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To determine the iron concentration of administrated magnetic nano-particles in animals is essential for evaluation of arrival efficiency in target regions in biomagnetism studies. In this work, a high Tc superconucting quantum interference devices (SQUID) biosusceptometry with a scanning coil set is developed. The measurement principal is based on the AC susceptibility of magnetic nano-particles, and the low noise of 8 pT/√Hz at 400 Hz is characterized in unshielded environment. The dextran-coated magnetic nano-particle of 50 nm in diameter is administrated intravenously into Wistar male rats to demonstrate the in-vivo and fast examination feasibility of this instrument. The in-vivo results of heart region and liver region explained the reasonable biological phenomenon of magnetic nano-particles in animals. Good correlation of concentration-time curve between the induction-coupled-plasma (ICP) and in-vivo examination by AC susceptibility measurement in rat liver region until 4 hours after injection of magnetic naoparticle validates the in-vivo measurement of iron concentration. After 4 hrs post magnetic nanoparticle injection, the phagocyted magnetic nano-particles in liver tissue shows antiferromagnetism properties and explain the difference between the low in-vivo intensity by AC susceptibility measurement and high intensity by ICP.