Edinburgh Napier University

  Magnetic Resonance Spectroscopy (MRS) can give chemically specific information about
biological tissue and has become an active area of research in human metabolism in health
and disease. In particular, the (1)H MKS modality is suited to studies of the brain, and the
commonly studied metabolites that have important biochemical roles are choline, creative
and N-acetyl aspartate (NAA). The success of MRS as a medical diagnostic tool depends
heavily on accurate quantification of the data by computer analysis, in order to determine
frequencies for the identification of metabolites and peak areas to obtain metabolite
concentrations. MRS performed on standard clinical scanners has problems regarding the
homogeneity of the magnetic field across the region being measured and this results in a
broadening of the inherently Lorentzian peaks of metabolite resonances. T'he broadening
effect can be approximated by a Gaussian function and so observed peaks are of a Voigt
form. However, traditional computer modeling of MRS data usually assumes a purely
Lorentzian model. Although the Voigt function is theoretically the most appropriate
model for quantification, its complex form presents an unnecessary time burden when
analyzing multiple spectra. An accurate approximation to the Voigt function, consisting of
a linear summation of Lorentzian and Gaussian functions, is given in this work. The
maximum error in determining peak areas using the approximation is shown to be 0.72%
over the entire range of simulated Voigt line shapes. Furthermore, it is shown that the
approximation can be derived analytically from the actual Voigt function in the regions where the Lorentzian or Gaussian components are dominant.
The (1)H spectra of dilute solutions containing choline, creative and sodium acetate were
collected from an Elscint Prestige 1.9T and a GE Sigma EchoSpeed 1.ST clinical scanner
and quantified by fitting with model line shapes using a Levenberg-Marquardt non-linear
least squares routine. Compared with an approximated Voigt model, fitting with purely
Gaussian and Lorentzian models resulted in a increase of (% mean ± SE) that was as
much as 19.06 ± 0.39 and 405.48 ± 5.66 respectively. Similar trends were observed from fitting two in vivo spectra of a healthy volunteer, where using purely Gaussian and
Lorentzian models resulted in a % increase of that was around 3% and 20%
respectively. The relaxation times T(1) and T(2) of choline, creative and sodium acetate were
estimated from measurements performed on the two clinical scanners and compared with
estunates from similar measurements performed on a high resolution spectrometer. After
correcting spectra for relaxation effects, the choline/creative and the choline/sodium
acetate peak area ratios (mean ± SE) were 3.01 ± 0.02 and 3.03 ± 0.03 respectively on the
GE scanner compared to an ideal value of 3. Use of an inappropriate line shape model
leads to systematic errors not only in peak areas, but also area ratios of overlapping
spectral peaks. Compared to fitting with the approximated Voigt model, noticeable
discrepancies in the choline/creative area ratio was observed when fitting with the
Gaussian model (3.7%) and the Lorentzian model (15.7%). Similarly for the in vivo
spectra, a noticeable discrepancy in the NAA/creatine area ratio was observed when fitting
with the Gaussian model (4.8%) and the Lorentzian model (12.6%). Therefore the
approximated Voigt model returned more accurate fits and should be considered as the
standard line shape in the reporting of spectroscopic studies of human brain metabolites.
A small scale reproducibility study was performed on the two clinical scanners to
determined the precision to which metabolite peak areas could be measured, and greater
precision was observed on the GE scanner. The coefficient of variation (CV) of the
`within-run' reproducibility (no repositioning of solutions between measurements) was
within 3.7% for spectra of dilute solutions and 7.8% for in vivo spectra. The `within
session' reproducibility (repositioning of sample between measurements) was within 0.9%
for spectra of dilute solutions. The `between-days' reproducibility for spectra of dilute
solutions, quantified by a t-test, showed no significant differences between both the mean
peak areas for any of the metabolites or the choline/creative and the choline/sodium
acetate peak area ratios (p > 0.05 in all cases).