International Journal on Magnetic Particle Imaging (IJMPI)
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Measured PNS Thresholds in a Human Head MPI Solenoid from 200 Hz to 88.1 kHz
No full textTime-varying B-fields produced by MPI drive coils can cause peripheral nerve stimulation (PNS)[1] which can compromise subject comfort and safety[2]. Previous magnetostimulation experiments[3-5] observed deviations from the hyperbolic strength-duration relation predicted by the Fundamental Law of Magnetostimulation (FLM)[6]. We measure PNS thresholds in 8 healthy volunteers in a head solenoid over 16 frequencies between 200Hz to 88.1kHz to supplement existing literature. The head solenoid designed for our human-scale fMPI scanner[7] is a four-layer coil wound with hollow Cu wire (4mm OD, 2mm ID), consisting of 54 turns (111cm length, 27cm winding ID). A capacitor bank enabled rapid switching between 6 untuned frequencies (200Hz-700Hz), and 10 tuned frequencies (1.8kHz-88.1kHz). Stimulation waveforms consisted of 256 cycle sinusoids with exponential ramp-up envelopes (time constant 25 cycles for all frequencies). Stimulation thresholds were robustly determined by fitting sigmoid functions to subject responses over stimulation amplitude.
Measurements reveal departure from the hyperbolic FLM, showing a minimum threshold of ~4.97mT peak (3.51mT rms) at coil center between 17kHz-25kHz, increasing to ~6.44mT peak (4.55mT rms) at 66.8kHz contrasting the predicted asymptotic behavior. Only 4 out of 8 subjects reported stimulation at 88.1kHz (amplifier max 7mT peak). We fit error function CDFs to estimate mean thresholds across subjects (~6.86mT peak, 4.85mT rms at 88.1kHz). PNS thresholds may limit sensitivity in human head imaging[8]. This work provides measured PNS thresholds that critically inform drive field amplitudes for human head imaging, or drive frequency optimization for maximum sensitivity considering particle relaxation, Faraday detection, and drive amplitude at the PNS threshold
Focused small field of view magnetic particle imaging for the isolation and quantification of MPI signal in tumours
No full textTumour Associated Macrophages (TAMs) play a crucial role in breast cancer progression and have the potential to be used as a biomarker for patient prognosis. Magnetic particle imaging (MPI) is an emerging modality which can detect cells labelled with superparamagnetic iron oxide (SPIO) nanoparticles and can be used for non-invasive TAM assessment. However, MPI TAM detection is limited by its effective dynamic range. This limitation occurs when SPIO nanoparticles injected intravenously accumulate in the liver resulting in a large MPI signal which shadows regions of interest with lower signals (i.e the tumour) preventing their isolation and quantification. In this study we test a new reconstruction method which allows us to prescribe a small focused field of view (FOV) on lower signals of interest. We then demonstrate the success of this method with an in vivo tumour model and show enhanced image quality and successful isolation of MPI signal in mouse mammary tumours with different metastatic potentials (4T1 and E0771). Utilizing in vivo MPI, we did not see significant differences in the MPI signal for 4T1 tumours compared to E0771. These findings highlight the potential of MPI for in vivo TAM quantification offering a promising avenue for broader applications in cancer research and potentially overcoming constraints of MPI in other in vivo imaging contexts.
Dynamic image reconstruction in MPI with RESESOP-Kaczmarz
No full textIn Magnetic Particle Imaging (MPI), it is typically assumed that the studied specimen is stationary during the data acquisition. In practical applications however, the searched-for 3D distribution of the magnetic nanoparticles might show a dynamic behavior, caused by e.g. breathing or movement of the blood. Neglecting those dynamics during the reconstruction step results in motion artifacts and a reduced image quality. This article addresses the challenge of capturing high quality images in the presence of motion. A promising technique provides the Regularized Sequential Subspace Optimization (RESESOP) algorithm, which takes dynamics as model inexactness into account, significantly improving reconstruction compared to standard static algorithms like regularized Kaczmarz. Notably, this algorithm operates with minimal prior information and the method allows for subframe reconstruction, making it suitable for scenarios with rapid particle movement. The performance of the proposed method is demonstrated on both simulated and real data sets
Super-resolution Pulsed Magnetic Particle Imaging Using Shape Anisotropic Nanoparticles
No full textPulsed magnetic particle imaging (MPI) allows the use of long relaxation time magnetic particles to achieve high-resolution images. In this study, we proposed a super-resolution imaging method for pulsed MPI, which can further enhance its spatial resolution. We validated our approach using autonomously synthesized long relaxation time shape anisotropic nanoparticles on our in-house-built 1D MPI system. The results demonstrate a spatial resolution of 1.0 mm under a gradient field of 1.2 T/m
Magnetic Particle Fractionation and In-line Characterization for Enhancing Magnetic Particle Imaging Tracers
No full textRefining the size distribution of magnetic nanoparticles (MNPs) holds great potential for enhanced performance in magnetic particle imaging (MPI). This work demonstrates the coupling of a preparative-scale magnetic fieldcontrolledsize fractionation technique with in-line magnetic particle spectroscopy (MPS) for the real-time assessmentof magnetic nanoparticle downstream processing performance. In-line magnetic particle spectroscopymonitoring allowed for the concurrent evaluation of the amplitude spectrum and derived parameters including theharmonic ratio A5/A3, enabling real-time selection of fractions with desired characteristics during the fractionationprocess. Furthermore, magnetic particle spectroscopy offered increased sensitivity and more comprehensivecharacterization capabilities than conventional in-line analysis techniques such as light absorption spectroscopy.A theoretical model delineating the dependency of magnetic particle spectroscopy signal on core/shell size ratiowas developed. These results potentially have significant implications for prospective applications in the realm ofin-line magnetic particle spectroscopy analysis for real-time process control and quality assurance
Acoustomagnetics: proof of concept
No full textMagnetic nanoparticles (MNPs) play a pivotal role in various medical applications, including manipulating biomolecules, targeted drug delivery, and hyperthermia treatment. MNPs can undergo functionalization or be loaded with additional payloads to carry out specialized functions in treatment, sensing, or imaging. External sensing devices can detect the response of mobile magnetic nanoparticles in a stationary magnetic field. Magnetic Particle Imaging (MPI) is an emerging imaging technique that holds the potential for directly visualizing MNPs within the body. However, its clinical applications encounter limitations due to restricted penetration depth caused by substantial electromagnetic damping effects and the necessity for large, energy-intensive installations. To address these challenges, we leverage acoustic waves to stimulate the MNPs, aiming to overcome these limitations. A notable distinction in the acquired signal is observed when the DC-field is switched on
Microwave-assisted high-speed synthesis of superparamagnetic iron-oxide nano¬particles
No full textIn this work, the microwave-assisted synthesis of superparamagnetic iron oxide nanoparticles (SPIONs) for MPI is presented. Compared to conventional coprecipitation, microwave-assisted synthesis in specially designed microwave reactors enables good yields while reaction times can be reduced to a few minutes. To find the best synthesis route, various parameters such as the base used, reaction temperature, heating and cooling time were analysed. Various dextrans (molecular weight 10,000-100,000) were used to assess whether the methods can also be applied to other coating materials. The first results of this evaluation study are presented here
MPI field-free line scanning with dual electromagnet current adjustment
No full textMagnetic particle imaging (MPI) is a technology that directly detects the nonlinear response of magnetic nanoparticles (MNPs). It has garnered significant attention in medical imaging. MNPs are saturated except near a specific point known as the field-free region, established by applying a static magnetic field [1]. Recent research has underscored using the field-free line (FFL) method for scanning the field-of-view to enhance MPI sensitivity [2-4]. This paper reports on a performance evaluation of an MPI system employing two pairs of electromagnets capable of scanning the FFL by finely adjusting the distribution of current flowing through each electromagnet
A receive insert for non-human primate functional MPI (fMPI)
No full textWe designed and built a receive gradiometer coil for non-human primate (NHP) fMPI studies. The coil was integrated and tested within our human-size brain MPI system. We demonstrate the imaging resolution and sensitivity expected in NHP studies using a dilusion series sensitivity measurement (120 µg to 23 ng iron mass) and a NHP-brain-sized phantom with an iron concentration of 62.5 µg/mL, below the level expected in arterial blood in MPI (143.0 µg/mL). Using 6 mT drive amplitudes and producing one 12.8 cm field-of-view image every 4.7 seconds resulted in a detection limit (SNR = 1) of 19.5 ng of iron in 2D imaging and good image quality. This would correspond to a contrast-to-noise ratio (CNR) of 20 for a 25% change of cerebral blood volume (CBV) following activation in a 6 mm isotropic voxel. These promising preliminary sensitivity levels encourage the pursuit of the first in vivo brain function MPI experiments in primates
Towards an MPI-MRI-MEG fused neuroimaging system: Acquisition of MR images at 300 kHz with a newly developed compact OPM
No full textOptically pumped magnetometer (OPM) is a promising magnetic sensor alternative to SQUID that enables the measurement of very small magnetic signals. In addition, OPMs have the intrinsic advantage of not requiring cryogenic cooling. We have been developing a compact and portable OPM module with a pump-probe arrangement. Since sensitivity of OPM does not depend on frequency, it is suitable to be used as a receiving sensor for magnetic particle imaging (MPI) and ultra-low-field (ULF) MRI systems. In a previous study, we demonstrated the possibility of remotely detecting magnetic fields generated from super-paramagnetic iron oxide nanoparticles using an OPM with a flux transformer [1]. Here, we introduce our newly developed miniaturized OPM module having its noise floor less than 20 fT/Hz1/2 as well as the first magnetic shieldless OPM-based ULF-MRI scanner with an OPM operating at a Larmor frequency of 300 kHz