Fat suppression for 1H MRSI at 7T using spectrally selective adiabatic inversion recovery

Abstract

Proton magnetic resonance spectroscopic imaging ((1)H MRSI) at 7T offers many advantages, including increased SNR and spectral resolution. However, technical difficulties associated with operating at high fields, such as increased B(1) and B(0) inhomogeneity, severe chemical shift localization error, and converging T(1) values, make the suppression of the broad lipid peaks which can obscure targeted metabolite signals, particularly challenging. Conventional short tau inversion recovery can successfully suppress fat without restricting the selected volume, but only with significant metabolite signal loss. In this work, we have designed two new pulses for frequency-selective inversion recovery that achieve B(1)-insensitive fat suppression without degrading the signal from the major metabolites of interest. The first is a spectrally selective adiabatic pulse to be used in a volumetric (1)H MRSI sequence and the second is a spatial-spectral adiabatic pulse geared toward multi-slice (1)H MRSI. Partial interior volume selection may be used in addition to the pulses, to exclude areas with severe B(0) inhomogeneity. Some differences in the spectral profile as well as degree of suppression make each pulse valuable for different applications. 7T phantom and in vivo data show that both pulses significantly suppress fat, while leaving most of the metabolite signal intact.

Figure 1

(A) Real and imaginary components of the RF waveform and (B) spectral profile of the spectrally-selective adiabatic inversion pulse. The peak B₁ value of the pulse is well below the limit of our 7T RF amplifier, which is 17 μT. The transition bands for the spectral profile are thin enough to invert lipids while avoiding NAA. There is approximately 1% ripple in the inversion band.

Figure 2

Simulated inversion profile for the spectrally-selective adiabatic inversion pulse versus B₁ overdrive factor. Nominal B₁ for these simulations is set at the adiabatic threshold. Pulses may be overdriven by 290% before reaching the peak B₁ limit of 17 μT for the RF amplifier on our 7T magnet; however, the pulses maintain adiabaticity well beyond that limit.

Figure 3

(A) Real and imaginary components of the SPSP adiabatic inversion pulse. The peak B₁ value of the pulse is well below the 17 μT limit of our 7T RF amplifier. The associated oscillating gradient waveform is shown in Fig. 5. (B) Spectral profile and (C) main inversion band of spectral profile demonstrate that the selectivity as well as sideband distance are sufficient to invert lipids while avoiding most other metabolites of interest in the brain. (D) 2D spatial-spectral inversion profile shows the stability of the selected slice over a range of chemical shift frequencies.

Figure 4

Simulated inversion profiles for the adiabatic SPSP inversion pulse. (A) Spatial profile and (B) central inversion band of the spectral profile versus B₁ overdrive factor. Pulses may be overdriven by 150% before reaching the peak B₁ limit of our 7T RF amplifier.

Figure 5

(A) Modified PRESS sequence including fat-selective adiabatic inversion recovery. RF amplitude, phase and gradient waveforms for (B) the spectrally-selective adiabatic in-version pulse used in version 1 of the modified PRESS sequence and (C) the SPSP adiabatic inversion pulse used in version 2 of the modified PRESS sequence.

Figure 6

Single-voxel spectra from the GE MRS Sphere phantom are shown for (A) conventional PRESS, (B) conventional PRESS using standard non-selective STIR, and PRESS with selective adiabatic inversion recovery using (C) the spectral inversion recovery pulse and (D) the spatial-spectral inversion recovery pulse. Metabolite signals from the Cho, Cr and NAA singlet resonances are significantly reduced when non-selective STIR is used, but are unaffected by either selective adiabatic inversion recovery pulse. Spectra obtained from a phantom containing water and canola oil are shown for (E) conventional PRESS, (F) conventional PRESS using standard non-selective STIR, and PRESS with selective adiabatic inversion recovery using (G) the spectral inversion recovery pulse and (H) the spatial-spectral inversion recovery pulse. Both the spectral and spatial-spectral adiabatic inversion pulses provide a 7.5-fold reduction of the largest lipid peak at 1.3 ppm. Acquisition parameters are: 3.4 cc voxel, TI: 300 ms, TE/TR: 90/2000 ms, 4 NEX, 1:44 min scan time.

Figure 7

(A) Water image, (B) B₁ map and (C) B₀ map of a 1.4 cm slice of a normal human brain for which ¹H MRSI data was obtained at 7T. The 7×9 spectral grid within the prescribed PRESS box is shown in (A). The location of the spectral grid is also overlaid on the maps in (B) and (C) to show the expected B₁ and B₀ changes across the ROI. The resulting ¹H MRSI data are shown for (D) conventional PRESS, PRESS with selective fat suppression using (E) the spectral inversion recovery pulse and (F) the spatial-spectral inversion recovery pulse. Significant fat suppression is achieved with both the spectral and spatial-spectral adiabatic inversion pulses. All spectra are scaled to the same magnitude. Acquisition parameters are: single 1.4 cm slice, 20×20 cm FOV, 12×12 matrix (only central 7×9 portion within PRESS box is shown), 3.9 cc voxels, TI: 300 ms, TE/TR: 80/3000 ms, 1 NEX, 7:24 min scan time.

Figure 8

(A) Water image showing 4 selected voxels near the edge of the brain. Spectra from selected voxels are shown for (B) conventional PRESS, and PRESS with selective fat suppression using (C) the spectral inversion recovery pulse and (D) the spatial-spectral inversion recovery pulse. Both inversion pulses significantly suppress fat while not degrading signal from Cho (3.2 ppm), Cr (3.0 ppm) or NAA (2.0 ppm). Lipid Suppression Factor (LSF) achieved by the pulses is shown for the peripheral voxels in (C) and (D).