Research On Digital Lock-in Amplifier For The Detection Of Faint Magnetic Field In Mri System

In recent years, magnetic resonance imaging has developed rapidly, becoming one of the mostly used tools in medical inspection. However, faint electric and magnetic signals that exist in the background noise are always needed to be detected and dealt with during the research and experiment of MRI. For example, the detection of the induced magnetic field between the main magnet and the gradient coil in Siemens's 1.5T ESSENZA superconductive MRI system, so as to see how the center of main magnet overlap the one of gradient coil. Lock-in amplify is a widely used technology which can retrieve signal amplitude from background noise according the cross correlation principle. It gives full play to its advantages in detecting the faint signals of MRI system.This paper did research on a special kind of digital orthogonal Vector-based Lock-in Amplifier algorithm and verifies that the algorithm can be applied in the detection of faint signals in a noisy background. To finish the research, a lot of relevant references were read and the structure features of the currently widely used orthogonal vector-based lock-in amplifier were carefully analyzed.Taking advantages of the parallel working pattern of FPGA, this paper proposed a solution for the orthogonal vector-based lock-in amplifier in FPGA. With DDS (Direct Digital Frequency Synthesize), this amplifier precisely produces reference signals for the phase sensitive detector in digital lock-in amplifier. In this research, a set of hardware circuit was constructed firstly. And then a FPGA based digital lock-in amplifier which had passed the functional simulation verification in Modelsim with various computing modules was designed in Quartus II software. After several moderate improvements, the lock-in amplifier was tested in Siemens's 1.5T ESSENZA superconductive MRI instrument, and was proved to be useful and practical as expected.