MR diffusion tensor imaging and fiber tracking in spinal cord compression

Abstract

Background and purpose: Spinal cord damage can result in major functional disability. Alteration of the spinal cord structural integrity can be assessed by using diffusion tensor imaging methods. Our objective is to evaluate the diagnostic accuracy of apparent diffusion coefficient (ADC), fractional anisotropy (FA), and fiber tracking in both acute and slowly progressive spinal cord compressions.

Methods: Fifteen patients with clinical symptoms of acute (n = 2) or slowly progressive (n = 13) spinal cord compression and 11 healthy volunteers were prospectively selected. We performed T2-weighted fast spin echo (FSE) and diffusion tensor imaging by using a 1.5-T MR scanner. ADC and FA maps were computed. Regions of interest were placed at the cervical, upper and lower thoracic cord levels for the healthy subjects and on the area with abnormal T2-weighted signal intensity in the patients with cord compression. In three patients, we used fiber tracking to locate the areas of cord compression precisely. Data were analyzed by using a mixed model. The sensitivity (SE) and specificity (sp) of imaging (T2, ADC, and FA maps) in the detection of spinal cord abnormality were statistically evaluated.

Results: For the healthy subjects, averaged ADC values ranged from 0.96 10(-3) mm(2)/s to 1.05 10(-3) mm(2)/s and averaged FA values ranged from 0.745 to 0.751. Ten patients had decreased FA (0.67 +/- 0.087), and one had increased FA values (0.831); only two patients had increased ADC values (1.03 +/- 0.177). There was a statistically significant difference in the FA values between volunteers and patients (P = .012). FA had a much higher sensitivity (SE = 73.3%) and specificity (sp = 100%) in spinal cord abnormalities detection compared with T2-weighted FSE imaging (se = 46.7%, sp = 100%) and ADC (SE = 13.4%, sp = 80%).

Conclusions: FA has the highest sensitivity and specificity in the detection of acute spinal cord abnormalities. Spinal cord fiber tracking is a useful tool to focus measurements on the compressed spinal cord.

Fig 1.

A, MR imaging of a spinal cord compression due to a breast tumor metastasis in patient 1. Images are from C7–T10. Color-coded scale related to parameters values. Higher values are coded in red, medium values in green, and lower values in blue. T2-weighted imaging (first image) shows a high signal intensity (white arrow), ADC (second image) is slightly increased (blue-green areas; ADC = 1.1 10⁻³ mm²/s versus 0.84 10⁻³ mm²/s in normal cervicothoracic area) and FA (third image) is decreased (green areas; FA = 0.61 vs 0.75 in normal cervicothoracic area) after region of interest measurements. The region of interest in abnormal area was drawn by using the fiber tracking three-dimensional reconstruction. B, MR imaging of a spinal cord compression due to a breast tumor metastasis with epidural involvement in patient 1. Fiber tracking over b0 image shows a mass-effect on fibers tracts. The region of interest (green area) was drawn over the maximal level of compression (blue arrow) and then automatically reported on the coregistered ADC and FA maps to measure ADC and FA values in the compression site.

Fig 2.

Time course from d1 to d30 of the averaged FA parameter estimated from the compression sites in 11 patients with abnormal FA values. FA values decreased from the 1st to the 21st days, then slightly increased, both related to the extracellular water diffusivity: restricted diffusivity in acute stage and increased diffusivity in chronic stage. Normal values range from 0.69 to 0.8.

Fig 3.

Fiber tracking performed on a volunteer’s cervical spinal cord. Sagittal (A), axial (B), and coronal (C) views show tracts reconstructed over the b0 sequence. Main white matter tracts are visible on axial (B) and coronal (C) views of the three-dimensional reconstructions: two individualized posterior lemniscal tracts (arrowheads), and posterolateral corticospinal tracts (arrows). Other tracts are visible, but have to be more correlated with known anatomy.

Fig 4.

Three-dimensional reconstructions are mapped over the b0 images. Fiber tracking performed on the spinal cord volunteer shows some pitfalls of the FT method due to magnetic susceptibility effect of the DTI MR image: “hole” effect, gap in three-dimensional reconstruction (blue arrows).