(615e) Modeling the Response of Magnetic Nanoparticles Relaxing By the Neel Mechanism for Magnetic Particle Imaging

The behavior of magnetic nanoparticles in magnetic fields has been studied extensively for technologies like magnetic resonance imaging and magnetic fluid hyperthermia. However, an emerging biomedical imaging technology called Magnetic Particle Imaging (MPI),¹ which also makes use of magnetic nanoparticles as tracers is currently in need of models that can be applied to rationally design nanoparticle tracers and imaging sequences. MPI has been demonstrated in applications such as real-time cardiovascular imaging², cell labeling and tracking^3,4, and in spatially focused magnetic fluid hyperthermia.⁵ As the image is obtained by mapping the tracer distribution to a scanned magnetic gradient field, understanding the behavior of the nanoparticles particles under these conditions is vital for advancing this technology. The initial theory for MPI assumed that tracers respond instantaneously to the applied time-varying magnetic fields. However, in practice finite magnetic relaxation or response time has been found to cause blurring of the image, limiting the attainable resolution in MPI.^6,7 The currently used gold standard for MPI, ferucarbotran, is primarily made of clusters of small magnetic nanoparticles relaxing by the Néel mechanism. With growing interest in synthesizing large magnetic diameter Néel particles, it’s important to understand the effect of relaxation time on the MPI signal and resolution.

In this work we used the Landau-Lifshitz-Gilbert (LLG) equation⁸ to model the behavior of particles that respond to magnetic fields by the Néel mechanism. The model takes into account the profile of the applied field and the crystalline anisotropy of the particles without the need of fitting parameters. This model was applied to predict the behavior of nanoparticles in situations typically used to characterize their performance for MPI, such as evaluating the point spread function (PSF) using an x-space relaxometer⁷ and obtaining the nanoparticle harmonic spectra in a magnetic particle spectrometer (MPS).⁹ Insights learned from the LLG model in these situations will be discussed.

¹ B. Gleich and J. Weizenecker, Nature 435, 1214 (2005).

² J. Weizenecker, B. Gleich, J. Rahmer, H. Dahnke, and J. Borgert, Physics in Medicine and Biology 54, L1 (2009).

³ B. Zheng, M. P. von See, E. Yu, B. Gunel, K. Lu, T. Vazin, D. V. Schaffer, P. W. Goodwill, and S. M. Conolly, Theranostics 6, 291 (2016).

⁴ B. Zheng, T. Vazin, P. W. Goodwill, A. Conway, A. Verma, E. Ulku Saritas, D. Schaffer, and S. M. Conolly, Scientific Reports 5, 14055 (2015).

⁵ D. W. Hensley, Z. W. Tay, R. Dhavalikar, B. Zheng, P. Goodwill, C. Rinaldi, and S. Conolly, Physics in Medicine and Biology 62, 3483 (2017).

⁶ L. R. Croft, P. W. Goodwill, and S. M. Conolly, IEEE Transactions on Medical Imaging 31, 2335 (2012).

⁷ P. W. Goodwill, A. Tamrazian, L. R. Croft, C. D. Lu, E. M. Johnson, R. Pidaparthi, R. M. Ferguson, A. P. Khandhar, K. M. Krishnan, and S. M. Conolly, Applied Physics Letters 98, 262502 (2011).

⁸ T. L. Gilbert, IEEE Transactions on Magnetics 40, 3443 (2004).

⁹ N. Garraud, R. Dhavalikar, L. Maldonado-Camargo, D. P. Arnold, and C. Rinaldi, AIP Advances 7 (2017).