31P-MRS of the Human Heart at 7 T With an Integrated Whole-Body 31P Radiofrequency Transmit Coil.

Abstract

Phosphorus-31 magnetic resonance spectroscopy ( 31P-MRS) can be used to characterize the steady-state in vivo myocardial energy status via the phosphocreatine (PCr) over adenosine triphosphate (ATP) tissue concentration ratio. Although used in many studies, this modality suffers from low sensitivity and poor measurement precision. This work reports on the acquisition of 31P-MR spectra of the in vivo human heart using a 7 T MR system equipped with an integrated whole-body RF transmit coil for 31P spin excitation. Eight volunteers (male/female, n = 4/4) underwent 31P-MRS at 7 T twice, using a low-flip angle, short-repetition time 3D 31P-MR spectroscopic imaging sequence with a 20-mm isotropic resolution and a 21-min acquisition time. A 16-channel 31P receive array was used for signal reception. Myocardial voxels were identified on transversal cine proton MR images. Signals of PCr, phosphodiesters (PDE), and inorganic phosphate (P i) were quantified relative to ATP and corrected for partial saturation and blood signal contamination. Bland-Altman analyses were used to establish intersession measurement repeatability. A comparison of supine and prone positioning was made for two subjects, yielding similar results but superior comfort for lying supine. Mid-septal myocardial PCr/ATP was 1.85 ± 0.37, with a repeatability coefficient of 17.7%. The repeatability coefficients for P i/ATP and PDE/ATP were 142.7% and 51.6%, respectively. Using the spatially homogeneous 31P spin excitation with the integrated whole-body RF transmit coil, we made quantitative estimates of the myocardial energy status at a higher precision than previously achieved at lower main magnetic field strengths. The noninvasive nature and high precision of the presented 31P-MR spectroscopic imaging approach will allow for therapeutic efficacy monitoring with repeated measurements in longitudinal study designs and investigations of normal physiology in healthy volunteers.