Radiofrequency Power Amplifiers for NMR and MRI

Abstract

The radio frequency power amplifier (RFPA) can analogously be thought of as the heart of an NMR spectrometer or MRI scanner. Although executing a conceptually simple and fundamental task; i.e., making a “small” RF signal into a “big” RF signal, how an RFPA operationally deals with a complex RF pulse sequence design can have direct impact on the signal-to-noise ratio (SNR) or other parameters with manifestations that are clearly visible in data or MRI scans. An NMR spectrometer/MRI scanner is an extremely complex machine, an understanding of which requires knowledge of physics, chemistry, analog/digital/radio frequency electronics, digital signal processing, etc. One could spend a lifetime on any one of these disciplines relative to NMR/MRI and still fall far short of a total understanding of all there is to know. In addition, a focused expertise on a given individual discipline leaves one lacking in the others. A balanced compromise would be a sufficient working knowledge in several of the areas to gain a solid grasp on the NMR/MRI technology en masse. The objective of this article is to provide the NMR/MRI technologists, be they a technician, engineer, scientist, business center manager, or radiologist, a basic working knowledge of RFPAs such that they understand fundamentally what an RFPA does, what constitutes acceptable current state-of-the-art RFPA performance, how to verify that performance, and, if it does fall short on expectations, how that shortfall may impact system performance. Keywords: radio frequency power amplifier; transmitter; magnetic resonance imaging; nuclear magnetic resonance spectroscopy