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We developed a highly sensitive measurement system using a Cu pickup coil cooled at 77 K for magnetic nanoparticle imaging. The magnetic field noise of the system was as low as 90 fT/Hz1/2 at the resonant frequency of 3 kHz when the pickup coil was connected to a resonant capacitor. We also characterized the magnetic properties of the nanoparticles for use in magnetic nanoparticle imaging. We demonstrated the detection of nanoparticles using the third-harmonic signal generated by the nonlinear magnetization of nanoparticles. An excitation field having a frequency of 1 kHz and amplitude of 1.6 mT was applied to the magnetic particles, and the third harmonic signal at 3 kHz was detected. By separating the signal frequency from that of the excitation field, we could avoid the interference of the excitation field and were able to obtain a clear contour map of the signal field. We could detect 100 µg of magnetic nanoparticles located as far as 100 mm under the pickup coil.

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