Assessment of Brain Tissue Microstructure by Diffusion Tensor Distribution MRI: An Initial Survey of Various Pathologies
To explore the potential of the novel diffusion tensor distribution (DTD) MRI method for assessment of brain tissue microstructure in terms of nonparametric DTDs and derived parameter maps reporting on cell densities, shapes, orientations, and heterogeneity through a pilot study with single cases of neurocysticercosis, hydrocephalus, stroke, and radiation damage.
METHOD AND MATERIALS:
Four patients were scanned with a <5 min prototype diffusion-weighted (DW) sequence in conjunction to their regular MRI protocol on a GE MR750w 3T. DW images were acquired with spin echo-prepared EPI using TE=121ms, TR=3298ms, and in-plane resolution=3mm. DW was applied with four b-values up to 2000 s/mm2 for 37 isotropic and 43 directional encodings. Raw images were converted to per-voxel DTDs and metrics including means and (co)variances of tensor "size" (inversely related to cell density), shape, and orientation, as well as signal fractions from elongated cells (bin1, including WM), nearly isotropic cells (bin2, including GM), and free water (bin3, including CSF).
Inspection of the parameter maps revealed the following conspicuous features. 1) neurocysticercosis: site of parasite (high bin3_fraction) enclosed by cyst (high bin2_fraction) and edema (high bin2_fraction and bin2_size); 2) radiation: damaged area (high bin1_fraction and bin1_size) surrounded by edema (high bin2_fraction and bin2_size); recurrent tumor: site of removed tumor filled by fluid (high bin3_fraction) lined with a rim of tumor (high bin2_fraction and elevated bin2_size); hydrocephalus: enlarged ventricles rimmed by thin intact WM (high bin1_fraction with bin1_orientation consistent with WM tracts); acute stroke: ischemic tissue (high bin1_fraction, low bin1_size) surrounded by penumbra (high cov_size_shape) (see Figure 1).
Figure 1. Diffusion Tensor Distribution (DTD) parameter maps for a case of acute stroke (arrows).
The custom sequence for DTD can be applied as a minor addition to a clinical MRI protocol and provides novel microstructural parameter maps with conspicuous features for a range of brain pathologies, thereby encouraging studies with larger patient groups and comparison with current gold standards.
The DTD method may enable detailed characterization of tissue microstructure in a wide range of brain pathologies.