Multivoxel 3D proton spectroscopy in the brain at 1.5 versus 3.0 T: signal-to-noise ratio and resolution comparison

Abstract

Background and purpose: The new 3.0-T imagers theoretically yield double the signal-to-noise ratio (SNR) and spectral resolution of 1.5-T instruments. To assess the possible improvements for multivoxel 3D proton MR spectroscopy (1H-MRS) in the human brain, we compared the SNR and spectral resolution performance with both field strengths.

Methods: Three-dimensional 1H-MRS was performed in four 21-29-year-old subjects at 1.5 and 3.0 T. In each, a volume of interest of 9 x 9 x 3 cm was obtained within a field of view of 16 x 16 x 3 cm that was partitioned into four (0.75-cm-thick) 16 x 16-voxel sections, yielding 324 (0.75-cm3) signal voxels per examination.

Results: In an acquisition protocol of approximately 27 min, average voxel SNRs increased 23-46% at 3.0 versus 1.5 T in the same brain regions of the same subjects. SNRs for N-acetylaspartate, creatine, and choline, respectively, were as follows: 15.3 +/- 4, 8.2 +/- 2.2, and 8.0 +/- 2.0 at 1.5 T and 22.4 +/- 7.0, 10.1 +/- 3.5, and 10.1 +/- 3.6 at 3.0 T. Spectral resolution (metabolite linewidths) were 3.5 +/- 0.5 Hz at 1.5 T versus 6.1 +/- 1.5 Hz at 3.0 T in approximately 900 voxels. Spectral baselines were noticeably flatter at 3.0 T.

Conclusion: Expected gains in SNR and spectral resolution were not fully realized in a realistic experiment because of intrinsic and controllable factors. However, the 23-46% improvements obtained enable more reliable peak-area estimation and an 1H-MRS acquisition approximately 50% shorter at 3.0 versus 1.5 T.

fig 1.

Schematic of the hybrid HSI-CSI localization sequence. A 25.6- or 15-ms, 60- or 120-Hz water suppression at 1.5 or 3.0 T was followed by VOI excitation. The PRESS 5.12-ms 90° pulse also performed fourth-order HSI (HSI) along Z_IS with 3 or 4.5 mT/m gradients at 1.5 or 3.0 T, respectively, followed 26.07 and 98.69 ms later by 10.24 ms SLR 180° pulses with 1 mT/m. Localization along X_LR and Y_AP was 16 × 16 2D CSI. A 25.6- or 15-ms gaussian 180° BASING pulse was applied between the two SLR pulses for extra water suppression

fig 2.

Corresponding (within 2–3 mm IS) axial T1-weighted MR images from the section richest in anatomic features from the same subject. Both are superimposed with the outline of the 9 × 9-cm VOIs and the real part of the corresponding 9 × 9 ¹H-spectra matrices. The horizontal (1.8–3.5-ppm) scale is common to both fields. A, Examination at 1.5 T. B, Examination at 3.0 T.

fig 3.

Five spectra from the corresponding dashed boxes in figure 2 scaled to the same maximum NAA peak height. Note an improved Cr-Cho spectral resolution, better SNR, and flatter baseline between 2 and 3 ppm at 3.0 versus 1.5 T