Assessing excitation field frequency for various magnetic nanoparticles

Abstract

Magnetic detection is presently used in a handheld probe to identify metastasis bearing lymph nodes (LNs) for
a variety of oncology applications. This approach utilises the underlying assumption that injected magnetic
nanoparticles (MNPs) will migrate to LNs with the shortest path to a tumor and therefore will generate a localized
magnetic signal. Nonlinear magnetic detection is specific for magnetic nanoparticles and negates the influence
of human tissue and surgical instruments. Our nonlinear DiffMag principle uses a combination of an AC and DC
magnetic field to activate MNPs and records the consequent magnetic signal. MNP detection can be optimised to
maximise detection parameters, such as iron sensitivity and detection depth. Tuning the excitation field frequency
to physical properties of MNPs (such as particle diameter) leads to improved detection.
This study assesses the magnetic properties of various MNPs (SHP15, SHP20, SHP25, SHP30) and compare the
findings to clinically available MNP (Magtrace®). Magnetization response of these MNPs was acquired using the
SuperParamagnetic Quantifier (SPaQ) at various AC field frequencies (1, 2.5, 5, 7.5, 10, 12.5 and 15 kHz). Two features
capturing magnetization response (maximum signal difference and full width at half maximum) were extracted to
compare MNPs. Additional acquisition captured AC susceptibility (ACS) in the range 10Hz-1MHz.
SPaQ results show an optimal excitation frequency between 5 and 12.5 kHz for the various types of MNPs. ACS
results show small particles (SHP15) are Néel dominated, large MNPs (SHP30) are Brownian dominated and the
sizes in between show a combination of Néel and Brownian relaxation. The larger, Brownian dominated MNPs
perform best in nonlinear magnetic detection.