In this study, we obtained microstructural parameter (fractional anisotropy (FA)) of the cervical spinal cord in patients with suspected cervical compressive myelopathy (CSM) and compared DTI parameters (FA values) with the information obtained from conventional MRI sequences.
Our vision was to correlate DTI parameters (FA values) measured in patients’ cervical cord levels opposite to normal and affected discs, as well as to identify the best location to measure FA values within the cord that reflects the severity of myelopathy.
Our study showed that DTI is 93% more sensitive in detecting early myelopathic changes than the conventional MRI. These results are consistent with Facon et al. , Kara et al. , Yoo et al. , Banaszek et al.  and Nukala et al. .
The mean FA value of the spinal cord opposite to normal disc levels in our study was 0.742, compared to 0.745 as found by Facon et al,  and 0.734 by Uda et al, . On the other hand, other studies showed variability in the mean FA values of the spinal cord ranging from 0.65 by Kara et al.  to 0.58 by Banaszek et al. .
In Facon et al.’s  study, the normal FA measurements were made in healthy volunteers at three different levels (cervical, C2–C5; high thoracic, T1–T6; and low thoracic, T7–T12) by using regions of interest located on the spinal cord. Special attention was paid to avoid CSF partial volume effect . In our study, due to relative stability of the upper cervical region and lack of related discal lesions, we obtained a normal FA value for each patient at C2-C2/3 level, to use as internal reference to reduce the variability of FA values due to different ages and sex. Besides, we measured FA values opposite to all cervical disc levels not specific discs levels; we also paid special attention to avoid CSF partial volume effect.
In our study, we compared the mean FA values of the whole cord cross section opposite to normal cord level (C2) and most affected disc level. Our results showed highly significant reduction of FA values of the whole cord cross section, opposite to the level of the most affected disc (P value = 0.001).
This matches with Song et al. , Hori et al. , and Kerkovský et al.  who reported significant FA reduction between compressed and non-compressed cord.
The anterior part of the spinal cord is the most affected site by degenerative myelopathy changes detected on DTI .
The anterior portion of the cord white matter opposite the affected disc level is more vulnerable to compressive myelopathic changes because it lies just posterior to the compressing factors such as degenerated discs and ossification of a posterior longitudinal ligament. So, DTI parameters (FA and ADC) are more affected at this site. On the other hand, DTI parameters in the posterior white matter remained unaffected. Also, the lateral white matter opposite to the affected level shows no significant changes of FA values, as the inflammatory cells proliferation and gliosis may help in its protection or reversal of damage .
When the spinal cord is displaced posteriorly by the spondylotic bar, the dentate ligaments resist this displacement and their dural attachments act as a fixed point, and this tensile stress is transmitted to the lateral columns which are more subjected to CSM while the anterior columns and the posterior columns are relatively unstressed .
We compared the mean FA values opposite to normal cord level (C2) and most affected disc level at 3, 6, 9, and 12 o’clock and we found that the most significant reduction of FA values was consistently noted at 12 o’clock position (P value = 0.001). These findings agree with Yoo et al.  and Sąsiadek et al. , and disagree with the dentate theory suggested by Levine .
By comparing FA values of the whole cord section, at 12, 3, 6 and 9 o’clock below the most affected disc with FA values opposite to normal cord level (C2-C2/3), we found highly significant FA value reduction with P value = 0.001. This matches with Kamble et al.  who found that FA values are decreased below the site of injury, likely due to Wallerian degeneration.
One limitation of the applied MRI protocol in this study was the relatively long scan time compared to the conventional MRI examination protocol which might represent burden for patients suffering from neck pain. Also, the processing of the DTI images was time consuming as we calculated the FA values of the whole cord circumference and opposite 3, 6, 9, and 12 o’clock positions at multiple cervical disc levels.
Another limitation of this study was the relatively low signal to noise ratio of DTI images obtained with the 1.5-T machine used in the study. We think that performing the examination using 3-T machines may improve the signal to noise ratio with more accurate assessment of DTI parameters and also the scan time can be shortened.