A study of array snr and coupling as a function of the input impedance of preamplifier

摘要

Much of the current research in magnetic resonance engineering focuses onreducing the acquisition time for obtaining an image while simultaneously maximizingthe Signal to Noise ratio (SNR) of the image. It is known that improvement in imagingtime or resolution is obtained at the cost of SNR. Therefore wherever possible, RF coilengineers design the coil in such a manner so as to maximize SNR for that coil design. Inone such design consideration, most coil designers prefer placing low impedance preamplifiersnear the coil. The further the pre-amplifiers are from the coil, the greater willbe the signal loss due to transmission and higher will be its input impedance as perceivedat the coil which would degrade inter-coil isolation. Owing to the current trend of usingincreasing number of receiver channels (32, 64 or 128) for parallel imaging, placing thepreamplifiers near the coil would greatly complicate the coil construction.The primary objective of this research was to find the relation between SNR andreferred preamp impedance and whether preamps need to be placed on the coil, or if theycan be placed outside the magnet at the end of a transmission line which would simplifythe construction of large count array. In addition, SNR was studied as a function of coildesign parameters - coil loading, array coil separation, and system frequency. Boththeoretical and experimental methods were used to undertake this investigation. Apopular electromagnetic modeling technique, finite difference time domain (FDTD), was used to model SNR in arrays of two 3 inch loop coils at 3T and 1.5T. Results were alsoverified through bench measurement at 3T and 1.5T and by evaluating SNR. To verifythe robustness of our results and to assess the possibility of using low cost standard 50ohm preamps, we carried out additional bench measurements at 4.7T. Resultsdemonstrated that preamplifier placement is critical at low field strength. At higher fieldstrength, SNR degradation due to preamplifier placement was less owing to heavier coilloading.