A modern approach to the treatment of localized disease involves the use of advanced polymeric or ceramic implant materials for controlled-rate drug delivery. These implants are dynamic systems that maintain drug concentrations within the optimal therapeutic window via complex hydration, swelling, and degradation processes. To optimize the performance of these materials, however, requires a fundamental understanding of the mechanisms that govern drug release. Magnetic resonance imaging (MRI) provides a means of non-invasively characterizing the microstructure and transport properties in this type of material, and has proven to be an invaluable tool for their advancement. Calcium polyphosphate (CPP) is a biomaterial that has shown promise as a degradable matrix for drug delivery and bone defect repair. Release rates are potentially governed by hydrogelation, swelling, and polymer chain scission. CPP bioceramics have previously been studied using models for drug elution, but these tend to be simplistic and unable to explain the many interrelated mechanisms. Structural analysis techniques have also been applied, but these tend to be inherently destructive and unable to characterize the material in situ. With the aim of characterizing degradation/drug release mechanisms, a non-invasive approach based on MRI was developed and optimized for imaging two existing types of CPP device. Techniques included mapping of the T1 and T2 relaxation times and the apparent diffusion coefficient (ADC), which together provide sensitivity to local fluid transport parameters. The non-destructive nature of MRI permitted longitudinal observation, and structural degradation effects were investigated by correlation with concurrent drug elution measurements. Temporal variation in the release mechanisms was treated by analyzing elution in stages. Large variation between samples was found, but on average, drug elution that was controlled by a structural-relaxation mechanism appeared correlated with the gradual formation of a highly-mobile ``free'' water component within the disk. Other characteristics, such as swelling rate, did not appear to correlate with drug release at all. While the data did not implicate a singular, governing scheme for drug release from CPP bioceramics, the approach did yield an assessment of the relative importance of the various contributing mechanisms.

During the past two decades, techniques and devices were developed to transmit and receive signals with a phased array instead of a single coil in the MRI (Magnetic Resonance Imaging) system. The two techniques to simultaneously transmit and receive RF signals using phased arrays are called parallel excitation (pTx) and parallel imaging (PI), respectively. These two techniques lead to shorter transmit pulses for higher imaging quality and faster data acquisition correspondingly. This dissertation focuses on improving the efficiency of the pTx pulse design and the PI reconstruction in MRI. Both PI and pTx benefit from the increased number of elements of the array. However, efficiency concerns may arise which include: (1) In PI, the computation cost of the reconstructions and the achievable acceleration factors and (2) in pTx, the pulse design speed and memory cost. The work presented in this dissertation addresses these issues. First, a correlation based channel reduction algorithm is developed to reduce the computation cost of PI reconstruction. In conventional k-domain methods, the individual channel data is reconstructed via linear interpolation of the neighbourhood data from all channels. In this proposed algorithm, we choose only a subset of the channels based on the spatial correlation. The results have shown that the computation cost can be significantly reduced with similar or higher reconstruction accuracy. Then, a new parallel imaging method named parallel imaging using localized receive arrays with Sinc interpolation(PILARS) is proposed to improve the actual acceleration factor and to reduce the computation cost. It employs the local support of individual coils and pre-determines the magnitude of the reconstruction coefficients. Thus, it requires much less auto-calibration signals (ACS) data and achieves higher acceleration factors. The results show that this method can increase the acceleration factor and the reconstruction speed while achieving the same level of reconstruction quality. Finally, a fast pTx pulse design method is proposed to accelerate the design speed. This method is based on the spatial domain pulse design method and can be used to accelerate similar methods. We substitute the two computational expensive matrix- vector multiplications in the conjugate gradient (CG) solver with gridding and fast Fourier transform (FFT). Theoretical and simulation results have shown that the design speed can be improved by 10 times. Meanwhile, the memory cost is reduced by 103 times. This breaks the memory burden of implementing pulse designs on GPU which enables further accelerations.

A study of methods for reducing the acoustic noise in magnetic resonance imaging (MRI) scanners is presented in this thesis. The structural-acoustic coupling mechanism of MRI scanners was investigated using a method of structural-acoustic modal analysis. Mathematical expressions of generalized radiation impedances of gradient coil ducts with perforated panel inserts were developed and the effects of the perforated panel inserts on the acoustic noise in the duct were discussed. The possibility of using micro-perforated panel (MPP) absorbers in MRI scanners to reduce the acoustic noise was then investigated through analytical and computational modeling. A comprehensive experimental study was conducted after the analytical and computational investigation. Finally, design methods and procedures were developed specifically for the MPP absorbers in MRI scanners. Design considerations and recommendations were given as well. Several major conclusions can be made from this research. Firstly, the method of structural-acoustic modal analysis is effective for finding the structural-acoustic coupling modes which should be avoided in the design of MRI scanners. Secondly, a perforated panel insert produces significant effects on the radiation impedance of gradient coil ducts and MRI scanner bores. This attribute partly contributes to its capability of reducing the acoustic noise in a duct. Thirdly, the effectiveness of MPP absorbers in MRI scanners can be accurately predicted using a combination of theoretical analysis and computational modeling. Moreover, it has been proved that well designed MPP absorbers are effective in reducing the acoustic noise in MRI scanners. Lastly, the presented design methods and recommendations for the MPP absorbers can be relatively easily used by MRI designers or engineers to tackle the acoustic noise problem in MRI scanners. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2010-07-13 11:46:03.585

MRI-guided prostate needle biopsy requires compensation for organ motion between target planning and needle placement. Two questions are studied and answered in this work: is rigid registration sufficient in tracking the targets with a maximum error of 3 mm (smaller than average prostate tumor size) and how many intra-operative slices are required to obtain this accuracy? We developed rigid and deformable multislice- to-volume registration algorithms for tracking the biopsy targets within the prostate. Three orthogonal plus additional transverse intra-operative slices were acquired in the approximate center of the prostate and registered with a high-resolution target planning volume. Simulated intra-operative data, phantom data, and MRI-guided robotic prostate biopsy data were used to assess tracking accuracy. Registration tests on simulated intra-operative data with 3, 4, and 5 slices were performed to evaluate the effect on registration error and time. Results: Using three orthogonal slices pro- vides sufficient accuracy. Convergence test results on phantom images demonstrated 100% success rate for initial misalignment of 5mm. Average registration errors for the patient data were 2.55mm and 2.05mm for the rigid and deformable algorithms, respectively. The algorithm was able to capture rigid biopsy target displacements of maximum 8mm and non-rigid displacements of maximum 1.5mm. Rigid tracking appears to be promising. Deformable registration does not seem warranted. / Thesis (Master, Electrical & Computer Engineering) -- Queen's University, 2010-07-13 16:41:52.223

Magnetic resonance imaging (MRI) has revolutionized the field of cognitive neuroscience as it allows researchers to noninvasively map brain function in response to stimulus or task demands. However, the acquisition of MR images generates substantial acoustic noise, so that imaging studies of speech, language and hearing are problematic. One proven solution for reducing acoustic noise in MRI scanners is the use of micro-perforated panels placed in the bore of the scanner. They can be applied to existing scanners with minimal cost and are suitable for sterile environments. Although these panels result in quantifiably lower noise levels, measured with microphones in an empty MRI, the improvement has not been quantified with a patient in the scanner bore, which dramatically affects the acoustic noise field. This thesis tested the reduction of noise inside the MRI environment using a previously designed micro-perforated acoustic absorber panel. These panels resulted in quantifiably lower noise levels with a volunteer in the scanner bore, however the reduction was not sufficient for significant differences in volunteer perceptions. Volunteers were generally unable to perceive a difference in noise between scans with and without absorbers and no reduction of fatigue was observed. Also no significant change in cortical activity was found between scans done with and without absorbers during an auditory function MRI study. Further testing could include designing a micro-perforated acoustic absorber for a specific scan sequence for maximum attenuation. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2012-09-25 16:54:41.118

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Covert stroke (CS) comprises lesions in the brain often associated by risk factors such as a diet high in fat, salt, cholesterol and sugar (HFSCS). Developing a rodent model for CS incorporating these characteristics is useful for developing and testing interventions. The purpose of this thesis was to determine if magnetic resonance (MR) can detect brain abnormalities to confirm this model will have the desired anatomical effects. Ex vivo MR showed brain abnormalities for rats with the induced lesions and fed the HFSCS diet. Spectra acquired on the fixed livers had an average percent area under the fat peak relative to the water peak of (20±4)% for HFSCS and (2±2)% for control. In vivo MR images had significant differences between surgeries to induce the lesions (p=0.04). These results show that applying MR identified abnormalities in the rat model and therefore is important in the development of this CS rodent model.