Novel proton exchange rate MRI presents unique contrast in brains of ischemic stroke patients

Background: To map and quantify the proton exchange rate (k(ex)) of brain tissues using improved omega plots in ischemic stroke patients and to investigate whether k(ex) can serve as a potential endogenous surrogate imaging biomarker for detecting the metabolic state and the pathologic changes due to ischemic stroke. New Method: Three sets of Z-spectra were acquired from seventeen ischemic stroke patients using a spin echoecho planar imaging sequence with pre-saturation chemical exchange saturation transfer (CEST) pulse at B-1 of 1.5, 2.5, and 3.5 mu T, respectively. Pixel-wise k(ex) was calculated from improved omega plot of water direct saturation (DS)-removed Z-spectral signals. Results: The derived k(ex) maps can differentiate infarcts from contralateral normal brain tissues with significantly increased signal (893 +/- 52 s(-1) vs. 739 +/- 34 s(-1), P < 0.001). Comparison with Existing Method(s): The k(ex) maps were found to be different from conventional contrasts from diffusion-weighted imaging (DWI), CEST, and semi-solid magnetization transfer (MT) MRI. In brief, k(ex) MRI showed larger lesion areas than DWI with different degrees and different lesion contrast compared to CEST and MT. Conclusions: In this preliminary translational research, the k(ex) MRI based on DS-removed omega plots has been demonstrated for in vivo imaging of clinical ischemic stroke patients. As a noninvasive and unique MRI contrast, k(ex) MRI at 3 T may serve as a potential surrogate imaging biomarker for the metabolic changes of stroke and help for monitoring the evolution and the treatment of stroke.