ERETIC

Magnetic resonance spectroscopy (MRS) is an established technique to assess metabolite distributions in vivo. Quantitative values are often reported as metabolite ratios or as ratios of metabolites to internal water (relative quantification). While many influence factors such as differences in coil load, B1 homogeneity or receive gain are directly considered, this approach has the major disadvantage of ambiguities whenever several metabolite concentrations, the tissue water content or relaxation times change simultaneously, as is the case for most diseases. Hence the development of a reliable reference standard that enables the assessment of tissue concentrations in mM (absolute quantification) is indispensable to precisely monitor metabolic changes for clinical diagnostics and physiological studies.

This project aims at reporting tissue metabolite concentrations in mM from in-vivo MR spectroscopy using the ERETIC (Electric REference To access In vivo Concentrations) method. To that a synthetic reference signal is injected into the receive coil during the acquisition of spectra and used as calibration standard. The primary advantage of ERETIC in comparison to phantom calibration methods is that it corrects for differences in coil loading and matching as well as receiver gain adjustments.

The method has been implemented on a clinical 3T MR scanner using an optical signal transmission to eliminate random fluctuations in ERETIC signal due to parasitic coupling to other RF sources. An auxiliary RF channel was used for signal transmission; thus standard channels remain available for either proton decoupling or parallel transmission. The ERETIC pulse shape, phase, decay time constant, amplitude and frequency can be adjusted directly at the scanner console. Phase cycling for single voxel spectra and phase encoding for the acquisition of MRSI spectra can also be applied, thus fully implementing a synthetic NMR signal. Frequency updates during a scan with multiple averages result in a simultaneous update of the ERETIC signal. Variations in the ERETIC signal depending on patient loading were avoided by placing the loop as close as possible to the receiver coil in order to couple the signal inductively and to avoid electric coupling effects.

So far, ERETIC was combined with 1H single voxel brain spectroscopy at 3T using a transmit/receive coil. Brain 1H MRS quantification results from healthy volunteers based on the ERETIC approach are in good accordance with the internal water reference and show a similar standard deviation. These results suggest that ERETIC is a promising approach to quantitative MRS in disorders in which assumptions about tissue water concentrations and relaxation times are unreliable.

In ongoing projects ERETIC shall be combined with MRSI, 13C and 31P MRS as well as 2D MRS and validated against tissue extracts.

Enlarged view: ERETIC: setup, appearance in the spectrum and cross validation with internal water reference in 5 healthy subjects
ERETIC: setup, appearance in the spectrum and cross validation with internal water reference in 5 healthy subjects

Publications:

  • Heinzer-Schweizer S, De Zanche N, Pavan M, Mens G, Henning A, Boesiger P. In-vivo measurement of absolute metabolite concentrations using the ERETIC method. Proc Intl Soc Mag Reson Med 2009;17.
  • Schweizer S, De Zanche N, Mens G, Henning A, Boesiger P. Advances in the ERETIC method for the quantification of in-vivo 1H and 31P spectra. Proc Intl Soc Mag Reson Med 2008;16.

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