Diagnostic Imaging Online
June 4, 2002
3D diffusion tensor imaging shows promise in early brain imaging study
High-resolution images of subcortical white matter tracts and deep intracranial structures such as the brain stem are possible using 3D diffusion tensor imaging (DTI), according to researchers at the Kennedy Krieger Institute in Baltimore.
In the past, DTI data were gathered using multislice acquisition, a technique limited in its ability to study small white matter structures. 3D DTI techniques offer several improvements, including the ability to acquire high-resolution data sets and all segmented echo-planar imaging readouts at the same moment in the cardiac cycle.
"The technique provides an extremely high resolution and is inherently isotropic. If you use conventional multislice to acquire an image of the brain stem and you move from an axial to a sagittal slice, you will be able to see the difference in quality between the two images very clearly. By using 3D DTI, you can reslice totally, without degradation in quality," said Xavier Golay, Ph.D., of Kennedy Krieger's F.M. Kirby Research Center for Functional Brain Imaging.
Golay and colleagues imaged three healthy subjects using a 1.5T scanner. They acquired DT images using a 3D segmented spin-echo echo-planar sequence with cardiac gating. The gating method allowed the researchers to reduce the technique's susceptibility to gross motion artifacts.
The researchers imaged two locations on the surface of the brain and one in the brain stem, then acquired multislice images of the brain stem for comparison. They were able to keep scanning time, including setup and anatomical scans, to less than 40 minutes.
The study, published in the May edition of Magnetic Resonance in Medicine, showed that Golay and colleagues were able to obtain high-resolution 3D images close to the surface of the brain and were able to reduce signal void due to motion artifacts. Work remains to be done to compensate for some of the technique's drawbacks, including its motion sensitivity and the amount of time it takes to perform the technique compared with multislice acquisition, according to Golay.
"The next improvement in this technique will be to combine it with parallel imaging techniques such as sensitivity encoding (SENSE) imaging, in order to make it whole-brain compatible in a reasonable amount of time," he said. "With these combined techniques, you could expect to have the acquisition of one entire brain in just three minutes per diffusion direction. The acquisition of a complete tensor data set, necessitating seven acquisitions (six independent diffusion directions and a reference scan) would take only about 20 minutes."
As for suppressing motion-related artifacts, Golay said that elaborate methods, such as measuring navigator echo and cardiac gating, had already been undertaken in the study. Combining real-time estimation of navigator echo with a reacquisition scheme that would reacquire a set of k-space that had been corrupted by motion-related artifacts could add to the radiologist's arsenal of motion-related artifact-suppression techniques.
The 3D DTI technique shows promise in delineating short-range cortico-cortical association fibers, which the researchers believe are involved in multiple sclerosis and adrenoleukodystrophy. But they are also aware of the technique's limits.
"This technique is mostly appropriate for high-resolution images of specific areas (such as subcortical white matter) and to delineate very subtle changes. It is not yet appropriate for whole-brain tractography (fiber-tracking)," Golay said.
3D DTI can provide high-resolution images of small white matter structures. Sagittally reconstructed diffusion-weighted images (A and B), and color-coded diffusion tensor map of the brain stem of healthy volunteers (blue = fibers running foot-to-head; green = left-to-right; red = perpendicular to the sagittal plane). In conventional multislice DTI, images are acquired axially, using a 3-mm slice thickness and a 1-mm2 in-plane resolution. In the 3D-DTI on the other side, the increased SNR due to the 3D acquisition technique allows radiologists to push the resolution down to 1 mm3, providing smooth multiplanar reconstruction in any direction, as demonstrated on the two zoomed circular regions of interest between the sagittally reconstructed multislice and 3D data sets. (Provided by X. Golay)
For more from the Diagnostic Imaging archives:
Diffusion MRI explores new indications
Diffusion tensor imaging promises to expand diagnostic frontier of fMRI
-- By Merlina Trevino
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