Establishing upper limits on neuronal activity-evoked pH changes with APT-CEST MRI at 7 T
Magnetic Resonance in Medicine
Purpose: To detect neuronal activity-evoked pH changes by amide proton transfer-chemical exchange saturation transfer (APT-CEST) MRI at 7 T. Methods: Three healthy subjects participated in the study. A low-power 3-dimensional APT-CEST sequence was optimized through the Bloch-McConnell equations. pH sensitivity of the sequence was estimated both in phantoms and in vivo. The feasibility of pH-functional MRI was tested in Bloch-McConnell-simulated data using the optimized sequence. In healthy
... cts, the visual stimuli were used to evoke transient pH changes in the visual cortex, and a 3-dimensional APT-CEST volume was acquired at the pH-sensitive frequency offset of 3.5 ppm every 12.6 s. Results: In theory, a three-component general linear model was capable of separating the effects of blood oxygenation level-dependent contrast and pH. The Bloch-McConnell equations indicated that a change in pH of 0.03 should be measurable at the experimentally determined temporal signal-to-noise ratio of 108. However, only a blood oxygenation level-dependent effect in the visual cortex could be discerned during the visual stimuli experiments performed in the healthy subjects. Conclusions: The results of this study suggest that if indeed there are any transient brain pH changes in response to visual stimuli, those are under 0.03 units pH change, which is extremely difficult to detect using the existent techniques.