Inferior Brain Lesions Monitored Using Diffusion Weighted Magnetic Resonance Imaging

1. Figure 1: Coordinate system used for mutual information registration. Inferior Brain Lesions Monitored Using Diffusion Weighted Magnetic Resonance Imaging Lars Ewell1, Naren Vijayakumar2, Jeffrey J. Rodriguez2 and Baldassarre Stea1 1) Department of Radiation Oncology, University of Arizona Medical Center, Tucson AZ 85724 USA 2) Department of Electrical and Computer Engineering, University of Arizona, Tucson AZ 85721 USA Mutual Information Since the images of the human brain considered here are subject to minimal deformation, a registration metric that utilizes rigid transformations and/or rotations is appropriate. Mutual information (MI) is the metric that has been chosen. For the axial scans in this study, a three-dimensional Cartesian coordinate system was chosen, such that the z axis is roughly parallel to the spine, with the x and y axes along the left-right and anterior-posterior, respectively. This choice is depicted in Figure 1. When registering two MRI scans via MI, we are thus searching for translations along the x and y axes, and rotations about the z axis, such that the two images are as optimally aligned as possible. DWMRI Scans The five image sets used for this study were obtained from an internal review board (IRB) approved protocol. They consist of a series of axial DWMRI scans taken from a superior position (top of skull) to an inferior one (bottom of cerebrum or below). A sample of both the EP and RD (DWMRI, b=0), as well as the T2 FLAIR images to which they are registered can be seen in Figure 2. ABSTRACT Radial Diffusion (RD) and Echo Planar (EP) Diffusion Weighted Magnetic Resonance Image (DWMRI) scans of five patients were compared in inferior brain locations. Registration to non-diffusion T2 weighted FLuid Attenuated Inversion Recovery (FLAIR) MRI scans was used for comparison. Mutual Information (MI) was utilized as a registration metric. RD DWMRI was, in general, found to have a better image registration (higher mutual information) to the T2 FLAIR images than EP DWMRI. This advantage increased for more inferior brain MRI slices. Introduction The use of DWMRI has increased recently, as the application has spread from use mainly in ischemia diagnosis, to monitoring therapy efficacy in radiation oncology1,2. When considering isotropic diffusion, the most common form of weighting is EP3. While this form has proven useful, lately there have been additional novel forms that have been proposed, such as RD4. In order to quantitatively compare these two different forms of DWMRI, we have used image registration to non-diffusion weighted scans, where mutual information5 has been chosen as the metric. Motivation When treating brain tumors with radiotherapy, the lesion is generally contoured by a clinician on a non-diffusion weighted image, such as T2 FLAIR. In order to calculate an ADC for the lesion, used to monitor therapy efficacy2, the contour must be transcribed to a diffusion weighted image. Therefore, the ability to utilize DWMRI to monitor therapy efficacy depends on the accuracy with which these contours can be transcribed. Although less common, gliomas can occur in more inferior brain locations, such as the occipital lobe6. In positions near magnetic inhomogeneities such as these and others, there are well known shortcomings of single-shot EP DWMRI: geometric distortions, blurring, poor spatial resolution and intra-voxel dephasing. In view of this fact, there is a need for a more robust form of DWMRI. With this in mind, we have compared EP DWMRI with RD DWMRI. Discussion As shown above, radial diffusion has a significantly higher value of mutual information than echo planar when registered with a T2 FLAIR image commonly used to contour brain lesions in preparation for radiation treatment. In general, this difference becomes more pronounced as the axial slice location in the brain moves from a superior position to an inferior one. Conclusion The ability to clearly image a DWMRI scan in an inferior portion of the brain is an increasingly important objective, as the use of DWMRI scans increase. While the current use of isotropic EP DWMRI scans is likely sufficient for superior locations in the cerebrum, the use of RD DWMRI for inferior regions is worth considering. Figure 2: Inferior (a, d, g), mid-brain (b, e, h) and superior (c, f, i) MRI slices of RD, EP (DWMRI, b=0) scans, and the T2 FLAIR images to which they are registered, respectively. Note: DWMRI (EP and RD) had window and level adjustments for clarity. The mutual information between these primary images, and the DWMRI scans was then calculated as in (3), as translations in the x and y directions, as well as rotations about the z axis were varied. A total of five patient scan sets were analyzed for this study. A typical plot of the MI from the two different forms of DWMRI as a function of slice number is shown in Figure 3. As can be seen in this figure, the MI between the T2 FLAIR primary images and the RD DWMRI scans is consistently higher than between T2 FLAIR and EP DWMRI scans. In addition, this difference generally gets larger as the axial slices go from a superior location to a more inferior one. The differences between the MI of the different registrations are displayed in Table 2. As can be seen in this table, the MI between the T2 FLAIR and RD DWMRI scans is approximately 0.5 higher than the MI between the T2 FLAIR and the EP DWMRI scans. In addition, the maximal difference generally occurs in the most inferior scans (highest slice number). 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