Field dependence study of in vivo brain ³¹P MRS up to 16.4T

In vivo ³¹P MRS provides a unique tool for studying bioenergetics of living organs. Although its utility has been limited by the relatively low ³¹P NMR sensitivity, increasing magnetic field strength (B₀) could significantly improve the quality and reliability of the ³¹P MR spectra for biomedical research. To quantitatively understand the field dependence of in vivo ³¹P MRS for brain applications, ³¹P NMR sensitivity of phosphocreatine (PCr) in rat brains was measured and compared at 9.4T and 16.4T. Additionally, the linewidths and T₁ relaxation times of PCr and adenosine triphosphate (ATP) resonances obtained from human and animal brains over a wide B₀ range from 4T, 7T, and 9.4T to 16.4T were examined and their field dependences were quantified. The results indicate an approximate 1.74-fold ³¹P signal-to-noise ratio (SNR) gain for PCr at 16.4T compared with 9.4T. An approximate power 1.4 dependence of ³¹P SNR on B₀ was concluded. Substantial improvements in spectral resolution and significantly shortened T₁ values of brain PCr and ATP were observed at high/ultrahigh fields, contributing to an additional sensitivity gain and spectral improvement. In summary, the overall findings from this study suggest that in vivo ³¹P MRS should greatly benefit from high/ultrahigh fields for noninvasive assessment of altered bioenergetics and metabolic processes associated with brain function and neurological diseases.