Towards 1H-MRSI of the human brain at 7T with slice-selective adiabatic refocusing pulses

Abstract

Objective: To explore the possibilities of proton spectroscopic imaging (1H-MRSI) of the human brain at 7 Tesla with adiabatic refocusing pulses.

Materials and methods: A combination of conventional slice selective excitation and two pairs of slice selective adiabatic refocusing pulses (semi-LASER) results in the formation of an echo from a localized volume. Depending on the used radio frequency (rf) coil efficiency and available rf power, the duration of the adiabatic full passage pulses (AFPs) is adapted to enable echo times down to 50 ms (head coil) or 30 ms (local surface coil).

Results: An AFP duration of 5 ms with a corresponding bandwidth of 5.1 kHz resulted in a chemical shift displacement error of 23% over 3.8 ppm at 7T. Using a local surface coil and an echo time down to 30 ms, we detected not only the three main metabolites (NAA, Cr and Cho), but also coupled signals from myo-inositol and glutamate/glutamine in spectra from 0.14 cc voxels with linewidths down to 10 Hz in 10 min measurement time.

Conclusions: The semi-LASER pulse sequence enables 1H-MRSI of the human brain at 7T for larger parts of the brain as well as small localized areas with both a high spectral and spatial resolution.

Fig. 1

The core of the semi-LASER spectroscopic imaging pulse sequence. Crusher gradients are positioned around every adiabatic full passage (AFP) pulse, with the largest pair around the final AFP pulse. Phase-encoding gradients in 2 or 3Ds are superimposed on the final crusher gradient

Fig. 2

Localization and excitation profile of semi-LASER ¹H-MRSI in an oil phantom at 7T. The white box in a represents the VOI of the MRSI experiment, the blue box is the size of the FOV. The gradient echo image is overlaid with a color-coded image of the integral of a Lorentzian fit to the oil resonance, showing an exact match of excited signal and VOI

Fig. 3

2D ¹H-MRSI of the brain of a healthy volunteer at 7T. In a the 2D FOV of the MRSI matrix is outlined in yellow (VOI in white) and overlaid on a transverse T₂-weighted TSE image. In the sagittal image inset, the position of this slice in the brain is indicated. The spectra from voxels inside the blue box are overlaid onto the T₂-weighted image in a spectral map b with range 1.5–4.3 ppm. The spectrum of the centre voxel of the spectral map (blue voxel) is enlarged in c. Color-coded overlays of the integral of Lorentzian fits to n-acetylaspartate (NAA), choline (Cho), creatine/phosphocreatine (Cr) and water are shown in d to g

Fig. 4

MRI and 3D ¹H-MRSI of a small part of the brain of a healthy volunteer with a local surface coil at 7T. In an axial gradient echo localizer image (a; r,l,a,p is right, left, anterior, and posterior, respectively) the plane of the spin echo images parallel to the coil conductors (b, TE 95ms and c, TE 11 ms) is indicated with the white line. The VOI of the 3D MRSI matrix is indicated with the white box in a–c. In two perpendicular spectral maps of the VOI of the 3D MRSI matrix the spectra are displayed from 1.8 to 4.3 ppm. In a plane perpendicular to the coil d the signal decreases with distance to the coil, mainly because of the B₁ reception profile. In a plane almost parallel to the coil e, the intensities of the different signals in the spectra are more homogeneous throughout the VOI. Voxels largely overlap, as the true size of a voxel is approximated by a 3.2 mm-radius sphere. The SNR of a single spectrum of the 3D dataset (location illustrated in g) still allows the identification of many different metabolite signals f. Spectral postprocessing existed of apodization (400ms Hamming window centered at 0 ms), Fourier transformation and manual zero-order phase correction