Design and implementation of a low-noise, low-power amplifier for a 3 tesla magnetic resonance imaging system

Abstract

Magnetic Resonance Imaging (MRI) is a vital diagnostic tool in modern medicine due to its high spatial resolution, excellent soft tissue contrast, and non-invasive nature. The signal quality in MRI systems is primarily determined by the signal to-noise ratio (SNR), which is crucial in high-field MRI systems like 3 T scanners. This thesis presents the design and implementation of a low-noise amplifier (LNA) operating at 123 MHz, corresponding to the Larmor frequency of hydrogen nuclei at 3 T. The LNA, based on a single-stage common-source topology using a GaAs pHEMT transistor, achieves a measured gain of 14.5 dB, a noise figure (NF) of 0.64 dB, an input reflection coefficient (S11) of −0.28 dB, and a 1 dB compression input power (Pin1dB) of −10.75 dBm. These results were validated against simu lations, where the design showed high performance, with a power consumption of only 12.6 mW. The amplifier meets the requirements for MRI receiver systems, ensuring low noise and power consumption. Additionally, another version was im plemented with input protection diodes for safe operation. This version achieves a measured gain of 14.9 dB, a noise figure (NF) of 0.79 dB, an input reflection coefficient (S11) of −0.32 dB, and a 1 dB compression input power (Pin1dB) of −9.65 dBm. This version also achieved 3.5 µs recovery time after 14 dBm of input power was applied for 1 ms. Key challenges such as impedance matching to short circuit, bias network, and input protection circuit are addressed in the design