Noise Parameter Extraction in the Design of Low Noise Amplifiers (LNA) for MRI

Introduction: A key component of the MRI system is the Low Noise Amplifier (LNA), the first stage of amplification of the very small MRI signal. There is a unique implementation of the LNA for array imaging where the input is configured to a low, real impedance (<3Ω). This is accomplished with a series-shunt LC circuit, the reactances of which are equal [1]. An alternative, but well established engineering method for the design of a LNA is to choose matching components that transform the source impedance to the optimum reflection coefficient (Γopt) of the transistor, producing minimum noise figure (NFmin). The necessary parameters to perform this transformation are given on manufacturer data sheets, typically for frequencies greater than 1 GHz. Operating frequencies for clinical MRI scanners are ≤ 128 MHz and small animal research systems in the range of 200-500 MHz. Consequently, critical design parameters are not available to the engineer. The work describes a technique to extract Γopt and NFmin from simulation and utilize them to design an optimized LNA. This design is compared to a LNA that uses the typical MRI array matching scheme of equal reactances in the input circuit.