Consistent depiction of the acidic ischemic lesion with APT MRI-Dual RF power evaluation of pH-sensitive image in acute stroke

Purpose Amide proton transfer-weighted (APTw) MRI provides a non-invasive pH-sensitive image, complementing perfusion and diffusion imaging for refined stratification of ischemic tissue. Although the commonly used magnetization transfer (MT) asymmetry (MTRasym) calculation reasonably corrects the direct RF saturation effect, it is susceptible to the concomitant semisolid macromolecular MT contribution. Therefore, this study aimed to compare the performance of MTRasym and magnetization transfer and relaxation-normalized APT (MRAPT) analyses under 2 representative experimental conditions. Methods Multiparametric MRI scans were performed in a rodent model of acute stroke, including relaxation, diffusion, and Z spectral images under 2 representative RF levels of 0.75 and 1.5 mu T. Both MTRasym and MRAPT values in the ischemic diffusion lesion and the contralateral normal areas were compared using correlation and Bland-Altman tests. In addition, the acidic lesion volumes were compared. Results MRAPT measurements from the diffusion lesion under the 2 conditions were highly correlated (R-2 = 0.97) versus MTRasym measures (R-2 = 0.58). The pH lesion sizes determined from MRAPT analysis were in good agreement (178 +/- 43 mm(3) vs. 186 +/- 55 mm(3) for B-1 of 0.75 and 1.5 mu T, respectively). Conclusions The study demonstrated that MRAPT analysis could be generalized to moderately different RF amplitudes, providing a more consistent depiction of acidic lesions than the MTRasym analysis.