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

Area of Science:

• Magnetic Resonance Imaging
• Biomedical Engineering
• Neuroimaging

Background:

• Chemical Exchange Saturation Transfer (CEST) MRI is sensitive to pH changes.
• Existing pH-enhanced (pH_enh) CEST imaging combines amide and guanidino proton signals but lacks optimized saturation parameters.
• Accurate pH imaging is crucial for assessing ischemic brain injury in stroke.

Purpose of the Study:

• To optimize saturation parameters for pH_enh CEST imaging.
• To develop a variant, pH_dual, for improved pH sensitivity and reduced background signal variations.
• To evaluate the potential of pH_dual for imaging acute ischemic brain injury in stroke.

Main Methods:

• Performed simulation and in vivo rodent stroke experiments using pH_enh MRI.
• Varied radiofrequency (RF) saturation powers for amide and guanidino protons to optimize contrast and minimize signal variations.
• Compared pH_dual imaging in acute stroke with amide-CEST, perfusion, and diffusion MRI techniques.

Main Results:

• Simulation showed unique pH sensitivities for amide and guanidino CEST signals.
• Optimized RF saturation power pairs (0.5 μT at 2.0 ppm & 1.0 μT at 3.6 ppm) maximized lesion/normal tissue contrast.
• Optimal contrast-to-variation ratio (CVR) was achieved with 0.7 μT at 2.0 ppm and 0.8 μT at 3.6 ppm.
• pH_dual demonstrated higher sensitivity than a three-point amide-CEST approach in acute stroke, showing distinct lesion patterns compared to diffusion and perfusion MRI.

Conclusions:

• pH_dual MRI enhances sensitivity for pH-weighted imaging.
• The optimized method provides a valuable tool for assessing tissue viability in stroke.
• pH_dual imaging offers improved characterization of ischemic brain injury.