In Vivo Quantification of Creatine Kinase Kinetics in Mouse Brain Using ³¹P-MRS at 7 T

³¹P-MRS is a method of choice for studying neuroenergetics in vivo, but its application in the mouse brain has been limited, often restricted to ultrahigh field (> 7 T) MRI scanners. Establishing its feasibility on more readily available preclinical 7-T scanners would create new opportunities to study metabolism and physiology in murine models of brain disorders. Here, we demonstrate that the apparent forward rate constant (k_f) of creatine kinase (CK) can be accurately quantified using a progressive saturation-transfer approach in the mouse brain at 7 T. We also find that a 20% reduction in respiration of anesthetized mice can lead to 36% increase in k_f attributable to a drop in cellular pH and mitochondrial ATP production. To achieve this, we used a test-retest analysis to assess the reliability and repeatability of ³¹P-MRS acquisition, analysis, and experimental design protocols. We report that many ³¹P-containing metabolites can be reliably measured using a localized 3D-ISIS sequence, which showed highest SNR amplitude, SNR consistency, and minimal T₂ relaxation signal loss. Our study identifies key physiological factors influencing mouse brain energy homeostasis in vivo and provides a methodological basis to guide future studies interested in implementing ³¹P-MRS on preclinical 7-T scanners.