Clinical use of diffusion-weighted imaging and DTI of the human spinal cord faces a number of technical challenges. The small size of the spinal cord necessitates the use of small voxel sizes for spatial resolution, at the expense of SNR. Images may be degraded as a result of macroscopic motion related to cerebrospinal-fluid pulsation, patient breathing/swallowing, or gross patient motion. Local-field inhomogeneity also contributes to image degradation.^1,2 Echo planar DTI can be performed in a reasonable amount of time for clinical use, and single-shot techniques may limit the adverse effect of in-plane bulk motion to some extent. New techniques using parallel imaging,³ as well as pulse triggering or cardiac gating, should further minimize image degradation. In the analysis of DTI maps, care must be taken to avoid including cerebrospinal fluid in the regions of interest, which would produce a misleading change in DTI metrics (decreased fractional anisotropy and increased mean diffusivity). The synergistic advantage of combining DTI of the cervical spinal cord at 3T with parallel imaging techniques using Siemens Medical's Tim technology is that together, they provide increased SNR, create increased temporal resolution (more rapid scanning), and overcome some artifacts, such as susceptibility and physiological motion. This synergy produces improved reproducibility and wider clinical applicability for DTI of the cervical spinal cord at 3T.

Figure 1. Diffusion tensor imaging maps through the cervical spinal cord at the C4/5 level comparing A (whisker plot), B (color fractional anisotropy), and C (black-and-white fractional anisotropy) at 1.5T (top row) and 3T (bottom row) [Magnetom Trio with Tim technology from Siemens Medical Solutions].

DTI has shown promise in the evaluation of white-matter–tract integrity, and has been demonstrated to be able to detect changes due to spinal-canal stenosis affecting the spinal cord,^4,5 typically consisting of decreased fractional anisotropy and increased mean diffusivity. These changes in fractional anisotropy and mean diffusivity may not be sufficient to differentiate between potentially reversible edema and irreversible gliosis in patients with spondylosis. The evaluation of the diffusion eigenvalues (from which fractional anisotropy is calculated) may, however, assist in identifying subgroups of patients with axonal damage (longitudinal diffusion eigenvalues) versus reversible edema (transverse diffusion eigenvalues).⁶ This has important implications for whether a patient would benefit from neurosurgical decompression instead of more conservative medical management. The improved SNR, shorter scanning times, and resultant decrease in motion artifacts at 3T allow more reliable and robust data sets than those obtained at 1.5T, permitting this analysis (Figure 1). The maps obtained at 3T demonstrate improved SNR and better delineation of the spinal cord's gray matter from white matter. The combination of higher SNR at 3T, parallel imaging, and shorter scan times improves image quality and decreases artifacts associated with scanning the spinal cord.

Figure 2. (A) Sagittal T2 weighted image in a patient with neck pain, demonstrating some abnormal increased T2 signal within the spinal cord at the C4/5 level (white arrow). (B) Diffusion tensor imaging and fiber tractography at 3T (Magnetom Trio with Tim technology from Siemens Medical Solutions) demonstrates abnormal diffusion within the posterior columns of the spinal cord in patients with MS.

A common clinical problem faced by the neuroradiologist is finding abnormal T2 signal within the cervical spinal cord and deciding whether the changes are due to degenerative spinal-canal stenosis or to primary demyelination, as seen in MS (Figure 2). In a patient with some mild degenerative spinal-canal stenosis, the differential diagnoses are cord edema and myelomalacia from degenerative spinal stenosis (versus primary demyelinating disease from MS). In degenerative spinal stenosis, the changes in diffusion usually affect the anterior spinal cord.

Evaluation of the spinal cord has become increasingly important in both the diagnosis of MS and the follow-up care of MS patients, as changes in the spinal-cord function of MS patients have been shown to correlate with clinical disability.⁷ DTI changes in MS patients recently have been detected in the cervical spinal cord using a sagittal DTI technique and evaluating large regions of interest using histogram analysis.^8,9 It is generally accepted, however, that the lesions of primary demyelination have a predilection for the posterior columns of the spinal cord; therefore, analysis of DTI metrics in different regions of the spinal cord may demonstrate spatial differences. Bot et al investigated the spinal cords of postmortem MS patients using high-field MRI with histopathologic correlation.¹⁰ Histopathology showed significant axonal loss, increased axonal diameter, and decreased myelin density in normal-appearing spinal cord in MS patients. Using DTI, fractional anisotropy is significantly lower in the normal-appearing spinal cord of MS patients in the lateral, posterior, and central cord, compared with controls.¹¹ Hence, DTI can help in differentiating MS from other causes of abnormal spinal-cord signal by demonstrating changes in fractional anisotropy and mean diffusivity within the posterior columns (versus changes occurring within the anterior columns in degenerative spinal-canal stenosis). Measurement of DTI metrics in the cervical spinal cord may prove useful not only in aiding the diagnosis of MS and correlating with clinical disability, but in monitoring disease progression and therapeutic effects, as well.

DTI at 3T also shows promise in the evaluation of other spinal-cord pathologies. Besides MS and degenerative spinal-canal stenosis, it has potential applications in other disorders affecting the cord, such as amyotrophic lateral sclerosis, spinal-cord mass lesions, spinal-cord trauma, and vascular lesions of the cord, as well as in the evaluation of recovery from spinal-cord injury.^2,12 Improvements in coil technology, coupled with optimization of pulse sequences and parallel imaging techniques at 3T, will soon make DTI even more accessible and reproducible in clinical practice. This, in turn, will benefit many patients with spinal-cord disease.

Meng Law, MD, is associate professor of neurosurgery and radiology at the New York University Medical Center.

References

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