Phase-sensitive detection of third-harmonic magnetization signal using magnetoresistive sensor-coupled asymmetric gradiometer

Human-scale magnetic particle imaging demands a high sensitivity in acquiring weak magnetization signal from nanoparticle tracers. The standard receive chain usually includes high-order filtering, multistage amplification, and broadband noise matching in order to improve signal quality. We incorporate these features into a conventional flux transformer circuit which employs a magnetoresistive sensor coupled with a gradiometric head coil. This approach provides low noise level to map the third harmonic response of a 0.5 mL Ferucarbotran sample under 16 mT/m DC gradient and 1.5 mT/µ0 excitation field at 2 kHz. However, high inductance of asymmetric gradiometer was responsible for a noisy output voltage of an empty bore, limiting the sensitivity up to 1 mgFe. We later adopted lock-in method to differentiate the signal of a few µgFe sample. Although harmonic noises are signal-disruptive for higher excitation fields, this technique recognized a 10 µgFe sample under 2.5 mT/µ0 in the current setup.