Incorporating DTI data as a constraint in deformation tensor morphometry between T1 MR images

Abstract

Deformation tensor morphometry provides a sensitive approach to detecting and mapping subtle volume changes in the brain from conventional high resolution T1W MRI data. However, it is limited in its ability to localize volume changes within sub-regions of uniform white matter in T1W MRI. In contrast, lower resolution DTI data provides valuable complementary microstructural information within white matter. An approach to incorporating information from DTI data into deformation tensor morphometry of conventional high resolution T1W imaging is described. A novel mutual information (MI) derived criteria is proposed, termed diffusion paired MI, using an approximation to collective many-channel MI between all images. This approximation avoids the evaluation of high dimensional joint probability distributions, but allows a combination of conventional and diffusion data in a single registration criteria. The local gradient of this measure is used to drive a viscous fluid registration between repeated DTI-MRI imaging studies. Results on example data from clinical studies of Alzheimer's disease illustrate the improved localization of tissue loss patterns within regions of white matter.

Fig. 1

An illustration of the derivation of different MI measures of similarity between multiple sets of images for conventional scalar images (top) and combined scalar and DTI data types (bottom). In conventional MRI data sets (T1W,PDW,T2W) there is appreciable shared information. For DTI data there is little shared information between individual diffusion direction maps. We can therefore consider the simplified relationship between DTI directional measurements separately paired with conventional MRI.

Fig. 2

Left: Sagittal and coronal slices though DTI and MRI data for the two studies of the subject analyzed in figure 3, showing the principal direction vectors (colour coded by direction) of the two DTI datasets overlayed onto the corresponding T1W MPRAGE studies. Right: Components of the force fields driving the studies into alignment, derived from conventional T1W MRI and DTI data. Note expanding ventricular boundary force in conventional MRI and additional forces within uniform regions of white matter from DTI data.

Fig. 3

A subject diagnosed with Alzheimer’s Dementia scanned twice with an interval of 9 months (MMSE 25, Age 61.7), exhibiting tissue loss and ventricular expansion. The scan pairs were fluidly aligned using T1 only (bottom right) and T1 with the full diffusion tensor (top right). The incorporation of the additional structural information on the internal white matter structure provided by DTI assists in constraining the local volume changes mapped by the fluid registration within a more focal region of white matter.)