Quantitative Modeling of In-Vivo Amide Proton Transfer Measurements in the Human Brain Indicates a Dominant Signal Contribution from Proteins with Short T2 Relaxation Times

Rachel Scheidegger^1,2, Elena Vinogradov^1,3, Weiying Dai^1,3, David C. Alsop^1,3

Amide proton transfer imaging has the potential to measure pH in-vivo based on the amide exchange rate. Using an APT saturation scheme with simultaneous saturation at two frequencies we were able to remove MT contamination from the images, allowing for accurate quantification of amide exchange rate, relaxation time and concentration in the healthy human brain as a function of saturation power. The exchange rate was measured to be 45Hz, consistent with previous animal experiments. The transverse relaxation time of 2ms was shorter than previously assumed and may indicate the APT signal has a contribution from bound proteins.