Amine / Amide Concentration Independent Detection ( AACID ) of Intracellular pH by CEST MRI at 9 . 4 T

Purpose: Zhou et. al first used exchanging amide protons (resonating at ~3.5ppm) associated with endogenous intracellular proteins to measure pHi in vivo . Recently, Jin et. al. reported significant pHi-dependent amine proton exchange (APEX) contrast at ~2.75ppm . Desmond et al. recently proposed a ratiometric approach to measure pH using amide (3.5ppm) and amine (2.0pmm) CEST effects. In the current study, a novel ratiometric technique is presented to directly measure absolute pHi using the ratio of CEST contrast from amine (~2.75 ppm) and amide (~3.5 ppm) protons: amine/amide concentration independent detection (AACID)