High Spatio-Temporal Resolution pHe Mapping of a Rat Glioma Derived From pH-Dependent Spin-Lattice Relaxivity

Introduction The extracellular pH (pHe) of cancers is acidic and inhibiting this acidity will inhibit metastases. Methods to image pHe in tumors based on relaxivity of a pH-dependent contrast reagent (CR), Gd-DOTA-4AmP were developed. We investigated the single injection of a mixture of DyDOTP with GdDOTA-4AmP. The ΔT1, and ΔT2* induced by the Gd-CR exhibited similar pH-dependence, while the pH-independent Dy-CR reduced T2* with negligible effects on the T1. Thus, with calibration, co-injection of this cocktail can enable dynamic calculation of spatially localized unique pH values. One concern with this approach was the low dynamic range afforded by gradient echo T2* imaging in vivo, because of high endogenous relaxivity. We have overcome this problem using Echo Planer Spectroscopic Imaging (EPSI); a technique that generates full 1H spectra of the water resonance for each pixel at high resolution.