The Investigation Of Several Technologies In Low-field Magnetic Resonance System

As a non-invasive diagnostic tool, Magnetic Resonance Imaging (MRI) has been widely used in the field of clinical diagnosis and scientific researches. Presently most of the advanced imaging methods and techniques were realized in the system of superconducting magnetic resonance, such as MRI-guided ultrasonic treating technique. However, the superconducting magnetic resonance (MR) scanner has not been popularly used in the hospital in China due to its high price and maintenance costs. The low-field opened MR scanner was widely employed due mainly to its low cost, but some key techniques were hard to realize because of its natural low signal-noise ratio (SNR) and low stability.The solution and experimental verification were proposed in this paper to solve some problems resulted from the development and application of our 0.3T/0.35T MRI system. Furthermore, the MRI-guided ultrasonic technique was investigated based on the 0.35T MRI system.The main contents of this thesis include,1. Method of NMR relaxation data analysis. The non-negative least-squares method (NNLS) and non-linear fitting method were used to analyze the NMR relaxation time. Meanwhile a new method was proposed. The procedure includes using NNLS to coarsely determine the relaxation time values. Then the relaxation time values are used as the initial parameters of the non-linear fitting computation. As a result, accurate relaxation time data can be conveniently obtained without any pre-knowledge of the sample. This method is verified and demonstrated by computer simulations and NMR experimental results.2. Method of compensating gradient eddy-current. First we introduced the formation of eddy-current and common methods of measuring gradient waveform and overcoming eddy-current in low-field MRI system. Then we proposed a new method to measure the gradient waveform using the digital receiver, introduce the detailed principle of receiving the audio signal and tested the performance of the audio receipt of the receiver. Finally, it is demonstrated that the gradient waveforms measured with the proposed receiver in our low-field MRI system, and the parameters of pre-emphasis are set according to the fitting result of the measured waveforms. As a result,the eddy currents were evidently reduced.3. Method of eliminating the jitter of digital gradient waveform. A method was proposed to eliminate the jitter of digital gradient waveform generator in MRI systems, which uses the internal clock of the digital to analog converter (DAC) to synchronize the start of pulse sequence generator. The principle of the proposed method was discussed in detail. In addition, its validity was demonstrated by MRI experiments. A variable delay solution was suggested to compensate the jitter during one scanning period.4. Method of maping the RF field distribution. A method of measuring the RF field distribution based Fourier Transformation was proposed. The measurement result is non-related with the uniformity of the samples and the asymmetry of one pixel in slice and in one domain was investigated.5. The research about the low-field MRI-guided ultrasonic technique. The principle of high-intensity focused ultrasound (HIFU) therapy and the hardware structure of the ultrasonic instrument in our lab were introduced. Then we investigated the rapid imaging sequence in low-field system and successfully guided and location of the target within two seconds. Through experiments we decided to use T₁ to represent temperature and have executed the T₁ mapping of the volunteers. Finally we testify the compatibility of the MRI system and HIFU system.