Synopsis

Keywords: Physics & Engineering: Hardware, Physics & Engineering: RF Safety

This lecture covers the basic concepts of small signal and power RF amplifiers, as well as the unique requirements for MR applications.

Introduction

An amplifier is an electronic circuit that takes an input signal and produces a copy of it with larger amplitude at the output. Amplifiers are used both in the receive chain and in the transmit chain of an MR scanner.
The lecture begins by covering the basic amplifier circuit unit and concepts like matching, gain, stability, linearity, bandwidth, and the various classes of amplifier. Pozar’s [1] and Bahl’s [2] books are comprehensive references, respectively, for general RF engineering topics and RF amplifiers.

Low-noise Amplifiers

As the first amplifier used to boost the MR signal picked up by the RF coil, the LNA must add as little noise as possible because of how noise figures in a chain of amplifiers add (the Friis formula). While for telecommunications it is usually preferred to have a small reflection at the LNA’s input (i.e., 50Ω matching), for MR arrays we prefer a large reflection so that preamp decoupling can be implemented. The need for a reflection and minimum noise figure (often < 0.5dB) results in a matching strategy that does not transfer maximum power from the coil to the LNA. Other requirements for LNAs used in MR that can vary from those used in telecommunications include linearity (or dynamic range) and bandwidth.
Some excellent references on the topic of LNAs include [3], Chapter 7 of [4] and [5].

RF Power Amplifiers

The RFPA is needed to produce strong B₁ fields for spin excitation. Because most MR applications are pulsed, RFPAs should be able to provide high power with low distortion and with a limited duty cycle (e.g., 10%). Between pulses the amplifier must not inject noise that could degrade SNR during reception, and gain must be highly stable (e.g., 0.3 dB in magnitude and 3º in phase over 30 min or more). These requirements are rarely satisfied by off-the-shelf RFPAs designed for telecommunications because they are designed for continuous operation (no noise blanking circuit) and do not require high stability. For in-vivo MR applications, output RF power must also be monitored in real time to ensure SAR limits [6] are not exceeded.
References on the topic of RFPAs include Chapter 6 of [4], [7], and previous years’ educational talks such as [8].

Acknowledgements

No acknowledgement found.

References

[1] D. M. Pozar, Microwave engineering, 4th ed. Hoboken, NJ: Wiley, 2012.
[2] I. J. Bahl, Fundamentals of RF and microwave transistor amplifiers. Hoboken, N.J: Wiley, 2009.
[3] G. Gonzalez, Microwave transistor amplifiers: analysis and design, 2nd ed. Upper Saddle River, N.J: Prentice Hall, 1997.
[4] A. Webb, Ed., Magnetic resonance technology: hardware and system component design. in New developments in NMR, no. 7. Cambridge, UK: Royal Society of Chemistry, 2016.
[5] P. J. Fish, Electronic noise and low noise design. in Macmillan new electronics series. Houndmills, Basingstoke: Macmillan Press, 1993.
[6] IEC 60601-2-33 Medical electrical equipment. Part 2-33, Particular requirements for the basic safety and essential performance of magnetic resonance equipment for medical diagnosis, Edition 4.0. Geneva, Switzerland: International Electrotechnical Commission, 2022.
[7] M. K. Kazimierczuk, RF power amplifiers, Second edition. Chichester, West Sussex, United Kingdom: John Wiley & Sons Inc, 2015.
[8] M. Twieg, “MRI Transmitter Amplifier Systems,” in Proceedings of the International Society for Magnetic Resonance in Medicine, 27th annual meeting, Montreal, 2019, p. E8179.