An Improved Magnetization Saturation Transfer Approach---T1nom for Rapidly Measuring and Quantifying CK Activity in the Rat Brain

Fei Du^1,2, Qiang Xiong³, Xiao-Hong Zhu¹, Wei Chen¹

The chemical exchange reactions (PCr↔ATP↔Pi) catalyzed by the creatine kinase (CK) and ATPase enzymes play key roles in maintaining brain function. In vivo 31P-MRS combined with magnetization saturation transfer (ST) provides a unique tool for assessing the reaction rate constants (kf). The accurate quantification of kf usually requires a long repetition time (tr) for approaching a steady-state saturation condition, however, suffers from limited sensitivity for biomedical application. This problem could be partially solved by shortening tr and allowing more signal averages. However, the partial saturation effect independent of kf makes kf quantification more complicate. Therefore, it is crucial to develop different approaches able to rapidly measuring and quantifying CK and ATPase enzyme activities in vivo. In this work, we present an improved magnetization ST approach, i.e.T1nom approach, for correcting the partial saturation effect with short tr. The results indicate a simple, linear relation between kf and the magnetization ratio of control and saturated 31P spectra. This approach was tested in the rat brain.