Diffusion tensor imaging and diffusion tensor tractography have been used successfully to assess the demyelinating or inflammatory disease burden [12, 13]. DTI can be used to assess the integrity of fiber tract and to guide the management of the spinal cord tumors [14, 15]. Also, DTI and DTT can be used to assess the functional state and prognosis of traumatic spinal cord lesions [9].
In this study, we found a statistical significant difference between the mean FA, RA, VR, E2 and E3 values between the lesion level and both control levels. We found also no statistical significant difference between the mean of ADC and E1 at these levels.
Our result for traumatic cord lesions, the FA and RA showed statistically significant reduction with no statistically significant raised ADC or E1 values. In agree with our result and on studying traumatic spine lesions by Alizadeh, they attributed that the decrease in FA may indicate degeneration of the fiber architecture, specifically the myelin will permit diffusion perpendicular to the neuronal axis, thus lowering the degree of FA. Although not statistically significant, the increase in MD may be attributed to an increase in cord edema [16].
Also, Facon previously found in their cohort no significant change in ADC values with a statistically significant difference for FA in patients with spinal cord injury and compression [17]. Werring and Beaulieu explained these findings in degenerated sciatic frog nerve and in human brain, a loss of anisotropy was noted, but average diffusivity may remain unchanged owing to an accumulation of cellular debris from a breakdown of the longitudinal axonal structure, while glial proliferation may hinder water movement in a parallel direction [18, 19]. In vitro study was carried out on spinal cord specimens of Indian calf (Bos primigenius indicus). The specimens consisted of the spinal cord surrounded by the vertebral column and paraspinal muscles. In the compression injury sites, FA, RA and VR were significantly altered, and more importantly, there was a progressive decrease in the values as the severity of compression increased. On the contrary, the ADC and E1 values again did not show significant changes at the sites of pathology [20].
This agreed with our findings in each group of pathology studied separately, as Shanmuganathan findings with the FA and RA showing statistical significant differences in spinal cord injury with the greatest reduction were found at the site of spinal cord injury. Also they found significantly increased VR in the hemorrhagic injury site compared to those in control subjects, denoting an increased isotropic behavior at these sites. A high VR means isotropic diffusion. An increase in VR at the site of injury is consistent with corresponding decreases seen in FA and RA when compared with control subjects. The RA followed FA trends and E1 mirror ADC value [10].
Loss of myelin integrity, which is typically secondary to demyelinating lesions, can be assessed with DTI. Demyelinating lesions demonstrate reduced FA values owing to a loss of anisotropy, as well as increased RD values secondary to an increase in water movement in the short axis, which occurs because the normal myelin barrier is damaged. On the contrary, after successful treatment, a decrease in RD and an increase in FA values occur with remyelination, indicating a positive response to treatment [21].
by Kang’s result, which was previously described, was in agreement with our results. They observed significant increase in minor eigenvalues E2 and E3, signifying the increase in transverse diffusion and loss of anisotropism. The preservation of E1, or longitudinal diffusion, reflects the maintenance of axonal integrity [22]. Klawiter described a linear trend for increased λ⊥ with increased demyelination and axonal loss within MS spinal cords [23]. In their study on neuromyelitis optica patient, Qian showed the reduced FA and increased increase in λ⟂ (diffusivity perpendicular to the axonal fibers) in their cohort. This suggests that the most relevant pathological mechanism occurring in the spinal tracts of these patients was probably demyelination [24], while Naismith in their study showed highly increased radial diffusivity, which has been associated with severe demyelination and axonal loss, correlating with the development of persistent T1 hypointensities or “black holes” in gadolinium enhancing brain lesions [25].
Klawiter described that, where λ|| and λ⊥ may be distended in a similar quantity, anisotropy may continue comparatively unchanged and so less informative. This is in distinction to other conditions where a differential increase in λ⊥ or decrease in λ|| may result in reduced anisotropy [23]. As Song found in their animal studies, we have reported that demyelination causes an increase in λ⟂ without changing λ|| [26]. Wang reported in their DTI animal study, which is in combination with biochemical validation, suggesting that increased λ⟂ with no change in λ|| represents an early sign of non-cystic white matter injury with reduced myelination before severe damage in structural integrity and necrosis [27].
When comparing the quantitative DTI indices between the two study groups of pathology, the results showed no statistically significant difference in the measured quantitative parameters for all indices. In agreeing with El Maati who found that the ADC and FA means of the tumor group were not significantly different from those of the non-tumor subjects (p = 0.539) [28]. In their study, Facon and Hassan showed decrease in FA value with variable ADC according to the type of extra-medullary tumor compressing the spinal cord [17, 29]. In agreeing with Facon, the FA measurements proved to have high sensitivity and specificity in the detection of spinal cord pathology in patients with extra-medullary spinal canal tumors [17].
In our findings, we support that the value of RA follows the trend of FA value and inverse to VR, while E1 mirrors ADC. The main contributing mechanism for FA reduction is the raised transverse diffusivity along the minor eigenvectors, which indicates the relative anisotropy and lowers the anisotropism. The preservation of E1, or longitudinal diffusion, reflects the maintenance of axonal integrity.