Diffusion tensor imaging promises to expand diagnostic frontier of fMRI
The potential influence of diffusion tensor imaging (DTI) on MRI practice permeated scientific sessions at this year's meeting of the International Society for Magnetic Resonance in Medicine in Honolulu.
DTI held the floor in the advanced physics sessions where the proof of principle for several new DTI imaging protocols was described. It showed up in technical exhibits demonstrating software that maps the presence, direction, and thickness of cerebral white matter fibers. The buzz was mostly based on anecdotal evidence, however.
Neurosurgeons at Massachusetts General Hospital are using DTI tractography to plan brain surgeries, according to Dr. A. Gregory Sorensen, a professor of radiology. He uses DTI to track white matter fibers that connect cerebral sensory regions, much like functional blood oxygen level-dependent imaging is used to delineate eloquent tissue. Neurosurgeons work around those regions to reduce damage to sensory and motor functions. MGH staff is encouraged by this early work, but its clinical impact has yet to be quantified.
DTI is also finding a place at the University of California, San Francisco, where it has been added to the standard MR/MR spectroscopy prostate imaging protocol, according to Daniel B. Vigneron, Ph.D, an associate professor of radiology. His study of 37 patients, reported on Wednesday at the ISMRM, suggests that DTI addresses the shortcomings of anatomic MRI and functional MRS for prostate cancer imaging, especially after therapy.
The single-shot fast-spin-echo DTI sequence adds two to four minutes to the standard prostate imaging protocol, according to Vigneron. It features a b-value of 606 gradient directions and 1.8 x 0.9-mm in-plan resolution. Seven to nine 4-mm slices are usually sufficient to cover the prostate.
Significant differences in average diffusion coefficient and anisotropy were observed in regions known to be cancer because of a positive biopsy, a reduced T2-weighted response during anatomic MRI, and elevated MRS choline and creatine/citrine ratios.
The DTI measures were especially relevant for cancers that spanned the transition zone, Vigneron said. The precise spread of cancer beyond the prostate capsule is otherwise difficult to ascertain because of the limited resolution of MRS and the relatively low contrast resolution of the T2-weighted MRIs, according to Vigneron.
The sharp resolution possible with DTI is especially valuable for tracking the effects of therapy, he said. The difference between residual cancer and necrotic tissues can be seen.
These preliminary findings require confirmation with larger clinical trials, especially ones that measure DTI's effect on patient management and outcomes. Practical experience at UCSF, however, suggests that that DTI will be a useful addition to prostate imaging, according to Vigneron.
