International Journal on Magnetic Particle Imaging (IJMPI)
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A sensitive, stable, continuously rotating FFL MPI system for functional imaging of the rat brain
No full textMagnetic particle imaging noninvasively maps the distribution of superparamagnetic iron oxide nanoparticles with high sensitivity. Since the particles are confined to the blood pool within the brain, it may be well-suited for cerebral blood volume (CBV)-based functional neuroimaging with MPI (fMPI). Here, we present a magnetic particle imaging system designed to detect the CBV modulation at the hemodynamic timescale (~5 sec) in rodents. It has the capacity to record sufficiently fast image time-series for several hours continuously. The time-series imaging was achieved with an optimized drive coil that maintains ~0.01% per minute current magnitude stability. An electrical slip ring and rotary union for cooling water allows continuous mechanical rotation of the 2.83 T/m Field-Free Line (FFL) permanent magnets and shift coils. The system achieves a 6.7 ng Fe detection limit (SNR = 5) in a single 5 sec image in the time-series, a spatial resolution of 3.0 mm in a 3 cm diameter field of view. The designs have been made open-source to enable replication of this device.
Int. J. Mag. Part. Imag. 8(2), 2022, Article ID: 2212001, DOI: 10.18416/IJMPI.2022.221200
Averaging Randomized Kaczmarz for Magnetic Particle Imaging
No full textMagnetic particle imaging (MPI) is a promising modality for medical imaging with high resolution and sensitivity. In this paper, the averaging randomized Kaczmarz (ARK) reconstruction method is proposed to accelerate the system matrix reconstruction process. Compared with the traditional Kaczmarz method, the ARK algorithm improves the image quality and reconstruction resolution. The iterative parameters of the ARK are selected, and further optimization needs to be studied
High-resolution MPI with spatially resolved measurement on field free lines
No full textIn magnetic particle imaging (MPI), 1D projected signals can be collected by exciting magnetic particles on a field free line (FFL) with a homogeneous excitation field. The movements and rotations of FFL with projection reconstructions generate 2D and 3D images of magnetic nanoparticles. The image resolution is heavily relying on the wideness of FFL, which is limited by the currently available maximal gradient strength. We proposed an additional gradient field with the same direction as the FFL for pulsed excitation and 1D spatial encoding. The spatial encoding steps include different gradient excitation profiles along with the FFL. System matrix for 1D image reconstruction is based on the relaxation-induced decay signal during the flat portion of pulsed square-wave excitation. For larger magnetic particles, our simulation shows that the pulsed excitation field with a greater flat portion generates a 1D bar phantom image with higher correlation and higher spatial resolution. With parallel FFL movements, high-resolution 2D images of human brain-sized Shepp-Logan phantom and clinical transverse MRA datasets are reconstructed by spatially resolved measurement of magnetic nanoparticles on FFLs
Characterization of magnetic nanoparticles for narrow-band magnetic particle imaging
No full textMagnetic particle imaging (MPI) is a promising imaging modality. Most MPI approaches are based on a wide-band detection, whereas our recently proposed narrow-band MPI (nbMPI) system only uses the harmonic. In this contribution we present our investigation of different multicore magnetic nanoparticle (MNP) systems regarding their suitability as tracers for our nbMPI system. In contrast to wideband MPI where many harmonics of sufficient amplitude are required, in our nbMPI approach a high amplitude of the 3rd harmonic is essential. In addition to MPI measurements with complex phantoms, our investigation included the measurement of the static magnetization curve with a magnetic property measurement system MPMS3, of the complex ac susceptibility and the harmonic spectra with MPS. Of the studied MNP systems, perimag® and synomag®-D-70 from micromod GmbH provided the best performance. Both MNP systems have a comparably high saturation magnetization and sufficiently low relaxation times providing a good nbMPI imaging performance
An Algorithm for computing optimal SNR-thresholds of a single-sided FFP MPI device
No full textDue to the topology of a single-sided MPI device, the magnetic field suffers from inhomogeneities which are limiting the penetration depth. Additionally, the sensitivity profile of the receive coils compromises the measured signal. Therefore, the signal spectrum shows a relatively high noise level. To achieve a sufficient reconstruction and reduce the reconstruction time, only the frequency components are used, that have a sufficiently high signal to noise ratio (SNR). The algorithm presented in this paper allows for computing optimal SNR-thresholds which promise reconstructed images with high quality. For now, the algorithm is limited to 2D-reconstruction, but it already promises to enhance the existing reconstruction algorithm for the single-sided field free point MPI device significantly
Enhanced characterization of a Magnetic Particle Imaging tracer combining field-flow fractionation and Magnetic Particle Spectroscopy
No full textFor a more detailed assessment of the performance of magnetic nanoparticles used as tracer in Magnetic Particle Imaging, we applied centrifugal flow-field fractionation (CF3) to separate a tracer according to their density and mass. The fractions were then magnetically and physically characterized by MPS, DLS, MALS and UV/Vis. Combining these findings with the results of magnetic characterization will allow a better understanding of the underlying mechanisms of MPI signal generation and tracer performance.  
Data augmentation for training a neural network for image reconstruction in MPI
No full textNeural networks need to be trained with immense datasets for successful image reconstruction. Acquiring these datasets may be a difficult task, especially in medical imaging. Data augmentation techniques are used to enlarge an available dataset by synthesizing new data. In this work, it is proposed to use the single measurements of a system matrix measurement in magnetic particle imaging for training a neural network for image reconstruction. Before training, mixup augmentation is used to create linear combinations of the single measurements and thus, enlarging the training dataset. Image reconstruction results using neural networks trained with an augmented system matrix are compared to images that have been reconstructed using the conventional system-matrix-based approach
Accelerated Kaczmarz for Convergence Speed-up in Multi-Contrast Magnetic Particle Imaging
No full textMagnetic Particle Imaging (MPI) is a tracer based medical imaging modality with great potential due to its high sensitivity, high spatio-temporal resolution, and ability to quantify the tracer concentration. Image reconstruction in MPI is an ill-posed problem, which can be addressed by the use of regularization methods. The corresponding optimization problem is most commonly solved using the Kaczmarz algorithm. Reconstruction using the Kaczmarz method for single-contrast MPI is very efficient as it produces the desired images fast with a small number of iterations. For multi-contrast MPI, however, the regular Kaczmarz algorithm fails to obtain good quality images without channel leakage when using a small number of iterations. In this work, we propose to use an accelerated Kaczmarz method in order to reduce the reconstruction time needed to achieve a good separation of the channels and a good image quality in multi-contrast MPI
An analytical equilibrium solution to the Néel relaxation Fokker-Planck equation
No full textIn this work, an analytical equilibrium solution to the Néel relaxation Fokker-Planck equation with a uniaxial anisotropy is presented. The solution is compared with a recently developed mathematical model that explains the spatial structure of the system function better than the Langevin function based model. The results presented show small relative errors on a large scale of typical particle parameters, so that both models can be considered approximately equivalent. The parameter choice for which both models diverge is outlined. The advantage of the proposed solution is its fast calculation in contrast to the numerical solution of the Fokker-Planck equation
Highly Flexible Human Aneurysm Models for Realistic Flow Experiments with MPI and MRI
No full text4D flow experiments, as known from MRI, are useful to examine the fluid dynamics in complex vessel geometries with high accuracy. The transfer to a positive contrasted fast and sensitive modality, such as MPI, offers new insights into dynamic particle flow under real-time visualization. In particular, measurements in 3D printed vascular phantoms are beneficial since they allow studies of the complicated flow-related interrelations causing the development of pathologies and also provide a quality control for the development of more realistic vascular phantoms. The presented framework provides an easy-to-use processing from extraction of desired vessel structures from 3D MRI or CT data over preparation them for 3D printing and further processing to final measurements