Multislice CEST MRI improves the spatial assessment of tumor pH

Abstract

Purpose Multislice maps of extracellular pH (pHe) are needed to interrogate the heterogeneities of tumors and normal organs. To address this need, we have developed a multislice acidoCEST MRI acquisition method with a CEST spectrum fitting method that measures in vivo pHe over a range of 6.3 to 7.4. Methods The Phase Offset MultiPlanar (POMP) method was adapted for CEST-FISP MRI to acquire multiple image slices with a single CEST saturation pulse. The Bloch-McConnell equations were modified to include pH based on a calibration of pH and chemical exchange rate for the contrast agent, iopamidol. These equations were used to estimate the pixelwise pHe values throughout the multislice acidoCEST MR images of the tumor, kidney, bladder and other tissues of a MDA-MB-231 tumor model. Results Multislice acidoCEST MRI successfully mapped a gradient of pHe from 6.73 to 6.81 units from the tumor core to rim, and mapped a gradient of pHe 6.56 to 6.97 across the mouse kidney. The bladder was found to be pHe 6.3. Conclusion AcidoCEST MRI with POMP acquisition and Bloch-McConnel analysis can map pHe in multiple imaging slices through the tumor, kidney, and bladder. This multislice evaluation facilitates assessments of spatial heterogeneity of tissue pHe.

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A comparison of exogenous and endogenous CESTMRI methods for evaluating in vivo pH

“…AcidoCEST MRI results were fit using previously published methods (11,23,24,(26)(27)(28). We first averaged the four preinjection images at each saturation frequency.…”