High‐resolution mapping of human brain metabolites by free induction decay ¹H MRSI at 7 T

Abstract

This work describes a new approach for high-spatial-resolution (1)H MRSI of the human brain at 7 T. (1)H MRSI at 7 T using conventional approaches, such as point-resolved spectroscopy and stimulated echo acquisition mode with volume head coils, is limited by technical difficulties, including chemical shift displacement errors, B(0)/B(1) inhomogeneities, a high specific absorption rate and decreased T(2) relaxation times. The method presented here is based on free induction decay acquisition with an ultrashort acquisition delay (TE*) of 1.3 ms. This allows full signal detection with negligible T(2) decay or J-modulation. Chemical shift displacement errors were reduced to below 5% per part per million in the in-slice direction and were eliminated in-plane. The B(1) sensitivity was reduced significantly and further corrected using flip angle maps. Specific absorption rate requirements were well below the limit (~20 % = 0.7 W/kg). The suppression of subcutaneous lipid signals was achieved by substantially improving the point-spread function. High-quality metabolic mapping of five important brain metabolites was achieved with high in-plane resolution (64 × 64 matrix with a 3.4 × 3.4 × 12 mm(3) nominal voxel size) in four healthy subjects. The ultrashort TE* increased the signal-to-noise ratio of J-coupled resonances, such as glutamate and myo-inositol, several-fold to enable the mapping of even these metabolites with high resolution. Four measurement repetitions in one healthy volunteer provided proof of the good reproducibility of this method. The high spatial resolution allowed the visualization of several anatomical structures on metabolic maps. Free induction decay MRSI is insensitive to T(2) decay, J-modulation, B(1) inhomogeneities and chemical shift displacement errors, and overcomes specific absorption rate restrictions at ultrahigh magnetic fields. This makes it a promising method for high-resolution (1)H MRSI at 7 T and above.

3D GABA imaging with real-time motion correction, shim update and reacquisition of adiabatic spiral MRSI

“…Yet, our VOI selection limits investigations only to a rectangular box, which complicates detection of these neurotransmitters in the cortex. Although, this is a limitation of most commonly available MRSI techniques (Kreis, 2004), there are alternative approaches with more brain coverage allowing detection of brain metabolites in cortical areas (Adalsteinsson et al, 1998; Bilgic et al, 2013; Bogner et al, 2012; Maudsley et al, 2009) and were applied successfully even to MEGA-edited 2D-MRSI (Zhu et al, 2011). Full brain coverage MRSI requires extremely good lipid suppression and/or lipid removal to eliminate the ringing of very strong lipid signals coming from bone marrow and subcutaneous fat which dominate the signal of brain metabolites.…”

High‐resolution 1H‐MRSI of the brain using SPICE: Data acquisition and image reconstruction

“…Therefore, further optimizations of the pulse sequences are desirable and currently being investigated. They include incorporating LASER-type of pulses (45,46) to reduce chemical shift displacement errors for improved spatial localization, and developing spin-echo and FID acquisitions to achieve shorter echo times (8,47) for improved SNR and detection of short-T 2 and J-coupled metabolites.…”

Density‐weighted concentric circle trajectories for high resolution brain magnetic resonance spectroscopic imaging at 7T

“…All used MRSI sequences are based on a single‐slice FID sequence 3, 4 with a three‐lobe sinc excitation pulse. Water‐suppression enhanced through T 1 ‐effects was used prior to signal acquisition.…”

“…Due to improvements in both sensitivity and spectral separation of metabolite resonances, MRSI is one of the MRI methods that should particularly benefit from ultra‐high static magnetic field strength ( $B_0\ge 7$ T), but technical challenges associated with B 0 / B 1 inhomogeneites, chemical shift displacement errors, specific absorption rate limits, and water/lipid suppression have long prevented a widespread application in patient studies 2, 3, 4, 5, 6. Only recent preliminary clinical applications of MRSI have been shown at 7T 7.…”