Ultra-high field MRS of rodents

Rats and mice may serve as subjects for modeling various pathological conditions, which can be studied by localized MR spectroscopy. In contrast to MR imaging, which relies on properties of water related to its interaction with biomacromolecules, spectroscopy is capable of providing information on concentrations of chemical entities present in a living tissue. Measurements at high magnetic fields benefit from the higher signal-to-noise ratio (SNR) and increased spectral dispersion, which improves quantification accuracy and precision. The benefits are expected to be important especially for metabolites having low concentration or overlapping spectral lines and for compounds giving complex Jcoupled spectral patterns. These advantages, however, are partially offset by a decrease of T2* with increasing static magnetic field [1-3]. The increase of spectral dispersion at ultra-high magnetic fields is particularly useful for proton (H) MRS of rodent brain, which is complicated by overlapping peaks of a number of metabolites [4]. Fig. 1 compares proton MR spectra of rat brain measured at 9.4 Tesla and at 14.1 Tesla [5]. Two novel features can be seen at 14.1 T, in particular, a group of spectral lines resonating between 4.2 and 4.4 ppm, and the narrowing (in ppm) of the spectral lines of GABA, glutamate and N-acetylaspartate in the spectral range from 1.8 ppm to 2.6 ppm. The improved spectral dispersion enabled to discern several low-intensity peaks in the spectral region from 3.5 ppm to 4.2 ppm, which were assigned to lactate (4.11 ppm), glucose (3.85 ppm) and glycerophosphocholine (3.67 and 3.87 ppm). The resolved resonances at 4.31 and 4.27 ppm, which typically are saturated at lower fields by water suppression pulses, and additional peaks/peak shoulders of GPC resolved between 3.6 ppm and 3.9 ppm may substantially improve the Fig.1. 9.4 T (a) and 14.1 T (b) proton MR spectrum of a rat brain. The expanded region (c) shows assignment of small peaks in the 3.2 – 4.4 ppm region. accuracy of the GPC and phosphocholine