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

## TL;DR

This study shows how to use 31P-MRS on 7-T scanners to measure creatine kinase activity in mouse brains, revealing how respiration changes affect brain energy metabolism.

## Contribution

Demonstrates the feasibility of quantifying creatine kinase kinetics in mouse brains using 7-T 31P-MRS and identifies physiological factors affecting brain energy homeostasis.

## Key findings

- A 20% reduction in respiration leads to a 36% increase in the apparent forward rate constant of creatine kinase.
- Localized 3D-ISIS sequence provides reliable measurements of 31P-containing metabolites with high SNR and minimal signal loss.
- Physiological factors like pH and mitochondrial ATP production significantly influence mouse brain energy homeostasis.

## Abstract

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.

We demonstrate the feasibility of using 31P‐MRS to quantify creatine kinase activity in the mouse brain on preclinical 7‐T scanners. A 20% drop in respiration increased the apparent forward rate constant by 36%, likely due to altered pH and mitochondrial ATP production. Our study establishes reliable 31P‐MRS protocols and identifies key factors affecting brain energy homeostasis, providing a foundation for future research in murine models of brain disorders.