NMR relaxation in systems with magnetic nanoparticles: a temperature study

Iron oxide magnetic nanoparticles based low-field MR thermometry

This paper reports on a highly accurate approach of magnetic resonance (MR) thermometry using iron oxide magnetic nanoparticles (MNPs) as temperature sensors. An empirical model for the description of the temperature dependent R ₂ relaxation rate is proposed by taking into account the temperature sensitivity of the MNP magnetization. The temperature sensitivity of the MNP magnetization (η) and the temperature sensitivity of the R ₂ relaxation rate (κ) are simulated with the proposed empirical models to investigate their dependence on the magnetic field and the particle size. Simulation results show the existence of optimal magnetic fields H_oη and H_oκ that maximize the temperature sensitivities η and κ. Furthermore, simulations and experiments demonstrate that the optimal magnetic field H_oη (H_oκ ) decreases with increasing the particle size. Experiments on temperature dependent R ₂ relaxation rate are performed at different magnetic fields for MNP samples with different iron concentrations. Experimental results show that the proposed MR thermometry using MNPs as temperature sensors allows a temperature estimation accuracy of about 0.05 °C. We believe that the achieved approach of highly accurate MR thermometry is of great interest and significance to biomedicine and biology.

Effect of Polymer Surface Modification of Superparamagnetic Iron Oxide Nanoparticle Dispersions in High Salinity Environments

Superparamagnetic nanoparticles (SPIONs) can be used as nuclear magnetic resonance (NMR) signal enhancement agents for petroleum exploration. This enhancement effect is uniform if SPIONs are monodisperse in size and in composition; yet it is challenging to synthesize monodisperse particles that do not aggregate in high salinity petroleum brine. Here, we report a method to synthesize individual SPIONs coated with tunable surface coating densities of poly(2-acrylamido-2-methyl-1-propanesulfonic acid (pAMPS) with a catechol end-group (pAMPS*). To establish parameters under which pAMPS*-coated SPIONS do not aggregate, we compared computational predictions with experimental results for variations in pAMPS* chain length and surface coverage. Using this combined theoretical and experimental approach, we show that singly dispersed SPIONs remained stabilized in petroleum brine for up to 75 h with high surface density pAMPS*.

Magnetic Heating Stimulated Cargo Release with Dose Control using Multifunctional MR and Thermosensitive Liposome

Rationale: Magnetic resonance imaging (MRI) is one of the most widely used diagnostic tools in the clinic. In this setting, real-time monitoring of therapy and tumor site would give the clinicians a handle to observe therapeutic response and to quantify drug amount to optimize the treatment. In this work, we developed a liposome-based cargo (cancer drugs) delivery strategy that could simultaneously monitor the real-time alternating magnetic field-induced cargo release from the change in MRI relaxation parameter R1 and the location and condition of liposome from the change in R2. The tumor site can then be monitored during the cargo release because liposomes would passively target the tumor site through the enhanced permeability and retention (EPR) effect. Physical insights from the experimental results and corresponding Monte Carlo spin dynamics simulations were also discussed. Methods: Superparamagnetic iron oxide (SPIO) nanoparticles, diethylenetriaminepentaacetic acid gadolinium(III) (Gd(III)-DTPA), and a model cancer drug (fluorescein) were co-loaded in PEGylated thermosensitive liposomes. The liposomes were characterized by transmission electron cryo-microscopy (cryoTEM), dynamic light scattering (DLS), and inductively coupled plasma optical emission spectrometry (ICP-OES). Alternating magnetic field (AMF) was used to create controlled mild hyperthermia (39-42°C) and facilitate controlled cargo (fluorescein) release from the thermosensitive liposomes. MRI relaxation parameters, R1 and R2, were measured at room temperature. The temporal variation in R1 was used to obtain the temporal profile of cargo release. Due to their similar sizes, both the gadolinium and cargo (model cancer drug fluorescein) would come out of the liposomes together as a result of heating. The temporal variation in R2 was used to monitor SPIO nanoparticles to enhance the tumor contrast. Monte Carlo spin dynamics simulations were performed by solving the Bloch equations and modeling SPIO nanoparticles as magnetized impenetrable spheres. Results: TEM images and DLS measurements showed the diameter of the liposome nanoparticle ~ 200 nm. AMF heating showed effective release of the model drug. It was found that R1 increased linearly by about 70% and then saturated as the cargo release process was completed, while R2 remained approximately constant with an initial 7%-drop and then recovered. The linear increase in R1 is consistent with the expected linear cargo release with time upon AMF heating. Monte Carlo spin dynamics simulations suggest that the initial temporal fluctuation of R2 is due to the plausible changes of SPIO aggregation and the slow non-recoverable degradation of liposomal membrane that increases water permeability with time by the heating process. The simulations show an order of magnitude increase in R2 at higher water permeability. Conclusion: We have performed MR parameter study of the release of a cargo (model cancer drug, fluorescein) by magnetic heating from thermosensitive multifunctional liposomes loaded with dual contrast agents. The size of the liposome nanoparticles loaded with model cancer drug (fluorescein), gadolinium chelate, and SPIO nanoparticles was appropriate for a variety of cancer therapies. A careful and detailed analysis with theoretical explanation and simulation was carried out to investigate the correlation between MRI relaxation parameters, R1 and R2, and different cargo release fractions. We have quantified the cargo release using R1, which shows a linear relation between each other. This result provides a strong basis for the dosage control of drug delivered. On the other hand, the fairly stable R2 with almost constant value suggests that it could be used to monitor the position and condition of the liposomal site, as SPIO nanoparticles mostly remained in the aqueous core of the liposome. Because our synthesized SPIO-encapsulated liposomes could be targeted to tumor site passively by the EPR effect, or actively through magnetofection, this study provides a solid ground for developing MR cancer theranostics in combination of this nanostructure and AMF heating strategy. Furthermore, our simulation results predict a sharp increase in R2 during the AMF heating, which opens up the exciting possibility of high-resolution, high-contrast real-time imaging of the liposomal site during the drug release process, provided AMF heating could be incorporated into an MRI setup. Our use of the clinically approved materials, along with confirmation by theoretical simulations, make this technique a promising candidate for translational MR cancer theranostics.

Magnetic nanoparticles: surface effects and properties related to biomedicine applications

Due to finite size effects, such as the high surface-to-volume ratio and different crystal structures, magnetic nanoparticles are found to exhibit interesting and considerably different magnetic properties than those found in their corresponding bulk materials. These nanoparticles can be synthesized in several ways (e.g., chemical and physical) with controllable sizes enabling their comparison to biological organisms from cells (10–100 μm), viruses, genes, down to proteins (3–50 nm). The optimization of the nanoparticles’ size, size distribution, agglomeration, coating, and shapes along with their unique magnetic properties prompted the application of nanoparticles of this type in diverse fields. Biomedicine is one of these fields where intensive research is currently being conducted. In this review, we will discuss the magnetic properties of nanoparticles which are directly related to their applications in biomedicine. We will focus mainly on surface effects and ferrite nanoparticles, and on one diagnostic application of magnetic nanoparticles as magnetic resonance imaging contrast agents.