The goal of this work was to demonstrate the feasibility of a column-specific analysis of the cervical spinal cord using quantitative MT- and DTI-derived quantities and to assess the associated cross-sectional, inter-rater, and test–retest variability. The assessment was performed across healthy volunteers, between the dorsal and lateral columns, and between the left and right lateral columns. The ability to reconstruct the fiber tracts with DTI in conjunction with coregistered MT data in the cervical spinal cord may increase the reliability of tract assessment in healthy and diseased spinal cords and have direct clinical relevance.
The study presented here differs from conventional ROI-based approaches in that MRI quantities are derived from tract reconstructions in a procedure known as spatially normalized tract profiling. Similar methodology has been applied to the corticospinal tracts and optic radiations in the brains of people with MS and healthy controls (23
). It may seem that there is little need to apply fiber tracking to the spinal cord, where fibers are known to run generally in a rostrocaudal direction; technically, it would certainly be possible to draw ROIs on each slice on the high-spatial-resolution maps obtained with MTCSF imaging. However, the reproducibility of tractography and its relative speed compared to manual ROI placement will help to translate the methodology into more routine clinical use. Since only a few ROIs need to be drawn per subject, the tract-based approach is expected to reduce the subjectivity of manual ROI placement.
Currently, no tractography-derived, individual-column MTCSF and DTI quantities are available in the literature for comparison with our results, but whole-cord values (5
) and ROI-based examination of the MTCSF values in the columns of the spinal cord (4
) have been reported. Our column-specific measurements of λ||
are at the high end of the previously reported whole-cord range of 1.50–2.26 μm2
/ms, and our λ
measurements are in the middle of their reported range of 0.40–0.92 μm2
/ms. The column-specific MD is in the middle of the whole-cord range (0.90–1.29 μm2
/ms), whereas the measured FA values are at the higher end of the range (0.43–0.83) and are consistent with FA values in the dense white matter tracts of the brain (24
). It should be noted that DTI-derived indices are sensitive to the sequence design parameters and SNR (28
) with the former having a large impact on MD, λ||
, and λ
and the latter playing a large role in determining the bias in FA. Thus, when comparing these values to the literature, care should be taken to verify the similarity of the sequences chosen for comparison.
Mean MTCSF values for the lateral and dorsal columns cannot be compared straightforwardly with previous measures at 1.5T due to their dependence on sequence design, pulse power, and relaxation rates (which are field dependent). Despite these considerations, the delineation between white and gray matter on the MTCSF images is clearly more apparent at 3T than at 1.5T (22
). To reduce cross-study variability, it would be useful to define a set of standard values for B1, irradiation offset and duration, and steady-state timing parameters. In this work, we chose to use an offset frequency = 1.5 kHz for our MTCSF calculation to visually maximize the discrimination between white and gray matter to facilitate ROI placement and tractography while still giving rise to a sufficient MT effect. While this value is suspected to be less than ideal for maximizing the MT effect, further studies and simulations may be undertaken to generate optimal scanning parameters.
A recent study explored the mean and standard deviation of MTCSF measurements derived from ROIs placed in the dorsal and lateral columns of the spinal cord in healthy volunteers and patients with multiple sclerosis (4
). The normal range of the white matter MTCSF values in that paper was 0.48–0.50, slightly lower than the MTCSF values derived from the tract profiles reported here (0.50–0.53).
Reproducible measurement of column-specific, multimodality MRI quantities in the cervical spinal cord allows investigation of column-specific disease processes. Since these columns are somatotopically organized (34
), disease confined to the lateral columns, for example, is more likely to impact motor function than sensory function. With the column-reconstruction method presented here, it may be possible to use profiles of each quantity to probe more specifically the structure-function relationship in health and disease. This may have applications in monitoring the effects of rehabilitative therapies or to detect column-specific damage prior to the onset of clinical symptoms.
All of the MRI quantities reported here have normalized differences <10%, however, two points of caution should be noted. First, even though the normalized difference does an adequate job of giving an impression of the percent variation in a dataset, it can be disproportionately affected by data where their means are close to zero, as is the case for test–retest λ
= 8.44%). Second, the variability presented here is sequence-and resolution-dependent. Therefore, it is conceivable that at lower resolution, where the signal-to-noise ratio is higher, the normalized differences could be smaller. However, this effect might be mitigated by worse partial-volume effects. Simulations are necessary to determine the optimal resolution to obtain reproducible data in the spinal cord.
Although parallel and perpendicular diffusivity values can be difficult to interpret in the context of the altered tissue microstructure that occurs in disease processes or in situations where multiple tracts cross (35
), this situation may be less troublesome in the spinal cord, where the individual columns can be more easily separated, than in the brain. One way to increase the reproducibility of perpendicular diffusivity measurements is to study the directional diffusivity directly. Recently, our group has examined the ability of high b-value diffusion-weighted imaging with q-space analysis to detect spinal-cord damage (15
). With appropriate coregistration techniques, such quantities can be examined along the column of interest in a manner similar to the one described here.
A surprising finding in our work is that both the diffusion and magnetization transfer indices that we studied do not show appreciable signal intensity differences between the lateral and dorsal columns. Histology shows that the axonal density differs by column, with the lateral column being comprised of fewer, larger, and more heavily myelinated axons than the dorsal column. With respect to MTCSF, these considerations may be offset by similar amounts of myelin in the dorsal and lateral columns (i.e. even though the individual axons of the dorsal column are surrounded by less myelin, there are more axons per voxel in the dorsal column).
Diffusion indices, on the other hand, are expected to be sensitive to the microenvironment of the tissue. However, this sensitivity is lowered by methodological limitations. The experiments presented here use a relatively low b-value (b = 500 s/mm2
) for diffusion weighting compared to the generally accepted range of 700–100 s/mm2
for DTI in the brain. The lower b-value was chosen because it provided sufficient detectable signal in the low SNR environment, while at the same time, permitting sufficient diffusion weighting to quantify diffusion anisotropy. It is conceivable that DTI (which assumes Gaussian diffusion) at low-b-values (<1500 s/mm2
) may not be capable of distinguishing the different diffusion environments since the signal attenuation in this regime is likely to be dominated by the fast diffusion which may not be sensitive to the highly restricted diffusion of water within WM fiber bundles. Recently, our group demonstrated that the signal attenuation due to water diffusion in the human spinal cord is not mono-exponential (15
) when measured over a wide range of b-values (up to 5000 s/mm2
. It is hypothesized that a tract-specific study of diffusion over a wider range of b-values will demonstrate the microstructural differences that are known to exist histologically between the two columns.
We note two additional limitations of the current study. The first is the choice of two different registration schemes: FLIRT to register DTI volumes to the initially acquired b0
and AIR to register the MT-weighted volume to the mean b0
. In principle, either AIR or FSL could have been used to register both MT and DTI datasets. In prior work, we have optimized FLIRT-based registration of diffusion-weighted volumes (15
). Although we did not perform a sophisticated analysis and comparison of registration algorithms, we found in the current work that AIR appeared to supply the most consistent registration of the MT-weighted data sets to the mean b0
acquisition. We note that the two-step process to fine-tune the registration could potentially be simplified using other registration algorithms and cost functions.
A second limitation of our results is that, even at 3T, the resolution obtained with the DTI acquisition remains coarse relative to the sizes of the spinal-cord structures we would like to assess. The spinal cord is approximately 1.5 cm in diameter, and the individual columns are substantially smaller — on the scale of our acquired voxels. Therefore, partial volume effects heavily impact the accuracy and potentially the sensitivity of the derived quantities to differences in tissue microstructure. We expect that the higher SNR and spatial resolution that will be available at higher field strengths will enhance the ability for DTI and MT to sensitively and accurately probe microstructure in both healthy and diseased spinal cords, not only in the cervical region but also in the thoracic and lumbar regions.