Design Of A New MRI Spectrometer And Research Of Key Techniques

Magnetic resonance imaging (MRI) is a new imaging method in the field of medical imaging. It uses radio frequency (RF) pulses to excite the hydrogen nucleuses thus producing nuclear resonance signals, which are then processed in computer to obtain image. With the advantages of multi-parameters imaging, high contrast, arbitrary slice imaging, no bone artifacts disturbance, and non-invasive etc, MRI has become one of the most widely used technologies.MRI system consists of computer, spectrometer, magnet, RF coils, gradient coils and power amplifiers. As the core of the whole MRI system, Spectrometer is in charge of generating, sending, receiving and processing of all kinds of RF signals and gradient signals. Spectrometer is a high technology device, whose functional divisity determines the ability of meeting user's requirements and whose performances directly influence images'quality. Currently, commercial MRI spectrometer can only be produced by a very few foreign companies so that most spectrometers of Chinese MRI systems are imported at high price. With the fact that our domestic spectrometer falls far behind foreign countries, it is rather important to research and design our own spectromer with high performances and low cost.Upon the fully understandings in MRI principles and previous deep explorations into spectrometer design, a new MRI spectrometer based on FPGA, DSP, ADC, DAC and other IC chips is proposed. In this thesis, we firtly introduce the overall design of our MRI spectrometer. Then, we will dicuss every particular module and its key techniques in the spectrometer, including RF generator, gradient generator and RF receiver. In sum, our work is organized as follows:1) A new digital MRI design is proposed. The hardware system is based on high performance FPGA, DSP, ADC, DAC and other IC chips. The design has many advantages such as small size, simple structure, low cost, high stability and timmer precision. On the other side, software system includes core modules used for command and data processing and other peripheral modules. Cooperating with hardware system, software system can satisfy many imaging requirements from users and obtain high quality images. Our spectrometer is applicable to both low field permanent magnet MRI system and high field superconduction MRI system.2) Propose a solution for digital radio frequency generator of MRI spectrometer. Using digital-up-convertor chips with high performance and multi-function, we can accomplish frequency synthesis, digital quadrature modulation and D/A conversion. Such solution has many properties such as fast modifying base-belt waveform data, frequency modulation, amplitude and phase modulation, high precision and stability, broad frqency convering, high integrated level, low cost and good compatibility.3) Design a gradient generator based on high performance DSP. All gradient calculation including rotation, pre-emphasis and shim are implemented in a single DSP. Such solution takes fully advantage of DSP's properties such as high speed and precision, easily programming, algorithm compatibility and code reusability.4) Design a RF receiver of MRI spectrometer. We use multi-functional receiving chip to accomplish A/D conversion and digital quadrature demodulation so that it has low cost, high demodulation phase precision, integrated level and stablility. By extracting and multiplely filtering the acquired nuclear magnetic signals, we have implemented the algorithm based on general filters and proposed an algorithm based on multistage wavelet decomposition. Both can preprocess the NMR signals well and gain the K-space data needed for imaging.