Amplifier Tutorials

8 RFPA SYSTEM APPLICATIONS; CURRENT AND FUTURE

The NMR/MRIS RFPA architecture has changed little in the past 20-25 years. Tube amplifiers currently represent a fairly cost effective modality for narrowband proton amplifiers in 1.5T and 3T. These are slowly being supplanted by solid state equivalents. At field strengths of 3T and higher, obtaining a uniform B1 field presents a substantial challenge. To alleviate this issue, multi channel RF amplifiers (typically anywhere from 8-16 amplifier channels) have been deployed for B1 shimming. The amplifier requirements and specifications do not vary much, only now the individual amplifier channels need to matched for Insertion Gain and Phase. Matrixed amplifiers (amplifiers that can switch between multichannel mode and combined amplifier mode) are now being used where there is a desire to use the RFPA in a conventional single output or switch to a multi-channel output mode. The primary challenges in multi channel (B1 shimming) RFPAs are the gain/phase matching of up to 16 independent channels, especially if the amplifiers are broadband. Another issue is the coupling between transmitter elements which can lead to high levels of reflected power into the RFPAs output and virtual VSWR fault triggering(i.e. reflected power that is not from an adverse load but from element to element coupled power)

There is an ongoing move to develop amplifiers that can be located either in the bore (high field compatible) or mounted just outside the bore. High efficiency amplifiers may find use as they run nearly twice as efficient as conventional Class AB amplifiers which may make them a suitable candidate for in-field RFPAs.