Magnetic nanoparticles (MNPs) can be used in various biomedical applications, such as magnetic drug targeting (MDT) or magnetic hyperthermia for cancer treatment. MNPs are injected into the body and accumulated with an external magnetic field at the tumor site. For therapy monitoring, the MNPs distribution has to be accurately mapped in real time. Magnetomotive ultrasound (MMUS) is a suitable technique for this purpose. It was already demonstrated that MMUS can map the accumulation process of MNPs during MDT. Moreover, inverse MMUS is an advanced technique to quantify MNP distribution. This method simulates the displacement of MNPs and surrounding tissues and compares the results to measured data. Understanding how magnetomotive displacement behaves in various elastic environments is crucial for advancing inverse MMUS and accurately quantifying MNP distributions. In this study, ultrasound tissue phantoms were fabricated with varying elastic parameters, incorporating MNPs. Subsequently, their MMUS displacement is assessed.

Introduction

One promising technique for monitoring magnetic hyperthermia is magnetorelaxometry (MRX), where magnetic moments of magnetic nanoparticles (MNPs) are aligned by an external magnetic field and the relaxation of their net magnetic moment is measured after the excitation field is switched off. In contrast to previously employed SQUID sensors, optically pumped magnetometers (OPMs) do not require cryogenic cooling, hence they are predestined for measuring relaxation signals in clinical applications. Based on relaxation signal parameters, the spatial MNP distribution is reconstructed. However, in hyperthermia applications relaxation signals are acquired at different temperatures. In this work, we present an OPM-based MRX setup for investigating temperature effects on relaxation signals.

Methods

Two commercially available, immobilized MNP samples (Perimag®, micromod and RCL-01, Resonant Circuits Limited) were placed between an excitation coil and an optical magnetic gradiometer (OMG, Twinleaf). Measurements were performed inside a shielded room and with a background magnetic field applied for operating the sensor. Two orientations of the background field and several magnetic flux densities of the excitation field were applied. During the measurement, the sample was continouosly heated to different temperatures using hot air and relaxation amplitude and time were evaluated.

Results

Our results show decreasing relaxation amplitudes and times for increasing MNP temperatures for both particles systems, all excitation field strengths and both background field orientations. After recooling, both relaxation parameters return to their initial values, indicating that the temperature effect is reversible. Preliminary results also showed a dependence of the temperature effect on the iron amount of the MNP sample. Investigations on that are currently ongoing.

Conclusion

Measurements with our OPM-based MRX setup revealed that the relaxation behavior of immobilized MNPs changes in dependence on the MNP temperature. That is why these temperature effects should be considered for reconstructions of the spatial MNP distribution, particularly for hyperthermia monitoring.

Introduction

Magnetic nanoparticles (MNP) offer promising biomedical applications like magnetic hyperthermia and magnetic drug targeting. In magnetic hyperthermia, MNP are injected into the tumor and are heated by applying an RF magnetic field (e.g. 100 kHz, 20 kA/m). The knowledge about the spatial distribution of MNP in the tissue is a key for safe and effi-cient treatments. However, currently available clinical tools are not able to provide this information due to the high iron concentration used. Research grade magnetorelaxometry imaging (MRXI) is especially suited for these applications. In MRXI, the magnetic relaxation signals of previously magnetized MNP is recorded with high sensitive magnetometers and the spatial MNP distribution is reconstructed by solving an ill-posed inverse problem. Currently, the need for ex-cessive shielding is preventing the broad applicability of MRXI, which we aim to address here.

Methods

The imaging setup is composed of 32 excitation coils, two synchronized dual channel optically pumped magnetometers (OMG, Twinleaf), and triaxial homogeneous field and field gradient coils. The imaging volume is 36x36x36 mm³. Our 3D printed rat phantom is loaded with immobilized RCL-MNP with a clinically relevant iron amount. The environ-mental field is compensated. The excitation coils used for magnetizing the MNP are switched on for 200 ms and the relaxation is measured for 400 ms. Each coil is pulsed individually and sequentially. We apply extensive noise removal techniques like time-synchronized averaging, feed forward techniques, software gradiometry and correlation based data analysis before extracting the relaxation parameters from the measurement data.

Results

We successfully demonstrated proof-of principle reconstructions of point-like MNP sources in our rat phantom. Imag-ing parameters like the resolution and the iron detection limit are currently under investigation.

Conclusion

We demonstrated proof of principle quantitative imaging of MNP in an unshielded environment using magnetorelaxo-metry and optically pumped magnetometers.

To perform magnetoencephalography (MEG) or electroencephalography (EEG) source analysis, the conductivities of the conductive head tissues must be known. The influence of inter-individual variations of tissue conductivities has been investigated in detail for EEG source analysis. Since MEG source analysis is assumed to be much less affected by these variations, this sensitivity has been investigated only in few studies, even though it might be important especially for a combined analysis of EEG and MEG.

MEG forward solutions of dipole sources regularly distributed in the gray matter compartment were simulated in a de-tailed five-compartment head model for three realistic sensor configurations using the FEM multipole approach. Subsequently, a generalized polynomial chaos approach (gPC) was employed to rapidly calculate MEG leadfields for varying tissue conductivities. Based on these gPC expansions, the sensitivity of MEG forward solutions towards tissue conductivity uncertainties and the influence on MEG source analysis was investigated for the three sensor configurations.

A strong influence of tissue conductivity uncertainties on the MEG forward solutions is found especially for quasi-radial sources, e.g., on top of gyri. However, as such sources only have a very weak MEG signal and can therefore usu-ally not be properly reconstructed, this has little practical implications. For quasi-tangential sources, a strong influence of the gray matter conductivity on the signal magnitude is found. The sensitivity of the signal topography towards tis-sue conductivity uncertainties strongly depends on the source location. Generally, the influence of tissue conductivity uncertainties on MEG forward solutions and source analysis is clearly weaker than for the EEG.

Even though the sensitivity of MEG source analysis towards tissue conductivity uncertainties is clearly weaker than for the EEG, it should not be completely neglected, especially in a combined analysis of EEG and MEG.

Determining the position and orientation of a medical instrument is essential for accurate procedures in endoscopy, surgery, and vascular interventions. Recently, a novel sensor based on torsional pendulum-like magneto-mechanical motion has been proposed. This sensor is passive, wireless and inductively coupled to a transmit-receive coil array. This setup allows the determination of all 6 degrees of freedom using the characteristic resonance of the sensor. Additional physical quantities such as temperature and pressure can be measured based on the frequency of the sensor, which mainly depends on the distance between the two involved permanent magnets. In this study, we analyze a sensor composed of two magnetic cylinders with variable magnet-to-magnet distance and a basic physical model based on a dipole assumption. Experimental analysis of the resonance frequency and comparison with the model values show both qualitative and quantitative agreement with an average relative error of only 0.8 %. Results indicate good agreement with the implemented model and show the suitability of our magnetic-mechanical resonator made from cylindrical permanent magnets for sensing applications.

Introduction

Magnetic nanoparticles embedded in fiber-based, drug-loaded scaffolds, hereafter called magnetic scaffolds, enable controlled drug release by on-demand magnetic hyperthermia induced degradation of the magnetic scaffold. To control the drug release, models that predict the degradation under specific hyperthermia applications are necessary. Here, the development of a testing system capable of multimodally loading the magnetic scaffolds to accelerate degradation and measure the resulting changes in the material properties is presented. The data gathered by the developed testing system will be used to develop the degradation prediction model.

Methods

An experimental setup was designed to simultaneously load the magnetic scaffolds with a thermal, mechanical, and a hydrodynamic load. The mechanical load, in the form of a cyclic uniaxial tensile strain on the fiber, and the hydrodynamic load, in the form of phosphate-buffered saline flowing at physiological conditions, were applied continuously. The thermal load was applied every three days for one hour analogous to the NanoTherm® treatment plan. Changes in the mechanical properties after multimodal loading were measured by performing tensile tests using a universal testing machine.

Results

No bulk temperature increase was observed at the applied magnetic field strength and frequency. A roughening of the magnetic scaffold’s surface was observed. This, however, did not have an impact on the measured mechanical properties. Thus, the impact of the thermal and hydrodynamic load was limited. The results of the tensile tests show that the changes in the mechanical properties of the fiber-based magnetic scaffold after the long-term multimodal load are consistent with the standard linear solid model.

Conclusion

With the developed testing setup, we can measure changes in the mechanical properties due to the multimodal loading of the fiber-based magnetic scaffolds. Thus, the groundwork for the development of a model to predict the degradation of the fiber-based, drug-loaded scaffolds due to magnetic hyperthermia applications was laid.

Systems based on smart materials have become an increasing research focus in recent years, as their properties provide opportunity for higher functional integration, improved energy efficiency and completely new functionalities. Magnetic shape memory alloys (MSM) provide a promising option here. However, they have received limited attention in the field of medical engineering. This paper therefore aims to provide an introduction into the (magneto-mechanical) properties of MSM and to give an overview of existing applications-oriented research, both in general and in medical engineering. In the reviewed literature, NiMnGa has been used in several prototypes of micro pumps for intracranial drug delivery and lab-on-a-chip systems. FePd, another MSM alloy, has aroused interest in research into temporary implants.