PRESS TE = 97 ms at 3T
We have developed a new MRS method, PRESS TE = 97 ms, that can measure 2HG, Gly, Cit, and other hard-to-detect metabolites at 3T with high precision (Choi et al. Nat Med 2012).
This method has several advantages over standard MRS:
- The 2HG signal at 2.25 ppm is narrow, improving 2HG selectivity with respect to the adjacent signals of glutamate (Glu) and glutamine (Gln). The Glu and Gln signals are also narrow and well detectable.
- Macromolecule signals are substantially attenuated due to the effects of their short TE, alleviating the spectral complexity.
- Third, the 2HG-optimized method allows detection of several other technically challenging brain metabolites such as glycine (Gly) and citrate (Cit) with good precision (low %SD).
- The signal of myo-inositol, a major obstacle in Gly detection, is extensively decreased and shows a two-peak pattern at 3.5 - 3.6 ppm; a Gly signal at 3.55 ppm can be easily resolved from the background myo-inositol signal.
- For Cit, the four strongly coupled spins give rise to a negative peak at 2.6 ppm. (The Cit signal overlaps with the NAA multiplet at ~2.6 ppm, but the NAA multiplet strength can be accurately estimated from the well-defined NAA singlet at 2.01 ppm. In tumors, NAA is decreased and the NAA signal at 2.6 ppm is essentially null [NAA6ppm/NAA2.01ppm ≈ 0.1]. )
High-Field MRS (7T)
High-field MRS benefits from enhancement of spectral resolution and signal gain. At 7T, the spectral analysis for 2HG remains complicated by the presence of the broad multiplets of GABA, Glu, and Gln and the complex baseline signals from macromolecules (MM). We have designed a new method for detection of 2HG in gliomas at 7T (Ganji et al. Magn Reson Med 2016). This method gives rise to a narrow 2HG signal at 2.25 ppm with negative polarity, thereby giving improved selectivity of 2HG with respect to the neighboring resonances compared to 3T. With this method, 2HG may be measurable without GABA contamination, which is important when 2HG elevation is moderate.
Echo planar spectroscopic imaging (EPSI)
EPSI provides an effective tool for high-resolution imaging of brain metabolites. We use a novel EPSI approach, a read-out gradient-alternated scheme, that can image brain metabolites with high spatial resolution, doubled spectral width, and low read-out gradient strength (low acoustic noise) compared to conventional EPSI acquisitions (An et al. ISMRM 2016). EPSI data is acquired following the aforementioned 2HG-tailored PRESS sequences at 3T and 7T. In vivo results from brain tumor patients are shown in the figures. The data were acquired with 6x6 mm2 resolution (slice thickness 14 mm), followed by zero filling by twofold.