Small Animal ³¹P Cardiac MR Spectroscopy: Sequence and Setup Optimization

To determine the metabolic composition of mouse myocardium in vivo, the phosphorus (³¹P) magnetic resonance spectroscopy (MRS) setup was investigated and optimized.

After assessment of coil properties, typical spectroscopy sequences were evaluated against each other using metabolite phantoms of adenosine triphosphate (ATP) and phosphocreatine (PCr) on a Bruker 7 T small animal scanner, with respect to signal-to-noise ratio (SNR) and spatial selectivity.

The free induction decay (FID) in combination with outer volume suppression (OVS) showed the best SNR compared to the other spectroscopy sequences. OVS achieved a signal suppression of adjacent metabolites of approximately 84%. The performance of the setup was demonstrated in beating mouse hearts before and after myocardial infarction. A PCr to ATP(γ) ratio change from 2.5 pre-infarction to 1.74 approximately one week post-infarction was observed.

Among the evaluated methods, FID with OVS provided the best balance between localization accuracy and acquisition efficiency for application in the beating mouse heart. Despite probable signal contamination from surrounding tissue, in vivo measurements revealed post-infarction metabolic changes, evidenced by the reduction in the PCr/ATP(γ) ratio.

Due to the dynamic nature and small size of the target organ, examination of the beating heart with ³¹P MRS requires a signal intensity- and noiseoptimized implementation. The FID with OVS approach allows clear detection of variations in the ATP to PCr ratio before and after myocardial infarction within an acceptable measurement time (approximately 6.5 min). Image-selected in vivo spectroscopy (ISIS), the only serious competitor to FID with OVS, was found to be less suited for in vivo myocardium investigations due to inherent limitations in signal intensity and SNR.