SNR optimized ³¹P functional MRS to detect mitochondrial and extracellular pH change during visual stimulation

Abstract

Summary

Energy metabolism of the human visual cortex was investigated by performing ³¹P functional MRS.

Introduction

The human brain is known to be the main glucose demanding organ of the human body and neuronal activity can increase this energy demand. In this study we investigate whether alterations in pH during activation of the brain can be observed with MRS, focusing on the mitochondrial inorganic phosphate (Pi) pool as potential marker of energy demand.

Methods

Six participants were scanned with 16 consecutive ³¹P‐MRSI scans, which were divided in 4 blocks of 8:36 minutes of either rest or visual stimulation. Since the signals from the mitochondrial compartments of Pi are low, multiple approaches to achieve high SNR ³¹P measurements were combined. This included: a close fitting ³¹P RF coil, a 7 T‐field strength, Ernst angle acquisitions and a stimulus with a large visual angle allowing large spectroscopy volumes containing activated tissue.

Results

The targeted resonance downfield of the main Pi peak could be distinguished, indicating the high SNR of the ³¹P spectra. The peak downfield of the main Pi peak is believed to be connected to mitochondrial performance. In addition, a BOLD effect in the PCr signal was observed as a signal increase of 2–3% during visual stimulation as compared to rest. When averaging data over multiple volunteers, a small subtle shift of about 0.1 ppm of the downfield Pi peak towards the main Pi peak could be observed in the first 4 minutes of visual stimulation, but no longer in the 4 to 8 minute scan window. Indications of a subtle shift during visual stimulation were found, but this effect remains small and should be further validated.

Conclusion

Overall, the downfield peak of Pi could be observed, revealing opportunities and considerations to measure specific acidity (pH) effects in the human visual cortex.

Abstract

Energy metabolism of the human visual cortex was investigated by performing ³¹P functional MRS, focused on mitochondrial inorganic phosphate (Pi _mi/ex) as potential marker of energy demand. Since the concentration is low, multiple approaches to acquire high SNR ³¹P measurements were combined. Results show that the mitochondrial Pi resonance could be detected. During visual stimulation a small subtle shift was observed, especially in the first 4 minutes. Overall, this reveals opportunities and considerations to measure pH effects in the visual cortex.