Research and design of open type permanent magnet type MRI spectrometer hardware system
|School||University of Electronic Science and Technology|
|Course||Pattern Recognition and Intelligent Systems|
|Keywords||MRI spectroscopy gradient waveform generator eddy currentcompensation the pulse sequence controller RF transmitter RF receiver|
Magnetic resonance imaging (MRI) establish on the basis of magnetic resonance theory. Compared to other medical devices, such as CT, XR, and so on, MRI has advantage of no electromagnetic radiation, any section imaging, and high resolution. Therefore, MRI has a very important role in modern medical imaging diagnosis.In recent decades, open permanent magnet type of MRI has made tremendous improvement and development. MRI of the open space design can effectively eliminate the claustrophobic sense of discomfort in the patient examination, but it also supplies space and convenience for doctors to involve in the treatment. At the same time, the low-field environment of the permanent magnet type does not exist organism’s security issues, and the maintenance costs and operation of the permanent magnet-type MRI are very cheap and simple.The spectrometer is the core component of the MRI equipment, furthermore, it controls the timing of the MRI system, signal processing and generation, the level of their performance affects the imaging quality. Abroad, only a few large companies can provide spectrometer products, the spectrometers system in our country are imported. Thus, the self-study and design about the spectrometer is great significance of magnetic resonance career development in China.This paper completed the overall program and key module of the open permanent magnet-type MRI spectrometer system hardware design and verifies its feasibility by simulation experiments, the main work are as follows:1、Proposing design scheme. According to the function of the spectrometer requirements, we analysis the demand of spectrometer, and the general scheme of the spectrometer hardware system which included spectrometer workflow design and functionality of the module, by the same time, we research the system several key module about spectrometer, each module has a specific design.2、The design of gradient waveform generator. In MRI spectrometer system, the gradient waveform generator is used to generate a linear gradient of the magnetic field, it has three-way to complete the spatial coordinates encoded on the detected object. An important indicator of the gradient waveform generator includes gradient switching rate and accuracy, which has a fast gradient switching rate and precise output accuracy, has an important effect on image quality. This paper uses the PCM1704, which is high accuracy, high signal to noise ratio of the audio digital-analog converter chip to achieve the output of the gradient signal, it has full scale of fast switching of±1.2mA in200ns, the24bit DAC conversion accuracy and good inhibitory effect about noise. We have a detailed discussion and analysis of eddy current compensation, and we have given the algorithm of simulation experiment.3、The design of pulse sequence controller. The pulse sequence controller design is an indispensable part of the spectrometer system, the general requirements of the trigger signal with high accuracy and good signal synchronization, according to the sequence requirements, the pulse sequence controller provides accurate pulse sequence trigger signal. Based on the above characteristics, using the high-performance FPGA which completes the implementation of the trigger signal is particularly important, we use EP3C40F484C6in Cyclone Ⅲ family to achieve the functionality. There is a problem on pulse sequence controller, it’s about the optimization of memory resources, because a large storage space is occupied by the event delay table, this paper has optimized the event delay from setting length encoding loop mechanism, and it makes memory to save a few hundred times of the storage space, reducing the waste of space resources.4、The design of RF transmitter and receiver. RF transmitter and RF receiver is also an important part of the spectrometer system design, the changing of frequency, phase and amplitude parameters of RF signal will directly determines the image quality. Sampling and filtering receiver detection of magnetic resonance signal are crucial.We uses AD9957and AD6655as RF signal emission and resonance signal reception, and its good performance to launch the transition is controllability, and synchronization of the received samples is effectively improved.5、 The discussion of experimental results.Finally, this paper gave experimental results of key technologies, and it’s proved that the design is scientific and rational, and the eddy current compensation simulation verify the feasibility of its design, which has laid a solid foundation for the future spectrometer prototype production.