The Gradient Waveform Generator For Magnetic Resonance Imaging

MR scanner is one of the key tools in medical diagnostic and scientific researches. Pulse Field Gradient (PFG) unit is an important part in the scanner. This thesis focuses on the study of pulse field gradient unit. Two types of Gradient Waveform Generators (GWG) are developed in this thesis, one is for teaching MRI and the other is for clinical MRI.The main contents of the thesis include:First, the MR scanner, including magnet system, spectrometer and computer system, is introduced briefly. The structure of integrated MR spectrometer is described. The hardware part includes pulse sequence generator, RF unit, receiver unit and pulse field gradient unit. Their architectures are all based on the PCI bus. Direct Digital Synthesize (DDS) is the pivotal technique used in the RF unit. Digital receiver is employed in the receiver unit in order to simplify the structure and eliminate the imaging peak.Second, the introduction of the Pulse Field Gradient is given in detail. The applications and the hardware including gradient waveform generator, gradient current amplifier and gradient coil are also presented. Then, some methods of measuring the gradient field are proposed. One of these methods, nominated one dimension imaging, is carried out to measure the gradient field. Finally, the mechanisms of eddy current, measurement of eddy current and the method of eddy current compensation are discussed.Third, low-cost Pulse Field Gradient unit for teaching MR scanner is presented. The first part is the structure and requirement of this unit. The second part is the introduction of the gradient waveform generator. PCI based gradient waveform generator adopted FPGA for its logic controlling. We pictured the hardware structure of gradient waveform generator. The introduction of FPGA and FPGA design flow is also described. The third part is the design and the performance of gradient current amplifier. At last, in order to demonstrate the performance of the proposed design, the output linearity, long-term thermal stability, noise level and the couplings between each gradient channels are tested. Finished the work above, the unit is integrated into the MR scanner. The results of the MRI experiment that used different samples are given.Fourth, a Gradient Waveform Generator for clinical MR scanner is described in detail. The configuration and function of GWG are recommended at first. Gradient Waveform Generator is the key component of the gradient unit. We gave a full description of the hardware architecture, circuits, and the design structure of the FPGA logic. According to the requirements, we appended the function of waveform preemphasis. The most important technique is how to achieve the preemphasis by employing the FPGA to produce a DSP core. At last, the performance of the presenteddesign is tested and listed. After integration into the MR scanner we can do some conventional MRI experiment and experiment for eddy current compensation.