Abstract
Measurement of lactate diffusion properties using diffusion-weighted magnetic resonance spectroscopy in vivo may allow elucidating brain lactate cellular compartmentation, which would be of great importance for neuroscience. However, measuring lactate signal is complicated by low signal-to-noise ratio due to low lactate concentration and J-modulation of its 1.3 ppm peak.In this work, we assess the benefits of using a diffusion-weighting spin echo block and spectrally selective refocusing pulses to suppress the effect of J-coupling on the 1.3 ppm lactate resonance, by not refocusing its coupling partner at 4.1 ppm. Two different kinds of spectrally selective pulses, either polychromatic or single-band, are evaluated in the mouse brain at 11.7 T. Almost complete suppression of J-modulation is shown, resulting in an approximately two-fold signal increase as compared to a reference STE-LASER sequence (for the specific diffusion times used in this work). Repeated measurements confirm that lactate diffusion-weighted signal attenuation is measured with an approximately two-fold precision.
Highlights
The idea has been proposed that measuring brain lactate (Lac) diffusion properties, using diffusion-weighted MR spectroscopy (DW-MRS), may shed some light about lactate distribution between extracellular and intracellular space, due to different diffusion properties within these two compartments [1]
Two series of independent experiments were conducted to evaluate the precision on lactate signal attenuation measurement: first, diffusion measurements using standard stimulated echo (STE)-LASER sequence (Fig. 1A), used as a reference, were compared to a ‘‘PC-spin echo (SE)-LASER” sequence, i.e. a sequence where the diffusion-weighted STE block is replaced by a diffusion-weighted SE block where refocusing is achieved with a polychromatic pulse (Fig. 1B and 1C); second, diffusion measurements using standard STE-LASER sequence were compared to SB-SE-LASER, i.e. a SE-LASER sequence where refocusing is achieved with a single-band (0.1–4.0 ppm range) refocusing pulse (Fig. 1B and 1D)
The STE-LASER used as reference is as described in the original article [11], where the STE block consists in three broad pulses and diffusion gradients, and the LASER block contains six adiabatic full passage (HS1) slice-selective pulses (Fig. 1A)
Summary
The idea has been proposed that measuring brain lactate (Lac) diffusion properties, using diffusion-weighted MR spectroscopy (DW-MRS), may shed some light about lactate distribution between extracellular and intracellular space, due to different diffusion properties within these two compartments [1]. Methods have been proposed to optimize detection of the 1.3-ppm lactate peak and separate it from macromolecules at 1.2–1.4 ppm, based on comparison/difference of spectra acquired at different echo times (TE) [8], or on MEscherGArwood (MEGA) J-editing [9] These methods require very long TE (>144 ms), which becomes impractical at high field because of shorter T2 relaxation times [10]. Incorporating diffusion gradients in such sequences is not trivial if one wants to avoid crossterms with other gradients
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