Magnetic resonance spectroscopy quality assessment at CUBIC and application to the study of the cerebellar deep nuclei in children with fetal alcohol spectrum disorder

Du Plessis, Lindie

Magnetic resonance spectroscopy quality assessment at CUBIC and application to the study of the cerebellar deep nuclei in children with fetal alcohol spectrum disorder

Master Thesis

2010

Publisher

University of Cape Town

Department

Department of Human Biology

Faculty

Faculty of Health Sciences

Abstract

In vivo magnetic resonance spectroscopy (MRS) is an imaging technique that allows the chemical study of human tissue non-invasively. The method holds great promise as a diagnostic tool once its reliability has been established. Inter-scanner variability has, however, hampered this from happening as results cannot easily be compared if acquired on different scanners. In this study a phantom was constructed to determine the localisation efficiency of the 3 T Siemens Allegra MRI scanner located at the Cape Universities Brain Imaging Centre (CUBIC). Sufficient localisation is the key to acquiring useful spectroscopic data as only the signal from a small volume of interest (VOI) is typically acquired. The phantom consisted of a Perspex cube located inside a larger Perspex sphere. Solutions of the cerebral metabolites N-acetyl aspartate (NAA) and choline (Cho) were placed in the inner cube and outer sphere respectively. The phantom was scanned at a range of voxel sizes and echo times in order to determine parameters that typically indicate the performance of the scanner in question. The resultant full width at half maximum (FWHM) and signal to noise ratio (SNR) values indicated that optimal results were obtained for a voxel with dimensions 20 x 20 x 20 mm3. The selection efficiency could not be measured due to limitations in the scanner, but two other performance parameters ' extra volume suppression (EVS) and contamination ' could be determined. The EVS showed that the scanner was able to eliminate the entire background signal from the out-of-voxel region when voxel sizes with dimensions (20 mm)3 and (30 mm)3 were used. This performance decreased to 96.2% for a voxel size of (50 mm)3. The contamination indicated that the unwanted signal, weighted by the respective proton densities of the chemicals, ranged from 12% in the (20 mm)3 voxel to 24% in the (50 mm)3 voxel. These ranges are well within acceptable limits for proton MRS. Analysis of the water suppression achieved in the scanner showed an efficiency of 98.84%, which is acceptable for proton spectroscopy. It was also found that manual iv shimming of the scanner improved the spectra obtained, as compared to the automated shimming performed by the scanner. The second objective of the study was to quantify absolute metabolite concentrations in the familiar SI units of mM as results were previously mostly expressed as metabolite ratios. The LCModel software was used to assess two methods of determining absolute metabolite concentrations and the procedure using water scaling consistently showed superior performance to a method using a calibration factor. The method employing water scaling was then applied to a study of fetal alcohol spectrum disorder (FASD) where the deep cerebellar nuclei of children with FASD and a control group were scanned. The cerebellar nuclei were of interest as children with FASD show a remarkably consistent deficit in eye blink conditioning (EBC). The cerebellar deep nuclei is known to play a critical role in the EBC response. The results show significant decreases in the myo-inositol (mI) and total choline (tCho) concentrations of children with FASD in the deep cerebellar nuclei compared to control children. The FAS/PFAS subjects have a mean mI concentration of 4.6 mM as compared to a mean of 5.3 mM in the controls. A Pearson correlation showed that there was a significant relationship between decreasing mI concentrations with increasing prenatal alcohol exposure. The mean tCho concentrations are 1.3 mM for FAS/PFAS and 1.5 mM for the controls. There was no significant differences between the heavily exposed group and either the FAS/PFAS or the control subjects for either metabolite. The decreased mI and tCho concentrations may indicate deficient calcium signalling or decreased cell membrane integrity ' both of which can explain the compromised cerebellar learning in FASD subjects.