Proton MR Spectroscopy in a 1T Open MR Imaging System

Technique

Approval was obtained for this study from the institutional review board of the University of Vermont. Water-suppressed SVS of the brain was performed on a high-field open Panorama 1T MR imaging scanner (Philips Healthcare, Best, the Netherlands) by using PRESS localization with a 4-channel solenoid technology SENSE coil. PRESS SVS was added to routine clinical MR imaging examinations either with contrast injection (3-plane T1-weighted turbo spin-echo pre- and postcontrast; axial T2-weighted turbo spin-echo; T2-weighted FLAIR; T2-weighted gradient-echo; and diffusion-weighted imaging, total scan-session duration = 31 minutes) or without contrast injection (sagittal T1-weighted FLAIR instead of 3-plane pre- and postcontrast, total scan-session duration = 20 minutes). Spectra were acquired on 9 patients (5 women, 4 men; 28–78 years of age; mean age, 42 years). Voxels were placed in the centrum semiovale bilaterally in patients with normal-appearing brain on MR imaging. All 9 of these patients had a clinical indication of headache, and the results of the brain MR imaging examinations were interpreted as normal for patient age by a board-certified neuroradiologist with certificate of added qualification. The centrum semiovale was chosen given institutional experience with acquiring normative data on ¹H-MR spectroscopy at 1.5T and 3T within the centrum semiovale.

Two patients who failed imaging at 3T due to claustrophobia and who had intracranial mass lesions were studied in the 1T open MR imaging scanner with PRESS SVS. One was a 38-year-old man with a cystic enhancing lesion in the right frontal lobe with focal mass effect and slight shift from right to left. He had presented with headache to his primary care physician who ordered the MR imaging examination. A voxel was placed over the intracranial mass lesion, and he became claustrophobic and could not tolerate a voxel placed on the contralateral side. The second patient was an immunocompromised 51-year-old man with a CD4 count of 14, who presented with decreased cognition to his primary care physician, who ordered MR imaging. Brain MR imaging showed a lesion in the right temporal subcortical white matter without focal mass effect or enhancement. Voxels were placed over the lesion and over the contralateral normal-appearing left temporal subcortical white matter.

Data were acquired with TR/TE of 1500/144 ms with SVS voxel sizes of 2 cm³ with 192 signal-intensity averages with 1-kHz sampling bandwidth and an acquisition time of 4 minutes, 48 seconds. Taking advantage of open magnet configuration, we performed automatic left-to-right and superior-to-inferior table movements, so that voxels were acquired at the magnet isocenter. Spectroscopic features available to the technologist for this system included all those available for a 1.5T MR imaging scanner (eg, outer volume suppression, spectroscopic imaging). While the manufacturer-supplied protocol has a voxel size of 15 × 15 × 15 mm, a voxel as small as 10 × 10 × 10 mm is possible with the available gradients; for this study however, the manufacturer-recommended voxel size was used on the basis of initial trials to acquire adequate spectral SNR within 5 minutes. Automatic shimming was performed in all cases, and the mean of the full width at half maximum for the water peak averaged over all subjects was 0.10 ppm (4 Hz). For water suppression, a train of four 50-ms narrowband pulses was applied before the excitation pulse in a manner similar to the water suppression through enhanced T1 effects (WET) technique.⁸ The pulse shapes and flip angles were optimized to minimize sensitivity to the T1 of water and to the overall radio-frequency field strength.⁹ To avoid inadvertent saturation of metabolites close to water, we shifted the center of the water suppression notch 1 ppm downfield, away from the peaks of interest. As a result of this frequency-shift strategy, the CH₂ peak of creatine at 3.92 ppm was clearly present in all spectra. All spectra were processed with SpectroView (Philips Healthcare): eddy current correction by using an unsuppressed water reference signal-intensity time-domain filtering, fast Fourier transform, phase correction, Marquardt-Levenberg nonlinear least squares fitting of peaks and baseline by using prior knowledge of expected metabolites, and subtraction of the fitted baseline. NAA/Cr and Cho/Cr ratios were recorded along with presence or absence of lactate peaks. Absolute metabolite concentrations were not determined.