A comparison of 2-hydroxyglutarate detection at 3 and 7T with long-TE semi-LASER

Abnormally high levels of the oncometabolite' 2-hydroxyglutarate (2-HG) occur in many grade II and III gliomas, and correlate with mutations in the genes of isocitrate dehydrogenase (IDH) isoforms. In vivo measurement of 2-HG in patients, using magnetic resonance spectroscopy (MRS), has largely been carried out at 3T, yet signal overlap continues to pose a challenge for 2-HG detection. To combat this, several groups have proposed MRS methods at ultra-high field (=7T) where theoretical increases in signal-to-noise ratio and spectral resolution could improve 2-HG detection. Long echo time (long-TE) semi-localization by adiabatic selective refocusing (semi-LASER) (TE = 110ms) is a promising method for improved 2-HG detection in vivo at either 3 or 7T owing to the use of broad-band adiabatic localization. Using previously published semi-LASER methods at 3 and 7T, this study directly compares the detectability of 2-HG in phantoms and in vivo across nine patients. Cramer-Rao lower bounds (CRLBs) of 2-HG fitting were found to be significantly lower at 7T (6 +/- 2%) relative to 3T (15 +/- 7%) (p = 0.0019), yet were larger at 7T in an IDH wild-type patient. Although no increase in SNR was detected at 7T (77 +/- 26) relative to 3T (77 +/- 30), the detection of 2-HG was greatly enhanced through an improved spectral profile and increased resolution at 7T. 7T had a large effect on pairwise fitting correlations between -aminobutyric acid (GABA) and 2-HG (p = 0.004), and resulted in smaller coefficients. The increased sensitivity for 2-HG detection using long-TE acquisition at 7T may allow for more rapid estimation of 2-HG (within a few spectral averages) together with other associated metabolic markers in glioma.