Abstract
PurposePhosphorus magnetic resonance spectroscopy (31P‐MRS) provides a unique tool for assessing cardiac energy metabolism, often quantified using the phosphocreatine (PCr)/adenosine triphosphate (ATP) ratio. Surface coils are typically used for excitation for 31P‐MRS, but they create an inhomogeneous excitation field across the myocardium, producing undesirable, spatially varying partial saturation. Therefore, we implemented adiabatic excitation in a 3D chemical shift imaging (CSI) sequence for cardiac 31P‐MRS at 7 Tesla (T).MethodsWe optimized an adiabatic half passage pulse with bandwidth sufficient to excite PCr and γ‐ATP together. In addition, the CSI sequence was modified to allow interleaved excitation of PCr and γ‐ATP, then 2,3‐DPG, to enable PCr/ATP determination with blood correction. Nine volunteers were scanned at 2 transmit voltages to confirm that measured PCr/ATP was independent of B1+ (i.e. over the adiabatic threshold). Six septal voxels were evaluated for each volunteer.ResultsPhantom experiments showed that adiabatic excitation can be reached at the depth of the heart using our pulse. The mean evaluated cardiac PCr/ATP ratio from all 9 volunteers corrected for blood signal was 2.14 ± 0.16. Comparing the two acquisitions with different voltages resulted in a minimal mean difference of −0.005.ConclusionAdiabatic excitation is possible in the human heart at 7 T, and gives consistent PCr/ATP ratios. Magn Reson Med 78:1667–1673, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Highlights
Phosphorus MR spectroscopy (31P-MRS) allows noninvasive assessment of concentrations and/or reaction kinetics of high-energy metabolites such as adenosine triphosphate (ATP) and phosphocreatine (PCr) in vivo [1,2,3,4,5]. 31P-MRS is of particular interest in cardiovascular medicine [6], as the PCr/ATP ratio in the heart changes in most major heart diseases, e.g., myocardial infarction [7], failing hypertrophied myocardium [8], or dilated cardiomyopathy [9,10], where it can even serve as a predictor of mortality [11]
We propose an interleaved ultra-short echo time (UTE)-chemical shift imaging (CSI) sequence with adiabatic excitation at 7 T to measure PCr/ATP in the human heart
High repeatability of the PCr/ATP measurement at 7 T was achieved with our technique, even though reduced transmit voltage was used in the second experiment
Summary
Phosphorus MR spectroscopy (31P-MRS) allows noninvasive assessment of concentrations and/or reaction kinetics of high-energy metabolites such as adenosine triphosphate (ATP) and phosphocreatine (PCr) in vivo [1,2,3,4,5]. 31P-MRS is of particular interest in cardiovascular medicine [6], as the PCr/ATP ratio in the heart changes in most major heart diseases, e.g., myocardial infarction [7], failing hypertrophied myocardium [8], or dilated cardiomyopathy [9,10], where it can even serve as a predictor of mortality [11]. Cardiac 31P-MRS is not yet recognized as a practical tool for clinical applications. This is primarily because of the inherently low signal-to-noise ratio (SNR) for 31P-MRS on clinical MR systems operating at less than or equal to 3 Tesla (T). To overcome this restraint, ultra-high fields (e.g., 7 T), leading to more than doubled SNR [4,15,16,17] and/or increased temporal resolution [18], are being used ever more often for 31P-MRS. Further improvement in SNR for cardiac 31P-MRS, accompanied with enhanced heart coverage, was recently demonstrated using a dedicated receive array combined with a singleloop transmit radiofrequency (RF) coil at 7 T [19,20]
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