A Preliminary Study of the Effects of Trigger Timing on Diffusion Tensor Imaging of the Human Spinal Cord
P. Summersa,
P. Staempflia,b,
T. Jaermanna,b,
S. Kwiecinskia and
S. Kolliasa
a Institute for Neuroradiology, University Hospital Zurich, Zurich, Switzerland
b Institute for Biomedical Engineering, ETHZ, Zurich, Switzerland
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Fig 1. Cranio-caudal movement of the medulla (A and B), C2 (C and D), and C6 (E and F) segments of the spinal cord in 4 healthy subjects. Velocity- and acceleration-time curves for all levels show a period of relative quiescence between 200 and 550 msec after peripheral trigger. Normalized velocity-time curves (G) and acceleration-time curves (H) at the C2 level (reflecting those of the other levels; data not shown) indicate that quiescence consistently lasted 40% of the cardiac cycle, with good consistency between subjects.
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Fig 2. DTI data covering 1 vertebral body (4 sections) acquired without restriction to the quiescence of spinal cord motion. Ghosting of CSF is apparent in the T2-weighted images (left column), whereas ghosting of subcutaneous fat and inconsistent spinal cord signal intensity are seen in the 6 diffusion-weighted acquisitions. In the average diffusion-weighted image (right column) the spinal cord is not well demonstrated.
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Fig 3. DTI data covering one vertebral body (4 sections) acquired entirely during the spinal cord quiescence for the same subject as Fig 2. Ghosting of CSF in the T2-weighted images is greatly reduced (left column), and the diffusion-weighted spinal cord signal intensity is more consistent. Some ghosting of subcutaneous fat is still apparent. Compared with Fig 2, the spinal cord is clearly seen in the average diffusion-weighted image (right column) for all sections.
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Fig 4. Comparison of diffusion tensor properties obtained without (top row) and with (bottom row) optimized cardiac triggering, illustrated in midspinal coronal reformat of axial section data. The CSF space and CSF-spinal cord interface are better defined in the low b-value data (D versus A) allowing the low FA in the CSF space to be better appreciated (F versus C). FA maps (B and E) color-coded to reflect the orientation of the first eigenvector (blue, caudal-cranial; green, anteroposterior; red, right-left) show greater consistency between sections when the proposed trigger window is used. This is also seen when examining the orientation of the eigenvectors directly overlaid on a gray-scale FA map (F versus C).
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Fig 5. Fiber-tracking results from a healthy control (A) with the entire cross-section of the spinal cord in both the superior (blue fibers) and inferior section (red fibers) used as seeding regions. Few tracks extend from end to end of the scan volume (covering slightly more than 2 vertebral bodies). Color coding the same fibers in accordance with local orientation of the first eigenvector (blue, rostral-caudal; red, left-right; green, anteroposterior), many of the fibers appear to converge on an emerging nerve root, suggesting that fiber crossing may be present. Restricting seeding to the lateral white matter in the in the most superior section (C) shows no penetration of these tracks into the central regions of the cord.
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Fig 6. A patient with cervical syringomyelia. Axial T2-weighted images (A and D), mean diffusion-weighted images (B and E) and color-coded fractional anisotropy maps (C and F), at C2 (top row, A–C) and C3 (second row D--F) levels through the syrinx. Some residual ghosting of the spinal cord is apparent, but the orientation of the first eigenvector appears preserved. Lateral view of fiber tracking after seeding of the inferior and superior sections (G) shows little evidence of the pathology. Setting a conservative threshold on the T2-weighted images to isolate the core of the syrinx (gray), the tracks originating in the inferior section (H) and superior section (I) are seen not to penetrate this region of the pathology.
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Fig 7. T2 (left column), and diffusion-weighted images (middle column) together with FA maps (right column) of a cervical spine lesion in a patient with multiple sclerosis. Top row, above lesion; middle row, top-most section showing T2 hyperintensity; bottom row, through widest extent of the lesion. Hyperintensity in the diffusion-weighted images follows that of the T2-weighted images, whereas in the center of the lesion is a unilateral reduction in FA.
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Fig 8. An intramedullary glioma extending from C2 to C4 displaying hyperintense signal intensity on T2-weighted images (A). The overlaid lines denote the levels corresponding to the axial sections in B–E. The lesion displays heterogeneous signal intensities on conventional T1 (B), EPI T2 (C), and diffusion–weighted (D) images as well as the FA map (E). Below the tumor (bottom row), a clear butterfly configuration attributed to the central gray matter of the cord is visible on T2 and diffusion-weighted images and FA map but not at the T1-weighted image. In fiber tracking, seeding of the entire superior and inferior cross-sections results in a veil of fibers around a region of low fractional anisotropy in the tumor into which tracks do not propagate, as seen when viewed in section (F, arrow). From external viewpoints, the thinning of the fiber volume is visible only when seeding is restricted to the white matter (G).
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