Improved Metabolite Quantification for Multiple Voxels in the Human Brain
Magnetic resonance spectroscopy (MRS) techniques can provide a wealth of additional information with respect to MR imaging by quantifying concentrations of MR-sensitive substances. For example, proton (1H) MRS in the human brain is often applied to obtain quantitative, local information about brain metabolites, such as creatine (Cr), glutamate (Glu), glutamine (Gln), etc. In general, metabolite quantification is enhanced using advanced acquisition strategies yielding high spectral quality and high signal-to-noise ratios (SNRs). A recent example for such an advanced acquisition method is the "spin echo full intensity acquired localized" (SPECIAL) MRS technique that benefits from signal acquisition at so-called short echo time (TE). The SPECIAL sequence was previously used in 1H single voxel studies on clinical platforms at 3 and 7 tesla (T), and improved metabolite quantification was demonstrated. However, many applications require spectra from multiple voxels as can be acquired in MR spectroscopic imaging (MRSI), e.g. to avoid sampling issues in tumor diagnostics. In MRSI, similar to MR imaging spatial encoding is used to resolve information from multiple spatial locations. In this study, 1H MRSI using SPECIAL at very short echo times (< 10 ms) in human brain was realized on a clinical scanner at 3T. It was determined whether an increased number of metabolites could be quantified in multiple voxels.
MR scans were performed on a 3T Verio system (Siemens Medical Solutions, Erlangen, Germany). 1H‑MRSI data were acquired for five volunteers using the SPECIAL technique with an echo time TE = 6.6 ms. Resulting spectra were of high data quality and showed no major artifacts (see Figure 1). Data analysis was performed for all voxels within the volume-of-interest (VOI). Eight metabolites were reliably detected in all usable 25 voxels from the VOI (excluding some voxels at the edges of the VOI). On average, between 8 and 14 metabolites were reliably quantified for each voxel.
Figure 1. 1H MRSI grid (left) and spectra (center) from the parietal lobe of a human volunteer acquired with the SPECIAL-MRSI sequence. The white square corresponds to the VOI, and the yellow square corresponds to the field-of-view (FOV) of the acquisition scheme. Sample spectra from a mainly white matter (WM) and a gray matter (GM)-rich voxel are shown enlarged (right). Note the high spectral quality of the data.
For the first time, 1H MRSI using the SPECIAL technique was realized on a clinical platform. The high data quality allowed the reliable quantification of several metabolites in multiple voxels. Schemes to reduce scan time are currently being explored to render the technique even more attractive for clinical application.
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