Since the early development of in vivo magnetic resonance spectroscopy (MRS) in the 1980's, a recurring problem associated with this technique has been the issue of how to analyze spectra in order to calculate me tabolite concentrations. Since the magnitude of the nuclear magnetization is directly proportional to the number of nuclei from which it originates, in principle there is a linear relationship between the voltage which is induced in the spectrometer receiver coil and the nuclear (i.e. molecular) concentration. However, a large number of additional factors, many of which are unknown, will also affect the amplitude of the detected signal, preventing the direct calculation of molecular concentrations from first principles.
Therefore, to date, all approaches to spectroscopic quantitation that have been published make use of the comparison of the amplitude of the signal to be detected with that of a known reference signal. Ideally, the reference signal originates from a well-characterized compound whose concentration is accurately known. Multiple different choices of reference signal have been proposed, either indigenous or exogenous compounds, with each method having its own particular advantages and disadvantages (1).
Russell H Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, 600 N Wolfe Street, Baltimore, MD 21287 and F.M Kirby Research Center for Functional Brain Imaging, Kennedy Krieger Institute, Baltimore, MD 21205. Author for Correspondence, Peter B. Barker: Phone (410) 955-1740, FAX (410) 614-2535, email: [email protected].
This article reviews some of the more commonly used methods for quantifying proton spectra of the human brain, with a particular emphasis on determining the concentration of N-acetyl aspartate (NAA). The article does not cover methods for quantifying spectra from other organ systems, or for heteronuclear spectroscopy, although many analysis methods for proton brain spectra may well be applicable in these other instances also. The article also does not cover, except in passing, the many numerical methods which have been developed for estimating spectral peak areas (2-4), which is a pre-requisite for quantitative analysis. Finally, a method for quantitative analysis of spectroscopy data from multi-coil receiver arrays is discussed, and an example of NAA measurements in low- and high-grade human brain tumors is presented.
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