On the voxel size and magnetic field strength dependence of spectral resolution in magnetic resonance spectroscopy

While the inherent low sensitivity of in vivo MR spectroscopy motivated a trend towards higher magnetic fields, B₀, it has since become apparent that this increase does not seem to translate into the anticipated improvement in spectral resolution. This is attributed to the decrease of the transverse relaxation time, T₂*, in vivo due to macro- and mesoscopic tissue susceptibility. Using spectral contrast-to-noise ratio (SCNR) arguments, we show that if in biological systems the linewidth (on the frequency scale) increases linearly with the field, the spectral resolution (in parts per million) improves approximately as the fifth-root of B₀ for chemically shifted lines and decreases as about B₀^4/5 (in hertz) for a structure of J-coupled multiplets. It is also shown that for any given B₀ there is a unique voxel size that is optimal in spectral resolution, linking the spectral and spatial resolutions. Since in practical applications the spatial resolution may be dictated by the target anatomy, nomograms to determine the B₀ required to achieve the desired spectral resolution at that voxel size are presented. More generally, the scaling of the nomograms to determine the achievable spectral and spatial resolutions at any given field is described.