| Magnetic resonance imaging (MRI) guided high intensity focused ultrasound (HIFU) treatment has attracted a lot of attention in recent years. However, there are still many key technical problems to be overcome before the MRI-guided HIFU becomes practical. The studies in this thesis will focus on MRI equipment building-up and imaging techniques for guidance of HIFU treatment. The MRI system is equipped with U-type permanent magnet to fulfill the requirements of guiding the HIFU treatments. To be specific, the following items will be covered:1. A digital receiver with fast gain switching capabilityBefore HIFU treatment, high resolution MRI images are probably required for precise localization and outline of diseases. However, a typical MRI digital receiver cannot always meet the requirement of dynamic range in high resolution imaging due to ADC resolution restriction. In order to increase the dynamic range of the digital receiver, a method of fast gain switching is proposed, and a corresponding receiver unit is implemented. The effectiveness of the fast gain switching method and the performance of the proposed digital receiver are verified and demonstrated experimentally.2. Digital Combination of signals from quadrature MRI receiver coilsIn order to ensure sufficient accuracy of lesion localization, signal-to-noise ratio (SNR) of MRI images should be taken into account. An effective method of improving the SNR is using quadrature receiver coils (I-coil and Q-coil) to detect MRI signals. However, the sensitivity imbalance between I and Q receive channels will lead to unexpected quality degradation of final images. In order to address this imbalance problem, we propose a digital combination method to optimize SNR of final images, in which the weighting coefficients of I and Q channels are determined dynamically in K-space. It is demonstrated that the digital combination method is effective to enhance SNR of the final images acquired with quadrature MRI receiver coils.3. Correction of gradient delayFast spin echo (FSE) imaging sequence is typically used to obtain T2-weighted MRI images, which is supposed to provide good contrast between tumor and normal tissues. Unfortunately the FSE sequence is rather susceptible to imperfections in equipment and configuration--gradient delay is one of them. In order to correct the effects of the gradient delay, a novel method is proposed, in which the gradient channels are always triggered earlier than the radio-frequency channels. An MRI pulse programmer providing this independent trigger delay is developed, and the correction of the gradient delay is experimentally demonstrated.4. Simultaneous multi-slice imagingT1-weighted MR images can provide important information in preoperative diagnosis and postoperative evaluation, especially for liver disease. Generally, in low-field MRI scanners, a shorter TR is used for T1-weighted imaging. As a result, the allowable slice number in one TR is limited. In this paper, simultaneous multi-slice imaging (SMI) method is utilized to overcome this difficulty. Theory and specific implementation details are described, and the results of computer simulation and MRI experiments are reported.5. MRI monitoring techniquesDuring HIFU treatment, temperal temperature evaluation of disease and surrounding tissues is important. In this thesis, MR thermometry with proton resonance frequency is investigated on a low-field MRI scanner with U-type permanent magnet. Nevertheless, high openness of the U-type construction will cause the scanner to be severely sensitive to the variation of external electromagnetic field. Another challenge induced by the high openness is significant variation of Bo field distribution. In order to reduce or eliminate these effects on thermometry imaging, a series of corrections are treated, including navigator echo and dynamic shimming. Finally, a thermometry experiment is executed with these corrections, and the results are compared with those obtained without the corrections. |
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