3D P MR spectroscopic imaging of the human brain at 3 T with a P receive array: An assessment of H decoupling, T relaxation times, H- P nuclear Overhauser effects and NAD.

P MR spectroscopic imaging (MRSI) is a versatile technique to study phospholipid precursors and energy metabolism in the healthy and diseased human brain. However, mainly due to its low sensitivity, P MRSI is currently limited to research purposes. To obtain 3D P MRSI spectra with improved signal-to-noise ratio on clinical 3 T MR systems, we used a coil combination consisting of a dual-tuned birdcage transmit coil and a P eight-channel phased-array receive insert. To further increase resolution and sensitivity we applied WALTZ4 H decoupling and continuous wave nuclear Overhauser effect (NOE) enhancement and acquired high-quality MRSI spectra with nominal voxel volumes of ~ 17.6 cm (effective voxel volume ~ 51 cm ) in a clinically relevant measurement time of ~ 13 minutes, without exceeding SAR limits. Steady-state NOE enhancements ranged from 15 ± 9% (γ-ATP) and 33 ± 3% (phosphocreatine) to 48 ± 11% (phosphoethanolamine). Because of these improvements, we resolved and detected all P signals of metabolites that have also been reported for ultrahigh field strengths, including resonances for NAD , NADH and extracellular inorganic phosphate. T times of extracellular inorganic phosphate were longer than for intracellular inorganic phosphate (3.8 ± 1.4s vs 1.8 ± 0.65 seconds). A comparison of measured T relaxation times and NOE enhancements at 3 T with published values between 1.5 and 9.4 T indicates that T relaxation of P metabolite spins in the human brain is dominated by dipolar relaxation for this field strength range. Even although intrinsic sensitivity is higher at ultrahigh fields, we demonstrate that at a clinical field strength of 3 T, similar P MRSI information content can be obtained using a sophisticated coil design combined with H decoupling and NOE enhancement.