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 particleimaging (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 imagingperformance and develop optimized MNPs. Biosensing applications also make use of the change in the relaxationresponse of MNPs after binding to a target agent. As MNP relaxation properties change with temperature andviscosity, noninvasive probing of these microenvironmental properties is possible. In this work, we present an untunedrelaxometer to measure the relaxation properties of the MNPs in a wide frequency and amplitude range. The developedrelaxometer can produce above 80 mTpp magnetic field at up to 60 kHz frequency, and above 14 mTpp at up to150 kHz frequency. An asymmetrical three-section gradiometer receive coil is used to cancel the direct coupled signalfrom the transmit coil. The position of one of the receive coil sections is manually tuned using a rotating knob forimproved decoupling. The tuning coil section has a lower number of turns compared to the other sections to decreasethe 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 theresolution for two different commercially available superparamagnetic iron oxide MNPs (Perimag and Synomag-D) in amultifrequency/multiamplitude measurement scheme. The magnetization properties of MNPs for arbitrary waveformscan be measured efficiently using the developed relaxometer.

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