Reduction of T2 Relaxation Rates due to Large Volume Fractions of Magnetic Nanoparticles for All Motional Regimes

Bashar Issa

doi:10.3390/app8010101

Abstract

The effect of high volume fraction of magnetic nanoparticles (MNP) on Magnetic Resonance Imaging (MRI) transverse relaxation rates (R2 = 1/T2 and R2* = 1/T2*) is investigated using Monte Carlo (MC) simulations. Theoretical models assume that particles occupy a small volume fraction of the sample space. Results presented in this work show that models based on both motional averaged (MAR) and static dephasing (SDR) regimes respectively underestimate and overestimate relaxation rates at large volume fractions. Furthermore, both R2* and R2* become echo-time dependent. This suggests that diffusion is involved with larger echo-times producing smaller relaxation rates due to better averaging of the magnetic field gradients. Findings emphasize the need for the models to be modified to take account of high particle concentration especially important for application involving clustering and trapping of nanoparticles inside cells. This is important in order to improve the design process of MNP Contrast Agents.

Highlights

The ability to synthesize nanoparticles and functionalize them have opened many areas for scientific and technical applications in recent years
The water spins are very efficient in sampling and averaging the magnetic field variations introduced by the small magnetic nanoparticles (MNP) as long as they are not interrupted by line of the particle surface of a magnetic particle of radius R and magnetization refocusing RF pulses M, orand
For low particle volume fraction, the magnetization of each particle was assumed to be independent of the magnetic field of other particles i.e., only depends on H0 through M ∼ χH0. This assumption is violated for the large particle volume fraction when the inter-particle spacing becomes very small and the magnetization of the particle becomes affected by the magnetic field of the adjacent particles

Summary

Introduction

The ability to synthesize nanoparticles and functionalize them have opened many areas for scientific and technical applications in recent years. Understanding the individual evolution of these parameters with scale, magnetic field, temperature, and their interaction, mutual and with the surrounding environment, is of paramount importance to the design and application of intelligent and multifunctional magnetic nanoparticles (MNP). MNPs and in particular superparamagnetic iron oxides (SPIO) have a wide range of applications such as magnetic particle imaging (MPI) [6], drug delivery [7], magnetic hyperthermia (MH) [8,9,10], MRI [11], cell tracking [12,13], magnetic biosensors [14,15], regenerative medicine and tissue engineering [16,17], etc. Modern synthesis techniques of nanoparticles control their physical and chemical properties to great precision in order to control and predict interactions and functionality of the MNP within tissues such as their biocompatibility and toxicity, spatial distribution, and the relaxation enhancement per unit concentration (i.e., relaxivity).

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Discussion

Conclusion