Abstract
Purpose/Introduction: With ERETIC (Electrical REference To access Invivo Concentrations) the areas of the metabolite resonances are converted into absolute concentrations by comparing them to an artificially generated signal [1, 2]. As a further development and simplification of the phantom replacement method [3] it has, especially in the presence of lesions or pathological disorders, considerable advantages compared to methods using internal references
[4]. Since Eddy currents, arising from switching field gradients, falsify the quantification results, it is highly desirable to be able to use ERETIC as reference also in eddy current corrected spectra. In this work, we combine ERETIC and the eddy current correction proposed by Klose [5] with no need for additional post processing.
Subjects and Methods: In the ERETIC setup (1H MRS) used here [2] the electrical reference signal is generated by the spectrometer itself. An amplitude modulated RF pulse is sent through an auxiliary channel and transmitted over
an optical system to the coil, where it is demodulated and coupled inductively to the receive coil over a small loop [Fig. 3]. To avoid lineshape distortions of the ERETIC peak in the eddy current corrected spectra [Fig. 2], the used RF pulse is modified. Directly after the water reference scan the time-dependent phase of the complex water signal is
calculated at every time point and added to the ERETIC signal by frequency modulation of the RF pulse. In a sense, an inverse Klose correction is thus applied to the ERETIC signal. This method was implemented in the control
software of a Philips Achieva 3T human MRI scanner (Philips Healthcare, Best, The Netherlands). All signals were corrected for eddy current effects using the Klose approach and fitted using LCModel with simulated basis sets.
Results: With the modifications for the ERETIC signal proposed here, the ERETIC peak is automatically in phase with all the other metabolites and shows no line distortion in eddy current corrected spectra [Fig. 4]. Therefore
the ERETIC peak can be simultaneously fitted with commercially available algorithms without the need of changing fit parameters from scan to scan.
Discussion/Conclusion: In conclusion, the usability of ERETIC was increased by implementing an automatic phase correction and eddy current correction compensation. With this, fitting and data handling become far less demanding,
which is an important step towards clinical applicability of ERETIC.
