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Spatially resolved metabolite determination in the brain

Especially interesting for

  • physicians
  • neuroscientists
  • manufacturers of MRI devices

Autologous substances in the brain – so-called “metabolites” – can serve as biomarkers, for example, in neurological diseases. The concentration of metabolites and their location within the brain can now be determined more precisely by combining the so-called “SPECIAL” Magnetic Resonance Spectroscopy (MRS) method with spectroscopic imaging at 3 tesla. In studies with volunteers using the 3T MR scanner of the PTB, it was possible to determine the concentration as well as the distribution of up to 14 different metabolites. This was the first time that the SPECIAL method was used in spectroscopic imaging on a clinical scanner.

Voxel arrangement for MRSI (left) and corresponding spectra (centre) from the parietal lobe of the brain of a volunteer; acquired with the SPECIAL MRSI sequence. Right: example of a spectrum of a voxel consisting mainly of white matter (WM). The spectral lines represent metabolites, their amplitudes represent their concentrations. The narrow linewidths and the defined spectral lines are proof of the high quality of the data.

Magnetic resonance imaging (MRI) relies on the nuclear spin of water protons – which are very numerous in the organism – in order to obtain highly resolved images of tissues. By selecting the parameters of the so-called “MR measurement sequences”, it is, however, also possible to differentiate and quantify biochemical substances in a spatially resolved way, based on different chemical environment of the protons within these metabolites. MRS (Magnetic Resonance Spectroscopy) is used to obtain quantitative information on metabolites in the human brain, i.e. neurotransmitters and amino acids. It benefits from high and ultrahigh magnetic field strengths (≥ 3 tesla), which are becoming increasingly common. Due to the low concentrations of metabolites, the signal strengths in MRS are, however, up to 10 000 times lower than in MRI. Thus, the requirements for spectral resolution and for the data quality of the measurement procedures for the quantification of metabolites are exceptionally high.

A new method, which meets these criteria, is the so-called “spin echo full intensity acquired localized (SPECIAL) technique” with which MRS signals can be measured relatively sensitively. It has already been demonstrated in studies with MR scanners at field strengths of 3 T and 7 T that this method can provide precise data concerning metabolites in single volume elements (voxels) of the brain. Many applications, however, require data from several voxels, for example, in order to obtain the distribution of metabolites in larger areas of the brain.

With the 3-tesla scanner of the PTB, the short echo times of the SPECIAL method have now been combined with the spatial selection of the magnetic resonance spectroscopic imaging (MRSI). This combination has made it possible to detect different metabolites in several voxels of a freely selectable brain region. In healthy volunteers, artefact-free spectra with high signal/noise ratio were recorded with an echo time of 6.6 milliseconds. Based on this, it was possible to determine the concentrations of 8 metabolites reliably; for certain voxels, 14 metabolites were even reliably quantified. The obstacle to a broad application in the medical field is the still rather long measuring duration (34 minutes for one MRSI dataset). Currently, efforts are being undertaken to reduce this long scan time.

Contact

Ralf Mekle
Department 8.1 Medical Metrology
Tel. +49(0)30 3481-7767
E-Mail: ralf.mekle(at)ptb.de

Scientific publication

Mekle, R.; Mlynarik, V.; Walaszek, B.; Gruetter, R.; Ittermann, B.; Schubert, F.: 1H SPECIAL-MRSI at Ultra-Short TE: Improved Metabolite Detection for Multiple Voxels in Human Brain at 3T. Proc. 19 ISMRM, Montreal, Canada, 7.–13.5.2011, Abstract 3422