Imaging specific absorption rate with MR thermometry using paramagnetic lanthanide complexes and in vivo GABA MR spectroscopy in movement disorders
Magnetic Resonance Imaging (MRI) is a popular imaging modality due to its ability to provide excellent soft tissue contrast without exposure to ionizing radiation. It can be used for temperature monitoring (thermometry) as well as for assessing the biochemistry in vivo (MRS). This dissertation focuses separately on the development, application and quantitation issues of these two aspects of MRI. Radiofrequency (RF)-induced tissue heating is a concern in MRI. The dosimetric quantity for monitoring RF heating is the Specific Absorption Rate (SAR) defined as the RF power absorbed per unit mass of tissue. A novel approach for imaging SAR from absolute temperature images obtained using a paramagnetic lanthanide complex-Thulium 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis (methylene phosphonate) (TmDOTP5-) was developed. The effects of a bare-ended, insulated conductor in a phantom were investigated by 3D SAR imaging. 3D SAR maps were also generated using a high SAR sequence while varying the pulse duration. The high spatial resolution SAR maps correctly identified the local SAR rise near the wire end and also revealed increasing SAR with increasing pulse duration in the high SAR sequence, as expected. These results demonstrate the potential of MR thermometry with paramagnetic lanthanide complexes for evaluating safety of implants, medical devices as well as different pulse sequences. The second part of the thesis is dedicated to the technique of measuring in vivo levels of the neurotransmitter γ-aminobutyric acid (GABA) using MRS. GABA is an inhibitory neurotransmitter in the brain which is involved in the control of fine movement and balance. GABA MRS with spectral editing was performed and GABA was quantified using custom fitting parameters in the tool LCModel to measure changes in movement disorders - particularly Parkinson's disease (PD) and sleep bruxism. Higher levels of thalamic GABA were detected in PD with correlation to disease severity indicating the possibility to use GABA MRS as a biomarker for PD progression. On the other hand, in the bruxers, lower levels of GABA correlating with higher levels of glutamate in the dorso-lateral prefrontal cortex were detected indicating disturbances in the GABAergic and glutamatergic pathways. Lastly, since GABA quantification is a much discussed topic in literature with no one, clear and best approach, an effort was made to compare some popular fitting approaches in LCModel. Semi-synthetic simulated GABA spectra were used to test the accuracy, sensitivity and specificity of methods, all of which handled the baseline and macromolecules in the GABA spectra differently. Overall, the approaches using a fully flexible baseline ranked best in the tested aspects.
Dydak, Purdue University.
Health sciences|Medical imaging
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