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

Abstract

Much of the current research in magnetic resonance engineering focuses on reducing the acquisition time for obtaining an image while simultaneously maximizing the Signal to Noise ratio (SNR) of the image. It is known that improvement in imaging time or resolution is obtained at the cost of SNR. Therefore wherever possible, RF coil engineers design the coil in such a manner so as to maximize SNR for that coil design. In one such design consideration, most coil designers prefer placing low impedance preamplifiers near the coil. The further the pre-amplifiers are from the coil, the greater will be the signal loss due to transmission and higher will be its input impedance as perceived at the coil which would degrade inter-coil isolation. Owing to the current trend of using increasing number of receiver channels (32, 64 or 128) for parallel imaging, placing the preamplifiers near the coil would greatly complicate the coil construction. The primary objective of this research was to find the relation between SNR and referred preamp impedance and whether preamps need to be placed on the coil, or if they can be placed outside the magnet at the end of a transmission line which would simplify the construction of large count array. In addition, SNR was studied as a function of coil design parameters - coil loading, array coil separation, and system frequency. Both theoretical and experimental methods were used to undertake this investigation. A popular 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 also verified through bench measurement at 3T and 1.5T and by evaluating SNR. To verify the robustness of our results and to assess the possibility of using low cost standard 50 ohm preamps, we carried out additional bench measurements at 4.7T. Results demonstrated that preamplifier placement is critical at low field strength. At higher field strength, SNR degradation due to preamplifier placement was less owing to heavier coil loading.