The Development Of Digital Magnetic Resonance Imaging Spectrometer Based On One Frequency Conversion Technology

MRI (Magnetic Resonance Imaging, MRI) is a high and new technology which refers to the produce of objects’resonance launch and resonance reception on a combined effect of static magnetic field, gradient field and radio-frequency field by collect data and image reconstruction to active visualization of objects. Spectrometer is the control core of magnetic resonance imaging system, which includes digital radio frequency transmission part and radio frequency receive parts. This paper is devoted to designing and implementing a magnetic resonance spectrometer with low cost and good performance for low and medium intensity magnetic field.It has practical meaning and economic outlook,and has great significance for the promotion and application of the spectrometer for low and medium intensity magnetic field.Spectrometer is mainly composed of a computer, multifunctional data acquisition cards, a transmitter and a receiver. This study focuses on designing and implementing transceiver system that based on one frequency conversion technology, designing system clock circuit of spectrometer at the same time. Multifunctional data acquisition card PCIe-6323controls transmission system. The transmitter adopts DDS(direct digital synthesis) technology and implements fully digital orthogonal modulation and one up-conversion with one piece of AD9857.It solves not only the difficult problem of phase control in analog transmitter, but also the error of analog mixing in two up-conversion. Multifunctional data acquisition card PCI-7200controls receiving system.The receiver adopts digital direct down-conversion technology and implements quadrature demodulate and digital one down-conversion with one piece of AD9244and one piece of AD6620.It avoids complex processing of phase correction of two orthogonal signals in analogue technique, improves the overall performance of the receiver, and solves the error of analog mixing in two down-conversion.System clock circuit implements the unified allocation of entire system clock and guarantees synchronous working of the parts in the system, improves frequency stability and time resolution of the system, and reduces the time and frequency error with a TCXO-20M as the system clock source, a clock divider and a frequency multiplier at the same time. Synchronization of transmitting and receiving can realize in the case of delay calculation and phase compensation are completed.Finally, the paper finished the commissioning test of spectrometer system. From the debugging results can be seen that the design can implement a digital transmission system which based on direct digital synthesis.It can modulate the radio frequency selected layer signal that meets the requirements and has wide frequency range,and has many advantages like sending any envelope modulations, quickly switching frequency and amplitude,modulating well and so on.In addition,the design can implement a digital receiving system which based on digital direct down-conversion.It can demodulate the original signal quickly and accurately. The proper operation of the system validates the rationality and accuracy of the design and distribution of system clock.