Amide proton transfer (
APT) MRI is sensitive to ischemic tissue
acidosis and has been increasingly used as a research tool to investigate disrupted tissue metabolism during
acute stroke. However, magnetization transfer asymmetry (MTR(asym)) analysis is often used for calculating
APT contrast, which only provides pH-weighted images. In addition to pH-dependent
APT contrast, in vivo MTR(asym) is subject to a baseline shift (ΔMTR'(asym)) attributable to the slightly asymmetric magnetization transfer (MT) effect. Additionally,
APT contrast approximately scales with T(1) relaxation time. Tissue relaxation time may also affect the experimentally obtainable
APT contrast via saturation efficiency and RF spillover effects. In this study, we acquired perfusion, diffusion, relaxation and pH-weighted
APT MRI data, and spectroscopy (MRS) in an animal model of
acute ischemic stroke. We modeled in vivo MTR(asym) as a superposition of pH-dependent
APT contrast and a baseline shift ΔMTR'(asym) (i.e., MTR(asym)=APTR(pH)+ΔMTR'(asym)), and quantified tissue pH. We found pH of the contralateral normal tissue to be 7.03±0.05 and the ipsilateral ischemic tissue pH was 6.44±0.24, which correlated with tissue perfusion and diffusion rates. In summary, our study established an endogenous and quantitative pH imaging technique for improved characterization of ischemic tissue acidification and metabolism disruption.