. They concluded that, as a result of trauma, there was resultant descending as well as ascending Wallerian degeneration, and this fact leads to altered DTI metrics. Similar findings were also found by Mohamed, et al.  who conducted a study on DTI in children with spinal trauma.
The reason for the absence of any variation in DTI parameters above or below the level of injury in our study could probably be due to the timing of the imaging which was held soon after trauma in majority of our cases, whereas other studies have followed up the patients for a longer period of time sufficient for spread of axonal degeneration.
Using Spearman’s correlation test between FA and ASIA impairment scale (A, B, C, D, E), we found that the lower the FA values, the worse the clinical condition (r value= − 0.487). In the study of D’souza, et al. , they found a decrease in FA value reflecting axonal injury, and correlation with clinical status has been detected; the lower the FA values, the more severe is the injury with more serious neurological dysfunction and worse outcome.
The apparent diffusion coefficient (ADC) value was another parameter that we used in the current study; it refers to the overall diffusivity of the tissues whatever the number of barriers to water motion such as myelinated axons, cellular membranes, and extracellular molecules. It is extremely sensitive to the abnormalities typically seen in SCI; when combined with fiber tracking, the damaged areas of the spinal cord can be examined better than with T2-weighted imaging [22, 23].
We determined that the mean ADC value at the level of injury among cases (1.319 ± 0.378) was significantly lower as compared with values in controls (1.734 ± 0.768). No significant difference was observed in mean ADC values above or below the injury level among the cases and controls.
Previous studies have also shown a decrease in ADC values in patients with acute TSCI in accordance with our study as Shanmuganathan, et al.  who reported that ADC is significantly decreased in patients with acute TSCI and patients with spinal cord hemorrhage exhibiting the greatest decrease and Li XH, et al.  who also found that the mean value of ADC was significant decrease at 24h and 72 h in the severely injured group (p < 0.05).
Ellingson, et al.  have reported lower ADC values in the cervical cord of patients with TSCI. They suggested that these low ADC values were due to the decrease in overall diffusion magnitude occurring away from the injury site resulting from the restructuring of the axons and widespread spinal cord degeneration mainly in chronic cases.
However, some other studies reported that ADC is non-significantly different at the site of injury and the control group as the study by Czyz, et al.  who found that the difference was not statistically significant (p = 0.28), but suggested that ADC may serve a potential prognostic factor.
In the present study, due to the significant difference in the FA and ADC values at the site of trauma among patients as compared with controls, it is logical to conclude that DTI with these 2 parameters is a valuable instrument in assessment of the spinal cord following TSCI. And after analyzing the diagnostic accuracy of both parameters, we realized that FA is more sensitive than ADC (93.33% and 66.67% respectively) in diagnosing SCI and differentiating between patients and control groups.
Facon et al.  showed although ADC decreased in the majority of SCI patients, it was not as sensitive as FA in the detection of acute SCI. The authors Fiani et al.  and Song, et al.  suggested that the use of ADC should be restricted to chronic spinal cord compression. However, Shanmuganathan et al.  reported that ADC was the most sensitive marker of acute cervical cord injury and found it to be uniformly decreased in patients with cervical spine trauma.
Spinal cord DTI still has many limitations. The main limitation is its spatial resolution, which is affected by artifacts emerging from cardiac and respiratory motion and cerebrospinal fluid pulsation. Also, the technique is operator dependent especially FT that depends on a qualitative visual analysis by the radiologist. Furthermore, those patients with acute spinal trauma cannot bear up extra scan acquisition time in the MRI suite.
The main limitations in the current study are the small sample size and the difficulty of follow-up of the patients for assessment of axonal regeneration especially after treatment.