In vivo brain MRS at a 1.5T clinical scanner: Optimized derivative fast Fourier transform for high-resolution spectra from time signals encoded with and without water suppression

Abstract

We study single-voxel in vivo proton magnetic resonance spectroscopy (MRS) of white matter in the brain of a 25 year old healthy male volunteer. The free induction decay (FID) data of short length (0.5KB) are encoded at a long echo time (272 ms) with and without water suppression at a clinical scanner of a weak magnetic field (1.5T). For these FIDs, the fast Fourier transform (FFT) gives sparse, rough and metabolically uninformative spectra. In such spectra, resolution and signal to noise ratio (SNR) are poor. Exponential or Gaussian filters applied to the FIDs can improve SNR in the FFT spectra, but only at the expense of the worsened resolution. This impacts adversely on in vivo MRS for which both resolution and SNR of spectra need to be very good or excellent, without necessarily resorting to stronger magnetic fields. Such a long sought goal is at last within reach by means of the optimized derivative fast Fourier transform (dFFT), which dramatically outperforms the FFT in every facet of signal estimations. The optimized dFFT simultaneously improves resolution and SNR in derivative spectra. They are presently shown to be of comparably high quality irrespective of whether water is suppressed or not in the course of FID encodings. The ensuing benefits of utmost relevance in the clinic include a substantial shortening of the patient examination time. The implied significantly better cost-effectiveness should make in vivo MRS at low-field clinical scanners (1.5T) more affordable to ever larger circles of hospitals worldwide.