Thursday, June 19, 2014

pH Imaging in Stroke

I recently published a preliminary study on stroke imaging at the International Symposium on Magnetic Resonance in Medicine (ISMRM), the main MRI researchers' conference which was held in Milan, Italy.

My study evaluated the predictive capacity of water diffusion, blood perfusion and pH imaging for identifying tissue likely to become permanently damaged. MRI is able to evaluate damage due to stroke quite effectively 1 month after stroke onset (using FLAIR - Fluid Attenuated Inversion Recovery imaging), however, at this point it is far too late to influence patient treatment decisions. The study presented focuses on our ability to predict that final tissue outcome as soon as possible post stroke. My study was awarded an oral presentation (along with the top 15% of conference submissions). 

The following image is from my presentation and shows a FLAIR image demonstrating tissue damage due to stroke at follow-up imaging (left), a water diffusion weighted image (DWI) acquired in the acute phase hours after stroke onset (center) and a pH sensitive Amide Proton Transfer (APT) image (right) where dark values indicate possible tissue acidosis. The red lines mark the boundary of the tissue that was assessed by a clinician as being permanently damaged. The acute phase DWI and APT were coregistered (spatially alligned) into the same space as the follow-up FLAIR image and the clinician defined region-of-interest (ROI) was super-imposed onto these images as well. This provides an easy mechanism for the reader to see what tissue would go on to become permanently damaged on the acute phase (hours after stroke) DWI and APT images.

One of our lab's research goals is to evaluate the potential of using pH sensitive imaging with MRI as a possible imaging adjunct to the main existing methods for imaging acute stroke patients with MRI: primarily water diffusion and secondarily blood perfusion. When a region of brain tissue is subjected to reduced blood flow due to an ischaemic stroke, standard adenosine triphosphate (ATP, the energy molecule used by cells) production is reduced forcing the cells to rely on anaerobic ATP production which produce lactic acid as a byproduct thus resulting in tissue acidosis. The APT image on the right was able to identify the lower region of the red ROI as exhibiting tissue acidosis even though the water diffusion method (which is the gold standard MRI method for assessing acute stroke tissue damage) did not identify restricted diffusion in that same region. I was very fortunate to have my work included in the closing plenary talk on chemical exchange saturation transfer imaging on the final day of the conference.

The APT image above is pH sensitive. My colleague Dr. Yee Kai Tee has just published an article on pH imaging in acute stroke patients in the journal NMR in Biomedicine. As a co-author on the paper I am pleased to introduce the study here. It is focused on converting the measurements we acquire into quantitative pH maps, so that the individual pixels in the image correspond to an actual estimated pH value. You can access that article here.

Jacob Levman, PhD
Centre Of Excellence in Personalized Health Care
Institute of Biomedical Engineering
University of Oxford