Quantification of striatal dopaminergic uptake in Parkinson's disease: a new multimodal method combining SPECT DaT and MPRAGE
Smart, Karishma Lees
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Parkinson's disease (PD) is a neurodegenerative disease that causes degeneration of nigral dopaminergic terminals in the caudate and the putamen regions of the striatum in the basal ganglia. According to current practice, when an unequivocal clinical diagnosis of PD cannot be made, a single-photon emission computed tomography scan using the DaTscan radionuclide (SPECT DaT scan) is ordered. However, the assessment of SPECT DaT scans in the diagnosis of PD depends on the subjective judgment of a radiologist, which can pose problems for the accuracy of the diagnosis. Furthermore, as research studies generally do not quantify SPECT DaT scans when using them, their conclusions are not based on standardized data. The aim of this paper is to propose a method of quantification for SPECT DaT scans, to be employed in diagnostic and research environments. The methodology proposed in this thesis project will eventually be used for a much larger multimodal imaging project investigating the connectivity changes in the brain related to cognitive and affective symptoms in PD patients. Each of the 4 subjects in this project underwent a SPECT DaT scan and an MPRAGE scan (Magnetization Prepared Rapid Gradient Echo), an anatomical MRI (magnetic resonance image). The SPECT DaT scans and the MPRAGEs were coregistered, and then a voxel-based quantification of the caudate and the putamen in the left and the right hemispheres was performed in every subject. First, the percentages of voxels with intensities exceeding various pericalcarine baselines were calculated. A pericalcarine baseline was used because the pericalcarine gyrus in the occipital lobe has been shown to have little to no dopaminergic activity, particularly on SPECT DaT scans. Next, asymmetry indices (AI) were calculated for two of the thresholds whereby the ratio of the percentage of voxels in the right to the left hemispheric region was taken. Wilcoxon Signed-Rank tests and bootstrapping analyses were performed on both the caudate and the putamen in all four subjects to determine the significance of any detected asymmetry. The quantification of the data and the AI values revealed asymmetries in the voxel intensities between the left and right hemispheres. This asymmetry was consistent with each subject's side of physical symptom onset. According to the bootstrapping analyses, this asymmetry was significant in five of the eight comparisons. In summary, this methodology has potential to bring greater objectivity to the use of SPECT DaT scans in the diagnosis of PD and in research through its anatomically accurate, voxel-based quantification.