An arbitrary waveform magnetic nanoparticle relaxometer with an asymmetrical three-section gradiometric receive coil

Magnetic nanoparticles (MNPs) have a wide range of clinical applications for imaging, therapy, and biosensing. Superparamagnetic MNPs can be directly visualized with high spatiotemporal resolution using the magnetic particle imaging (MPI) modality. The image resolution of MPI depends on the relaxation properties of the MNPs. Therefore, characterization of MNP response under alternating magnetic field excitation is necessary to predict MPI imaging performance and develop optimized MNPs. Biosensing applications also make use of the change in the relaxation response of MNPs after binding to a target agent. As MNP relaxation properties change with temperature and viscosity, noninvasive probing of these microenvironmental properties is possible. In this work, we present an untuned relaxometer to measure the relaxation properties of the MNPs in a wide frequency and amplitude range. The developed relaxometer can produce above 80 mTpp magnetic field at up to 60 kHz frequency, and above 14 mTpp at up to 150 kHz frequency. An asymmetrical three-section gradiometer receive coil is used to cancel the direct coupled signal from the transmit coil. The position of one of the receive coil sections is manually tuned using a rotating knob for improved decoupling. The tuning coil section has a lower number of turns compared to the other sections to decrease the sensitivity to mechanical movement. By tuning the knob, the transmit-receive coupling can be decreased below -80 dB. We analyzed the x-space image resolution, harmonic levels, and effect of the number of used harmonics on the resolution for two different commercially available superparamagnetic iron oxide MNPs (Perimag and Synomag-D) in a multifrequency/multiamplitude measurement scheme. The magnetization properties of MNPs for arbitrary waveforms can be measured efficiently using the developed relaxometer.