Highly Accurate Quantification of Proton MR Spectroscopy in Rat Brain in Vivo at 16.4 T

H NMR Spectroscopy Spectra were acquired from 4 male adult Wistar rats that were anaesthetized by inhalation of 1.5-2 % concentration of isoflurane. All MR experiments were performed on a 16.4 T/26 cm horizontal bore Magnex magnet interfaced to a Bruker Biospec spectrometer with a 12 cm gradient coil (strength: 1000 mT/m, rise time: 212 µs). A home-built surface coil was used for transmitting RF pulses and receiving signals. To achieve an ultra-short TE, Gaussian shaped RF pulses (450 s) were employed in the STEAM sequence (TR, 4000 ms; TE, 2 ms; 400 averages; 2048 complex data points) with the VAPOR water suppression interleaved with three OVS modules. A 62 l voxel was positioned inside the brain encompassing the cortex and hippocampus. Field homogeneity was adjusted by calibrating all first- and second-order shim terms with FASTMAP, resulting in a tCr linewidth of 20 Hz (0.0293 ppm) in vivo. Spectrum Analysis All the acquired spectra were analyzed using LCModel [3]. In vitro spectra were collected from the following 20 metabolites in a buffered solution (pH = 7.2) at a temperature of 37 0C, kept constant by feedback-controlled heating pads: alanine (Ala), aspartate (Asp), Ascorbate (Asc), creatine (Cr), -aminobutyric acid (GABA), glucose (Glc), glutamate (Glu), glutamine (Gln), Glycine (Gly), glutathione (GSH), glycerophosphorylcholine (GPC), phosphorylcholine (PCho), myo-inositol (Ins), Histidine (His), lactate (Lac), N-acetylaspartate (NAA), Nacetylaspartylglutamate (NAAG), phosphocreatine (PCr), phosphorylethanolamine (PE), and taurine (Tau). These spectra were used as basis sets for the LCModel quantification of the in vivo data. Metabolite peaks were assigned by fitting with LCModel and verified using literature values [4]. Results A representative spectrum from a rat brain is shown in Fig 1, demonstrating excellent water suppression and high SNR, which made it possible to quantify, for the first time, the His peaks at 3.11 ppm, 3.22 ppm and 3.97 ppm in vivo, with Cramer-Rao lower bounds of less than 20 % in three of the four examined animals, indicating high quantification reliability. Especially the His peak at 3.11 ppm, which is usually concealed by the Crresonance, could be isolated. Discussion Due to the increased spectral resolution at the ultra-high field, the minimized J-modulation at the ultra-short echo time, and spectral fittings with LCModel based on basis sets from phantom solutions, the accurate and reliable quantification of in vivo brain metabolite spectra is possible. For the first time, this includes the reliable detection of the signal from His, which serves as precursor for Histamine and is connected to zinc excretion. Further improvements in the quantification accuracy will be possible by further reducing the echo time using asymmetric pulses and by taking account of the macro-molecular peaks in the quantification.