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Mapping pH using stimulated echoes formed via chemical exchange
Yu Zhao1, Zhongliang Zu2, Junzhong Xu2
1Vanderbilt University Institute of Imaging Science, Vanderbilt University Medical Center, Nashville, TN, USA; Shanghai Key Laboratory of Magnetic Resonance, School of Physics and Electronic Science, East China Normal University, Shanghai, China.
A modified RACETE (refocused acquisition of chemical exchange transferred excitations) sequence offers specific sensitivity to solute exchange rates, enabling robust pH mapping. This advancement overcomes confounding factors present in previous methods.
Area of Science:
- Magnetic Resonance Imaging
- Biophysical Chemistry
- Medical Physics
Background:
- Refocused acquisition of chemical exchange transferred excitations (RACETE) images solute-water exchange but lacks biophysical specificity.
- Existing methods are sensitive to multiple factors including exchange rates, relaxation rates, and solute concentration.
- This limits their utility for precise biophysical measurements.
Purpose of the Study:
- To modify the RACETE sequence for specific sensitivity to solute exchange rates.
- To develop a novel metric for accurate pH mapping.
- To overcome confounding factors affecting previous exchange imaging techniques.
Main Methods:
- Modified the RACETE sequence by splitting gradients for echo formation dependent on solute-water exchange.
- Developed a ratio metric, preserving irradiation power to isolate exchange rate.
- Addressed artifacts using phase cancellation and employed a fast gradient-echo readout.
Main Results:
- The ratio metric demonstrated a single-variable dependence on exchange rate, with minimal sensitivity to confounding factors.
- Numerical simulations and phantom experiments validated the method's accuracy.
- The technique showed robustness against B0 and B1 inhomogeneities and removed direct water excitation artifacts.
Conclusions:
- The developed method enables fast and specific pH mapping.
- It offers robustness against confounding effects common in other imaging techniques.
- This provides a more reliable tool for quantitative biophysical measurements.
