Compensatory Amide Proton Transfer Ratio (CAPTOR) Imaging to Improve the Specificity of Tissue Acidosis MRI

Introduction Amide proton transfer (APT) imaging is a specific form of chemical exchange saturation transfer (CEST) MRI that utilizes labile amide protons from endogenous proteins and peptides to probe microenvironment pH 1-2. In fact, APT MRI has been used to study acute ischemic acidosis, and recently postulated as a complementary tool to perfusion and diffusion MRI in imaging stroke 2,3. However, the conventional magnetization transfer (MT) asymmetry analysis contains not only pH-dependent APT effect, but also concomitant RF irradiation contributions such as intrinsically asymmetric MT. As such, APT imaging, in its current form, may be susceptible to non-negligible MT contribution and not specific to acidosis. Here, we propose a compensatory APT ratio (CAPTOR) method that better suppresses concomitant RF irradiation effects, and demonstrated its in vivo use via a global ischemic animal model. Further evaluation of CAPTOR MRI is currently undergoing before its clinical translation. Theory The amide proton transfer ratio (APTR) is given by asym asym ' MTR MTR APTR − = , in which MTRasym and MTR’asym