Tract probability maps in stereotaxic spaces: analyses of white matter anatomy and tract-specific quantification

Abstract

Diffusion tensor imaging (DTI) is an exciting new MRI modality that can reveal detailed anatomy of the white matter. DTI also allows us to approximate the 3D trajectories of major white matter bundles. By combining the identified tract coordinates with various types of MR parameter maps, such as T2 and diffusion properties, we can perform tract-specific analysis of these parameters. Unfortunately, 3D tract reconstruction is marred by noise, partial volume effects, and complicated axonal structures. Furthermore, changes in diffusion anisotropy under pathological conditions could alter the results of 3D tract reconstruction. In this study, we created a white matter parcellation atlas based on probabilistic maps of 11 major white matter tracts derived from the DTI data from 28 normal subjects. Using these probabilistic maps, automated tract-specific quantification of fractional anisotropy and mean diffusivity were performed. Excellent correlation was found between the automated and the individual tractography-based results. This tool allows efficient initial screening of the status of multiple white matter tracts.

Figures

Fig. 1

Reconstruction of cortical spinal tract in a multiple sclerosis patient. Lesions with low diffusion anisotropy are indicated by yellow (left hemisphere) and pink (right hemisphere) colors. The corticospinal tract is successfully reconstructed in the right hemisphere but not in the left, making it difficult to measure MR parameters along the corticospinal tract.

Fig. 2

ROI locations used for reconstructions of 11 white matter tracts in this paper. All tracts are reconstructed using a two-ROI approach, as previously published (Wakana et al., 2005). Abbreviations are: ATR: anterior thalamic radiation; CgC: cingulum in the cingulated cortex area; Cgh: cingulum in the hippocampal area; CST: cortitospinal tract; FMa: forceps major; FMi: forceps minor; IFO: inferior fronto-occipital fasciculus; SLF: superior longitudinal fasciculus; tSLF: the temporal projection of the SLF; UNC: uncinate fasciculus; DSCP: decussation of the superior cerebellar peduncle; POS: parieto-occipito sulcus. The tSLF shares the first ROI with SLF.

Fig. 3

Probabilistic maps of 11 white matter tracts. Results are superimposed on a single-subject JHU template. The 3D volume rendering of the averaged tract (A) and color-scaled probabilistic maps (B) are superimposed on 2D slices. Maximum intensity projection is used for the color intensity in (A). The color in (B) represents probability, as shown in the color bar.

Fig. 4

Comparison of individual and probabilistic methods for 11 tracts. Data from the both hemisphere are plotted together. Horizontal axis is FA value measured by probabilistic method and vertical axis is FA value measured by individual method. Dashed line is the identity line. Standard deviations of both methods are also shown.

Fig. 5

Comparison of FA measurements by individual and probabilistic methods for the corticospinal tract (CST) at each z-coordinate of the MNI template (A), and correlation plots of the left (B) and right (C) CST. Each point in (B) and (C) correspond to data points at each z coordinate. The origin of the z-coordinate (z = 0) is placed on the anterior commissure level. The averages and standard deviations were obtained from the 10 normal subjects.

Fig. 6

Pearson correlation analysis of probabilistic method (X axis) and individual method (Y axis) for FA measurements of 10 fiber tracts. All the correlation coefficients are greater than 0.82, except for UNC and SLF.

Fig. 6

Pearson correlation analysis of probabilistic method (X axis) and individual method (Y axis) for FA measurements of 10 fiber tracts. All the correlation coefficients are greater than 0.82, except for UNC and SLF.

Fig. 7

Probabilistic FA quantification of the corticospinal tract (CST) of an MS patient in the MNI coordinates. As shown in Fig. 1, a low FA region in the central semiovale interferes with CST reconstruction in this patient. The probabilistic approach is not influenced by this type of lesion in individuals. The probabilistic CST coordinates are superimposed on the MS patient data (A) and FA and Trace/3 are measured for the left and right hemisphere (B). In (C) and (D), FA and Trace/3 maps of the patient are shown at z = 20 mm (indicated by small arrows in (B)) where abnormality was found only in FA but not in Trace/3. The pink circles in (C) and (D) indicate probabilistic locations of the CST, defined by Pr > 0.2. White arrows indicate an apparent lesion in the FA map.

Fig. 8

Normalized images of subject #1, 3, 5, 7, 9 used in Fig. 4 – 6 to demonstrate registration quality. Three axial slices at z = 0, 12, and 32 are shown, which reveal the probabilistic locations of the IFO (green), the SLF (peach), the ATR (orange), and the CST (pink). The outer boundary defines the shape of the ICBM-152 template.