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.