Role of diffusion tensor imaging and tractography in spinal cord injury

Abstract

Spinal cord injuries pose grave medical and socioeconomic burdens warranting measures for early diagnosis, triaging, prognostication and therapeutics. Imaging has since long played a pivotal role in this regard, with continuing research and technological advancements opening newer frontiers. One such advanced Magnetic resonance (MR) technique is Diffusion tensor imaging (DTI) which assesses cord microstructure by tracking the movement of water molecules in biological tissues. DTI utilizes the principle of anisotropy exhibited by the normal compact white matter (WM) tracts of the cord, in which direction-dependent water molecular motion is seen along the axonal axis. Disruption of this complex structure in response to injury alters the movement of these molecules, interrupting anisotropy and thereby DTI metrics. Evaluation of DTI images can be done both by quantitative indices, of which fractional anisotropy (FA) and mean diffusivity (MD) are the most commonly used and by qualitative fiber tracking (tractography) methods in which three-dimensional WM tracts are reconstructed by algorithmic post-processing. Reduced FA is consistently seen at injury sites as a direct consequence of disturbance of anisotropy. Diffusivity values are however more variable with both high and low values recorded across studies. 3D tractography images allow visual assessment of cord integrity, morphology, and orientation. Significant correlation is found between DTI parameters and various spinal injury scores. Furthermore, DTI also helps in accurate lesion mapping and in assessing cord changes distant from injury epicenter providing a holistic evaluation. From its inception, consistent progress in the understanding and application of DTI has effectuated its clinical utility and impact. Incorporation into day-to-day diagnostics is however still challenging, due to suboptimal image acquisition, difficult post-processing, and lack of standardized protocols & image interpretation guidelines. Further research with technical validation, development of normative and disease data sets, and histological confirmation will help establish this novel technique in routine diagnostics.

Keywords: DTI, Diffusion tensor imaging; Diffusion tensor imaging; FA, Functional Anisotropy; Functional anisotropy; SCI, Spinal cord injury; Spinal cord injury; Tractography.

Fig. 1

27-year-old female with neck pain; Sagittal (a)T2W images of cervical spine with normal appearing cord; (b) Tractography image showing normal orientation and continuity of cord fibers; Colored (c) FA and (e) ADC maps with three region of interests (ROI)s placed at upper (ROI 10), middle (ROI 9) and lower (ROI 8) cervical levels showing values within the normal range (d) Reformatted color FA map with cord fibers in blue indicating superior-inferior fiber direction.

Fig. 2

Reformatted color FA map of the cervical cord with blue fibers indicating superior-inferior fiber direction.

Fig. 3

24-year-old female with history of road traffic accident; Sagittal (a)T2W and (b)STIR images of dorsal spine showing anterior wedging of D7 to 12 vertebral bodies with variable marrow edema (vertebrae between white dashed arrows in b) with fracture of the posterior elements of D8 (red asterisk in a). Focal intramedullary cord signal alteration at D7-D8 level involving almost the entire cord width; (c) Tractography image showing partial fiber interruption at injury epicenter (yellow arrow). Colored (d) ADC and (e) FA maps with three ROIs placed at injury epicenter (ROI 10), in the rostral cord (ROI 9), and in the caudal cord (ROI 8) showing increased diffusivity and decreased FA at site of injury with abnormal FA values extending for a much longer segment cranially than is apparent on T2W.

Fig. 4

Follow up case of a 50-year-old female with post-traumatic quadriparesis; (a)Sagittal T2 image showing short segment myelomalacia at C5–C6 level (yellow arrowhead) involving almost the entire cord width; Colored (b) ADC and (d) FA maps with three ROIs placed at injury epicenter (ROI 5), in the rostral cord (ROI 3) and in the caudal cord (ROI 4) showing increased diffusivity at and below the site of injury; decreased FA involving a much longer cord segment than is apparent on T2; (c) Tractography image showing partial-thickness fiber alteration and disruption with feathering (yellow arrowhead).