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

Abstract

31P 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, 31P MRSI is currently limited to research purposes. To obtain 3D 31P 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 31P eight‐channel phased‐array receive insert. To further increase resolution and sensitivity we applied WALTZ4 1H decoupling and continuous wave nuclear Overhauser effect (NOE) enhancement and acquired high‐quality MRSI spectra with nominal voxel volumes of ~ 17.6 cm3 (effective voxel volume ~ 51 cm3) 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 31P signals of metabolites that have also been reported for ultrahigh field strengths, including resonances for NAD+, NADH and extracellular inorganic phosphate. T1 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 T1 relaxation times and NOE enhancements at 3 T with published values between 1.5 and 9.4 T indicates that T1 relaxation of 31P 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 31P MRSI information content can be obtained using a sophisticated coil design combined with 1H decoupling and NOE enhancement.