Detecting activity-evoked pH changes in human brain

Localized pH changes have been suggested to occur in the brain during normal function. However, a lack of methods for non-invasively measuring pH with high spatial and temporal resolution limits current knowledge of brain pH dynamics. Here I report that a magnetic resonance imaging (MRI) strategy named T₁ relaxation in the rotating frame (T_1ρ) is sufficiently sensitive to detect widespread pH changes in the mouse and human brain evoked by systemically manipulating carbon dioxide (CO₂) or bicarbonate (HCO₃). Moreover, T_1ρ detected changes suggesting a localized acidosis in the human visual cortex induced by a flashing checkerboard. Lactate measurements and pH-sensitive ³¹P spectroscopy at the same site also suggest a localized acidosis. Consistent with the established role for pH in blood flow recruitment, T_1ρ correlated with blood oxygenation level dependent contrast (BOLD), although T_1ρ was directly sensitive to blood oxygen content. These observations provide the strongest evidence thus far for localized pH fluctuations in the human brain during normal function. Furthermore, they suggest a novel functional imaging strategy based on pH that is independent of traditional fMRI contrast mechanisms.

Possible sources of acidosis include local metabolism, which is likely to correlate with the degree of stimulation and the associated changes in local neural activity. Therefore, we hypothesized that T_1ρ and pH changes would increase with increasing stimulation frequency. To test this hypothesis, we used a full-field visual flashing checkerboard and varied the frequency between 1, 4, and 7Hz. The response was imaged with T_1ρ, BOLD, and ³¹P spectroscopy. Supporting our hypothesis, we found that increasing stimulation frequency increased responses measured by all three imaging modalities. The activation area detected by T_1ρ overlapped to a large degree with that detected by BOLD, although the T_1ρ response area was significantly smaller. ³¹P spectroscopy detected a greater acidosis with the higher stimulation frequencies. These observations suggest that, similar to the BOLD response, the magnitude of the T_1ρ and pH response depends on stimulation frequency and is thus likely to be activity-dependent.

Brain acidosis is the end product of energy metabolism. Metabolically active cells lower local pH, the detection of which could help pinpoint regions activated by sensory stimuli, emotion, or cognitive task. fMRI mostly relies on BOLD changes in the venous system while arterial spin labeling (ASL) enables changes in tissue perfusion resulting from local cerebral blood flow (CBF) changes. BOLD contrast can be significantly distant from the actual site of neuronal activity because it relies on changes of the local magnetic field within veins. The venous contribution results in a loss of spatial specificity and spatial resolution of the BOLD response. In addition, the hemodynamic response to brief periods of neural activity is delayed. However, ASL contrast originates predominantly from tissue and capillaries. Even though functional signal changes detected by ASL have superior spatial and temporal resolution as compared to BOLD contrast, ASL contrast still suffers from poor temporal resolution due to delays in the hemodynamic response resulting from neurovascular coupling. Therefore, the ability to measure pH dynamics may provide a more localized and direct measure of brain activity. We hypothesized that pH-sensitive T_1ρ response in the visual cortex will temporally precede the hemodynamic response measured by functional imaging including BOLD and ASL contrast since local acidosis evoked by neural activity may drive the hemodynamic response. To test this hypothesis, dynamic imaging was performed using

T_1ρ, BOLD, and ASL while viewing a phase-encoded expanding ring stimulus which induces travelling waves of neural activity in the visual cortex. We calculated the phase maps for the eccentricity across their occipital cortices for each of functional signal and compared the T_1ρ temporal resolution with the hemodynamic response. This study suggests that T_1ρ signal has a higher temporal resolution as compared to the hemodynamic response. This is further evidence that the T_1ρ signal is not sensitive to blood oxygenation or other blood factors that might alter T_1ρ.

In conclusion, T_1ρ imaging has the potential to provide a new functional imaging marker that may be more specific to the area of brain activity. Therefore, it is possible that by non-invasively detecting pH dynamics in the human brain, T_1ρ MRI could offer a novel, more direct approach to map brain function. A number of psychiatric and neurological disorders could potentially benefit from the ability to study dynamic pH changes.