3D 31P MR spectroscopic imaging of the human brain at 3 T with a 31P receive array: An assessment of 1H decoupling, T1 relaxation times, 1H-31P nuclear Overhauser effects and NAD+

P-31 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-31 MRSI is currently limited to research purposes. To obtain 3D P-31 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-31 eight-channel phased-array receive insert. To further increase resolution and sensitivity we applied WALTZ4 H-1 decoupling and continuous wave nuclear Overhauser effect (NOE) enhancement and acquired high-quality MRSI spectra with nominal voxel volumes of similar to 17.6 cm(3) (effective voxel volume similar to 51 cm(3)) in a clinically relevant measurement time of similar to 13 minutes, without exceeding SAR limits. Steady-state NOE enhancements ranged from 15 +/- 9% (gamma-ATP) and 33 +/- 3% (phosphocreatine) to 48 +/- 11% (phosphoethanolamine). Because of these improvements, we resolved and detected all P-31 signals of metabolites that have also been reported for ultrahigh field strengths, including resonances for NAD(+), NADH and extracellular inorganic phosphate. T-1 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-1 relaxation times and NOE enhancements at 3 T with published values between 1.5 and 9.4 T indicates that T-1 relaxation of P-31 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-31 MRSI information content can be obtained using a sophisticated coil design combined with H-1 decoupling and NOE enhancement.

Automated summary

P-31 MRSI is a versatile technique for studying phospholipid precursors and energy metabolism in the human brain, but its low sensitivity limits its clinical application. By using a sophisticated coil design and techniques such as H-1 decoupling and NOE enhancement, high-quality MRSI spectra can be obtained at a clinical field strength of 3 T, similar to ultrahigh field strengths.