Dual contrast CEST MRI for pH-weighted imaging in stroke.

Purpose: pH enhanced (pH ) CEST imaging combines the pH sensitivity from amide and guanidino signals, but the saturation parameters have not been optimized. We propose pH as a variant of pH that suppresses background signal variations, while enhancing pH sensitivity and potential for imaging ischemic brain injury of stroke.

Methods: Simulation and in vivo rodent stroke experiments of pH MRI were performed with varied RF saturation powers for both amide and guanidino protons to optimize the contrast between lesion/normal tissues, while simultaneously minimizing signal variations across different types of normal tissues. In acute stroke, contrast and volume ratio measured by pH imaging were compared with an amide-CEST approach, and perfusion and diffusion MRI.

Results: Simulation experiments indicated that amide and guanidino CEST signals exhibit unique sensitivities across different pH ranges, with pH producing greater sensitivity over a broader pH regime. The pH data of rodent stroke brain demonstrated that the lesion/normal tissue contrast was maximized for an RF saturation power pair of 0.5 μT at 2.0 ppm and 1.0 μT at 3.6 ppm, whereas an optimal contrast-to-variation ratio (CVR) was obtained with a 0.7 μT saturation at 2.0 ppm and 0.8 μT at 3.6 ppm. In acute stroke, CVR optimized pH (i.e., pH ) achieved a higher sensitivity than the three-point amide-CEST approach, and distinct patterns of lesion tissue compared to diffusion and perfusion MRI.

Conclusion: pH MRI improves the sensitivity of pH-weighted imaging and will be a valuable tool for assessing tissue viability in stroke.