Abstract
The design and validation of a magnetic particle spectrometer (MPS) system used to study the linear and nonlinear behavior of magnetic nanoparticle suspensions is presented. The MPS characterizes the suspension dynamic response, both due to relaxation and saturation effects, which depends on the magnetic particles and their environment. The system applies sinusoidal excitation magnetic fields varying in amplitude and frequency and can be configured for linear measurements (1 mT at up to 120 kHz) and nonlinear measurements (50 mT at up to 24 kHz). Time-resolved data acquisition at up to 4 MS/s combined with hardware and software-based signal processing allows for wide-band measurements up to 50 harmonics in nonlinear mode. By cross-calibrating the instrument with a known sample, the instantaneous sample magnetization can be quantitatively reconstructed. Validation of the two MPS modes are performed for iron oxide and cobalt ferrite suspensions, exhibiting Néel and Brownian relaxation, respectively.
Highlights
Magnetic particle spectrometer (MPS) development is motivated to assess magnetic suspension suitability for magnetic particle imaging (MPI).[1,2] MPI is an emerging biomedical imaging technique addressing drawbacks found in nuclear imaging by using non-radioactive tracers, i.e. the magnetic nanoparticles, with theoretically higher resolution in a short process time
The field is generated by a gapped solenoid excitation coil, with sinusoidal current supplied via a power amplifier fed by a computer-controlled data acquisition (DAQ) system (National Instruments PCI-6115, 12-Bit, 4 MS/s for multi-channel I/O) (Figure 1)
Linear and nonlinear measurements are performed on two in-house magnetic nanoparticle suspensions obtained by thermal decomposition.[11]
Summary
Magnetic particle spectrometer (MPS) development is motivated to assess magnetic suspension suitability for magnetic particle imaging (MPI).[1,2] MPI is an emerging biomedical imaging technique addressing drawbacks found in nuclear imaging by using non-radioactive tracers, i.e. the magnetic nanoparticles, with theoretically higher resolution in a short process time. MPI detects nanoparticle density spatially by probing locally their dynamic magnetization in a spatial selection gradient field and finds application in real-time cardiovascular imaging,[3] stem cell tracking[4,5] and hyperthermia.[6] The MPS described has no spatial scanning capability, but can assess both particle suspension relaxation and saturation, related to their performance for MPI.[7]. At higher applied field amplitudes, nonlinearity appears in the sample response due to magnetic saturation of the suspension. The measured spectrum presents odd harmonics of the fundamental frequency, characteristic of the suspension magnetic nonlinearity. This nonlinear MPS mode characterizes the nanoparticle suspension rotational dynamics, both in amplitude and in frequency, assessing both saturation and relaxation effects
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