Neurologic 3D MR spectroscopic imaging with low-power adiabatic pulses and fast spiral acquisition

Abstract

Purpose: To improve clinical three-dimensional (3D) MR spectroscopic imaging with more accurate localization and faster acquisition schemes.

Materials and methods: Institutional review board approval and patient informed consent were obtained. Data were acquired with a 3-T MR imager and a 32-channel head coil in phantoms, five healthy volunteers, and five patients with glioblastoma. Excitation was performed with localized adiabatic spin-echo refocusing (LASER) by using adiabatic gradient-offset independent adiabaticity wideband uniform rate and smooth truncation (GOIA-W[16,4]) pulses with 3.5-msec duration, 20-kHz bandwidth, 0.81-kHz amplitude, and 45-msec echo time. Interleaved constant-density spirals simultaneously encoded one frequency and two spatial dimensions. Conventional phase encoding (PE) (1-cm3 voxels) was performed after LASER excitation and was the reference standard. Spectra acquired with spiral encoding at similar and higher spatial resolution and with shorter imaging time were compared with those acquired with PE. Metabolite levels were fitted with software, and Bland-Altman analysis was performed.

Results: Clinical 3D MR spectroscopic images were acquired four times faster with spiral protocols than with the elliptical PE protocol at low spatial resolution (1 cm3). Higher-spatial-resolution images (0.39 cm3) were acquired twice as fast with spiral protocols compared with the low-spatial-resolution elliptical PE protocol. A minimum signal-to-noise ratio (SNR) of 5 was obtained with spiral protocols under these conditions and was considered clinically adequate to reliably distinguish metabolites from noise. The apparent SNR loss was not linear with decreasing voxel sizes because of longer local T2* times. Improvement of spectral line width from 4.8 Hz to 3.5 Hz was observed at high spatial resolution. The Bland-Altman agreement between spiral and PE data is characterized by narrow 95% confidence intervals for their differences (0.12, 0.18 of their means). GOIA-W(16,4) pulses minimize chemical-shift displacement error to 2.1%, reduce nonuniformity of excitation to 5%, and eliminate the need for outer volume suppression.

Conclusion: The proposed adiabatic spiral 3D MR spectroscopic imaging sequence can be performed in a standard clinical MR environment. Improvements in image quality and imaging time could enable more routine acquisition of spectroscopic data than is possible with current pulse sequences.

Figure 1:

A, Adiabatic spiral 3D MR spectroscopic imaging sequence. LASER based on GOIA-W(16,4) pulses is used to select the VOI and spiral waveforms on x and y gradients to simultaneously acquire two spatial dimensions and one frequency dimension. Phase-encoding gradients superimposed on the last LASER spoiler encode the z dimension. B, Section profiles of GOIA-W(16,4) pulses for on-resonance (0 ppm) and off-resonance (+1.75 ppm, −1.75 ppm) spins. C, K-space volume sampled with stack-of-spirals (black) and elliptical phase-encoding (red) protocols for 16 × 16 × 8 matrix. D–F, Spiral trajectories for, D, 16 × 16; E, 22 × 22; and, F, 30 × 30 matrixes; red dots are elliptical phase-encoding points. G, Spiral point spread function (PSF) corresponding to 30 × 30 matrix. H–I, Projections of 3D PSF along x, y (H), and z (I) directions for elliptical phase-encoding (ePE) and stack-of-spirals (spiral-acquired dimensions are specified inside brackets) protocols. J, PSF for weighted acquisition along z, using cos, cos² and Gauss weighting. K, Isosurfaces obtained with 3D PSF at full-width half-maximum. In all simulations, the FOV of 160 × 160 × 80 mm and spectral window of 1.2 kHz at 3 T were assumed.

Figure 2:

Elliptical phase-encoded and spiral (SP1-SP5) 3D MR spectroscopic imaging protocols in the double-layer and homogeneous brain phantoms. Left column: data from double-layer phantom show no signs of lipid contamination. Data from the homogeneous brain phantom are shown in the remaining columns. Entire spectral grids within the VOI (white rectangle), spectra from central voxels, and NAA maps are shown. NA = number of signals acquired, TA = acquisition time (specified in minutes, as obtained with a repetition time of 1000 msec).

Figure 3:

These 3D MR spectroscopic images of the brain were obtained in a volunteer with the elliptical phase-encoding protocol (PE) and SP2, SP3, and SP6. VOI spectral grids and examples of spectra from central voxels (red boxes) from two sections are shown. Sufficient SNR (>5) is obtained with spiral protocols at shorter acquisition times and higher spatial resolution. Spiral NAA maps show less partial volume effect by better delineation of ventricles. Acquisition time (TA) is specified in minutes (repetition time, 1200 msec).

Figure 4:

These 3D MR spectroscopic images were obtained in a patient with a brain tumor (glioblastoma) with the elliptical phase-encoding protocol (PE) and SP3 and SP4. Spectral grids and examples of spectra from voxels located in the healthy brain (blue boxes) and in the tumor (red boxes) are shown for two sections. Clinically adequate SNR (>5) is obtained with faster-acquisition and higher-spatial-resolution spiral protocols. Choline maps indicate that the active tumor and tumor necrosis are better delineated with high-spatial-resolution spiral protocols. Positions of FOV (yellow rectangle, green grid) and VOI (white rectangle) for each section are shown on axial and sagittal multiecho magnetization-prepared rapid acquisition gradient-echo cross sections. Acquisition time (TA) is specified in minutes (repetition time, 1000 msec).

Figure 5:

Overlays of spectra and LCModel fits for volunteers and patients with tumors. A, Measured spectra (blue) and LCModel fit (red) in a healthy volunteer obtained with a 16 × 16 × 8 matrix and 2-minute 58-second (repetition time, 1200 msec) spiral protocol. B, Measured spectrum (blue) and LCModel fit (red) in a patient with a brain tumor obtained with a 16 × 16 × 8 matrix and 2-minute 25-second (repetition time, 1000 msec) spiral protocol. C, D, Overlay of measured spectra recorded with all protocols in, C, a healthy volunteer and, D, a patient with a brain tumor. E, Overlay of LCModel fits for spectra from C. F, Overlay of LCModel fits for spectra from D. Similar voxel positions were chosen in all cases. PE = elliptical phase-encoding sequence.

Figure 6:

A–E, Bland-Altman plots indicate the limits of agreement between SP1–SP5 and elliptical phase-encoding (PE) measurements of NAA/Sum. F, Example of spectra normalized to area under spectral envelope (NAA/Sum). G–I, Mean values of NAA/Sum and choline-to-creatine (Cho/Cre) ratios are shown in, G, phantoms, H, volunteers, and, I, patients with tumors. Error bars = standard deviations.