With the recent non-significant risk classification of 7T MRI scanners by the US Food and Drug Administration, ultrahigh field in vivo
MR images and functional data with improved SNR and susceptibility contrast have been demonstrated at various institutions 14-16
. However, hardware and acquisition changes are necessary to accommodate the increased field strength 17,18
. For MR spectroscopic acquisitions at high field, enhanced sensitivity to magnetic susceptibility and uniformed excitation are two major fundamental challenges. Peak and average RF power requirements and coil performance issues also compound the acquisition process. Due to these challenges, prior 7 T MRS studies have focused single voxel acquisitions of relatively large 8 cm3
voxel sizes 5-7,19
or 2D acquisitions 20,21
. This approach has limited value for many applications due to the small spatial coverage across the brain and large voxel size. In this paper, we demonstrated the feasibility of acquiring 3D MRSI of the human brain at 7 T using SSRF and regular SLR with respective advantages and disadvantages.
A method employed by previous MRSI studies at lower fields to minimize the chemical shift artifact is to over prescribe the volume (OVERPRESS) and utilize VSS pulses to sharpen the edges of the PRESS-selected volume and saturate the outside 12
. With the increased field, power of these VSS is becoming an issue as each surface of the box required at least one pulse and additional ones if graphic saturation were prescribed. This required the redesign and implementation of optimized VSS pulses for 7 T MRSI. Using tools offered from the Electrical Engineering Department at Stanford University, a set of 0.12G peak power VSS were designed and implemented for the use at 7T. Based on visual inspection, it showed excellent spatial profile, yielding sharp edges for the PRESS selection. This is critical as no matter whether SSRF or SLR pulses were employed to select the PRESS volume, the high bandwidth, small transition band VSS pulses were required for out of volume suppression, sharpening the selected region and reducing chemical shift misregistration.
The use of SSRF is important due to the larger bandwidth requirement of the pulses to limit chemical shift artifact, which can be used in conjunction of OVERPRESS. The SSRF employed in this study also has adiabatic properties which allowed the acquisition to be much less sensitive to non-uniform excitation. The symmetric sweep property of the SSRF yielded B1 insensitivity; therefore, spectra remained consistent across the PRESS selection and all voxels were considered in the analyses. The customized low peak power and low SAR VSS pulses were also very important. At lower field, this may not be a concern because the SAR limit would allow multiple VSS to be played. However, at 7T, under a reasonable TR, only one VSS per band can be played within the SAR limit.
The linewidths in this 3D MR spectroscopic imaging study were higher than previously reported for single voxel 7 T MRS studies6
, due presumably to the much larger spatial coverage and increased Bo inhomogeneities for the MRSI data. The variation in spectral linewidth due to non-uniformity in the field may be improved by future improvements in higher order shimming hardware and software. Increased linewidths were observed near the edge of PRESS volume due to residual Bo inhomogeneities after higher order shimming.
The SSRF pulses provided the major advantage of having large bandwidth and adiabatic properties. This allowed high quality MRSI data to be acquired over a large 3-dimensional volume in the brain. However, a limitation to these pulses is the requirement of longer TE acquisitions due to the pulse length of the SSRF pulses. The two SSRF 180° pulses were each 30ms long compared to the 6.5ms pulse length of the SLR pulses. The SLR pulses allowed a TE of 35ms, in which short T2 metabolites were clearly observed. Due to modulation at higher field and properties of the SSRF, even at long TE, the Glx peak was clearly detected in the volunteer studies, shown in . Simulation has shown this peak is primarily Glu at this TE22
. This maybe of clinical interest, as Glu is readily detectable without editing techniques at 7T. Other short TE metabolites may also be visible, but due to the limited sweep width of the SSRF, they are not seen. Further studies are required to determine the modulation and corresponding modifications to the acquisition, which may further improve the visualization.
These initial 3D MRSI investigations of the human brain at 7T highlighted the need of optimal higher order shimming to address Bo variability across the volume. The top slice above the ventricle had the best shim, which can be observed visually and confirmed by the linewidth and SNR computed from the peak heights of the metabolites. The slices near and at the level of the nasal sinus air-tissue interface (moving from slice 2 to slice 3) showed worse SNR and larger linewidth due to high level of magnetic susceptibility. It is especially clear towards the edges of the PRESS box because the higher order shim region was manual selected with an ellipsoid covering most of the selection. The most inferior axial slice demonstrated the worst shim as it was the closest to the air-tissue interface and the resultant magnetic susceptibility shifts.
This initial investigation incorporated the design and application of new specialized rf pulse designs for obtaining high spatial resolution 3D MRSI at 7T from large selected volumes with increased B1 insensitivity and reduced chemical shift misregistration. The results of this study demonstrated the feasibility of this method to study metabolite distributions at 7T and highlighted the benefit of higher order shimming, low power very selective suppression pulses, and custom designed RF excitation pulses.