Automatic, log file-based process analysis of a clinical 1.5T MR scanner: a proof-of-concept study. / Automatische, Logfile-basierte Prozessanalyse an einem 1,5T-MRT-Scanner der klinischen Routine: Eine Machbarkeitsstudie.

PURPOSE: In light of the steadily increasing need for economical efficacy and capacity utilization it was the aim of this proof-of-concept work to implement an automated logfile-based analysis tool for MRI scanner utilization and to establish a process analysis. As a primary step, analyses of scanner and protocol utilization, parametrization of protocol processes, their durations, age dependency, and scan efficacy were to be tested. MATERIALS AND METHODS: Logfiles were continuously extracted from a 1.5 T MR scanner (Philips Achieva) and automatically explored for relevant scan parameters. Parameters were extracted into a database and logically combined to protocol parameters. Visualization was achieved using PowerBI (Microsoft, USA). Data aggregation comprised a day-based and protocol-based strategy. In addition, age- and regional-based testing was performed. The frequency of protocol usage was evaluated and those protocols with frequent usage compared regarding efficacy to those rarely used. RESULTS: After successful technical implementation, 3659 MR exams were available for further analysis. Out of a plethora of parameters, those relevant to the understanding of the scan process were identified. The initial results mirror the daily scanner usage and allow identifying, e. g., shortened scanner usage on Fridays or longer examination times in children. A scan efficacy of 69.6 ±â€Š17.6 % excluding preparation process was identified as a parameter with high potential to be optimized in daily routine. CONCLUSION: The logfile-based analysis of MR scanner processes was successfully introduced and holds the promise to be extended into a comprehensive analytic tool for the analysis and optimization of scanner processes. In combination with other variables from the departmental or institutional infrastructure or patient-specific information such tool may be developed into a intelligent steering tool. KEY POINTS: · The automated log file analysis of MR-scanner processes was successfully introduced. · The log file-analysis allows for a detailed analysis of scanner processes. · From a log file-analysis, there is potential benefit to users, applications specialists and developers. CITATION FORMAT: · Frydrychowicz A, Boppel T, Sieber V et al. Automatic, log file-based process analysis of a clinical 1.5T MR scanner: a proof-of-concept study. Fortschr Röntgenstr 2021; 193: 919 - 927.

Interactive Magnetic Catheter Steering With 3-D Real-Time Feedback Using Multi-Color Magnetic Particle Imaging.

Magnetic particle imaging (MPI) is an emerging tomographic method that enables sensitive and fast imaging. It does not require ionizing radiation and thus may be a safe alternative for tracking of devices in the catheterization laboratory. The 3-D real-time imaging capabilities of MPI have been demonstrated in vivo and recent improvements in fast online image reconstruction enable almost real-time data reconstruction and visualization. Moreover, based on the use of different magnetic particle types for catheter visualization and blood pool imaging, multi-color MPI enables reconstruction of separate images for the catheter and the vessels from simultaneously measured data. While these are important assets for interventional imaging, MPI field generators can furthermore apply strong forces on a magnetic catheter tip. It is the aim of this paper to give a first demonstration of the combination of real-time multi-color MPI with online reconstruction and interactive field control for the application of forces on a magnetic catheter model in a phantom experiment.

Magnetic Particle Imaging (MPI): Experimental Quantification of Vascular Stenosis Using Stationary Stenosis Phantoms.

Magnetic Particle Imaging (MPI) is able to provide high temporal and good spatial resolution, high signal-to-noise ratio and sensitivity. Furthermore, it is a truly quantitative method as its signal strength is proportional to the concentration of its tracer, superparamagnetic iron oxide nanoparticles (SPIOs). Because of that, MPI is proposed to be a promising future method for cardiovascular imaging. Here, an interesting application may be the quantification of vascular pathologies like stenosis by utilizing the proportionality of the SPIO concentration and the MPI signal strength. In this study, the feasibility of MPI based stenosis quantification is evaluated based on this application scenario. Nine different stenosis phantoms with a normal diameter of 10 mm each and different stenoses of 1-9 mm and ten reference phantoms with a straight diameter of 1-10 mm were filled with a 1% Resovist dilution and measured in a preclinical MPI-demonstrator. The MPI signal intensities of the reference phantoms were compared to each other and the change of signal intensity within each stenosis phantom was used to calculate the degree of stenosis. These values were then compared to the known diameters of each phantom. As a second measurement, the 5 mm stenosis phantom was used for a serial dilution measurement down to a Resovist dilution of 1:3200 (0.031%), which is lower than a first pass blood concentration of a Resovist bolus in the peripheral arteries of an average adult human of at least about 1:1000. The correlation of the stenosis values based on MPI signal intensity measurements and based on the known diameters showed a very good agreement, proving the high precision of quantitative MPI in this regard.

Multi-color magnetic particle imaging for cardiovascular interventions.

Magnetic particle imaging (MPI) uses magnetic fields to visualize the spatial distribution of superparamagnetic iron oxide nanoparticles (SPIOs). Guidance of cardiovascular interventions is seen as one possible application of MPI. To safely guide interventions, the vessel lumen as well as all required interventional devices have to be visualized and be discernible from each other. Until now, different tracer concentrations were used for discerning devices from blood in MPI, because only one type of SPIO could be imaged at a time. Recently, it was shown for 3D MPI that it is possible to separate different signal sources in one volume of interest, i.e. to visualize and discern different SPIOs or different binding states of the same SPIO. The approach was termed multi-color MPI. In this work, the use of multi-color MPI for differentiation of a SPIO coated guide wire (Terumo Radifocus 0.035″) from the lumen of a vessel phantom filled with diluted Resovist is demonstrated. This is achieved by recording dedicated system functions of the coating material containing solid Resovist and of liquid Resovist, which allows separation of their respective signal in the image reconstruction process. Assigning a color to the different signal sources results in a differentiation of guide wire and vessel phantom lumen into colored images.

Magnetic Particle Imaging: A Resovist Based Marking Technology for Guide Wires and Catheters for Vascular Interventions.

Magnetic particle imaging (MPI) is able to provide high temporal and good spatial resolution, high signal to noise ratio and sensitivity. Furthermore, it is a truly quantitative method as its signal strength is proportional to the concentration of its tracer, superparamagnetic iron oxide nanoparticles (SPIOs), over a wide range practically relevant concentrations. Thus, MPI is proposed as a promising future method for guidance of vascular interventions. To implement this, devices such as guide wires and catheters have to be discernible in MPI, which can be achieved by coating already commercially available devices with SPIOs. In this proof of principle study the feasibility of that approach is demonstrated. First, a Ferucarbotran-based SPIO-varnish was developed by embedding Ferucarbotran into an organic based solvent. Subsequently, the biocompatible varnish was applied to a commercially available guidewire and diagnostic catheter for vascular interventional purposes. In an interventional setting using a vessel phantom, the coating proved to be mechanically and chemically stable and thin enough to ensure normal handling as with uncoated devices. The devices were visualized in 3D on a preclinical MPI demonstrator using a system function based image reconstruction process. The system function was acquired with a probe of the dried varnish prior to the measurements. The devices were visualized with a very high temporal resolution and a simple catheter/guide wire maneuver was demonstrated.

Steering of Magnetic Devices With a Magnetic Particle Imaging System.

Small magnetic devices have been steered in arbitrary direction and with variable force using a preclinical demonstrator system for magnetic particle imaging (MPI). Fast localization due to the high imaging rate of over 40 volumes/s and strong forces due to the high field gradient of more than 1 T/m render an MPI system, a good platform for image-guided steering of magnetic devices. In this paper, these capabilities are demonstrated in phantom experiments, where a closed feedback loop has been realized to exert translational forces in horizontal and vertical direction on a magnetic device moving in a viscous medium. The MPI system allows for the controlled application of those forces by combining variable homogeneous fields with strong field gradients.

Magnetic particle imaging with tailored iron oxide nanoparticle tracers.

Magnetic particle imaging (MPI) shows promise for medical imaging, particularly in angiography of patients with chronic kidney disease. As the first biomedical imaging technique that truly depends on nanoscale materials properties, MPI requires highly optimized magnetic nanoparticle tracers to generate quality images. Until now, researchers have relied on tracers optimized for MRI T2(∗) -weighted imaging that are sub-optimal for MPI. Here, we describe new tracers tailored to MPI's unique physics, synthesized using an organic-phase process and functionalized to ensure biocompatibility and adequate in vivo circulation time. Tailored tracers showed up to 3 × greater signal-to-noise ratio and better spatial resolution than existing commercial tracers in MPI images of phantoms.

On the formulation of the image reconstruction problem in magnetic particle imaging.

In magnetic particle imaging (MPI), the spatial distribution of magnetic nanoparticles is determined by applying various static and dynamic magnetic fields. Due to the complex physical behavior of the nanoparticles, it is challenging to determine the MPI system matrix in practice. Since the first publication on MPI in 2005, different methods that rely on measurements or simulations for the determination of the MPI system matrix have been proposed. Some methods restrict the simulation to an idealized model to speed up data reconstruction by exploiting the structure of an idealized MPI system matrix. Recently, a method that processes the measurement data in x-space rather than frequency space has been proposed. In this work, we compare the different approaches for image reconstruction in MPI and show that the x-space and the frequency space reconstruction techniques are equivalent.

Perspectives on clinical magnetic particle imaging.

After realizing the worlds' first preclinical magnetic particle imaging (MPI) demonstrator, Philips is now realizing the worlds' first whole-body clinical prototype to prove the feasibility of MPI for clinical imaging. After a brief introduction of the basic MPI imaging process, this contribution presents an overview on the determining factors for key properties, i.e., spatial resolution, acquisition speed, sensitivity, and quantitativeness, and how these properties are influenced by scaling up from preclinical to clinical instrumentation. Furthermore, it is discussed how this scale up affects the physiological compatibility of the method as well as hardware parameters such as power requirements for drive field generation, selection and focus field generation, and the design of the receive chain of the MPI device.

Red blood cells as carriers in magnetic particle imaging.

Red blood cells (RBCs) represent intravascular carriers for drugs, biologics, and other therapeutic agents, characterized by their unique longevity in the bloodstream, availability, considerable surface and volume, high biocompatibility, and natural mechanisms for safe elimination. Recently, the potential of RBCs loaded with superparamagnetic iron oxide (SPIO) nanoparticles as a tracer material for magnetic particle imaging (MPI) to realize a blood-pool tracer agent with longer blood retention time for imaging of the circulatory system, has been investigated. MPI is a new tomographic imaging approach that can quantitatively map magnetic nanoparticle distributions in vivo. However, SPIO contrast agents, such as Resovist, have a short blood half-life due to rapid uptake by the reticuloendothelial system, which limits the applicability of such compounds for certain applications such as long-term monitoring. Here, we report the in vitro magnetic characterization study of human SPIO-loaded RBCs and the first MPI results obtained after intravenous injection of murine SPIO-loaded RBCs in an in vivo MPI experiment.

Micro-magnetic simulation study on the magnetic particle imaging performance of anisotropic mono-domain particles.

The performance of magnetic mono-domain particles is of crucial importance in magnetic particle imaging (MPI). So far, the behavior of mono-domain particles has been modeled within the framework of Langevin theory. This theory predicts the dependence of the MPI signal on the particle core size, but cannot account for the influence of the shape, i.e. the anisotropy of the particle core. In this study we present the first micro-magnetic ab initio simulation of spectra of anisotropic particles with different core diameters in an oscillating magnetic field at 25 and 100 kHz. We find that the MPI signal strongly depends on the anisotropy of the magnetic core. Thus, a difference of 3 nm between the principal axes of a prolate ellipsoid with the volume of a 30 nm sphere can result in a complete loss of the MPI signal. Smaller anisotropies, however, can increase the MPI performance of the particle. The simulations show that the effect of the anisotropy on the MPI signal depends on the frequency of the oscillating magnetic field. At 100 kHz, the optimal signal is found at smaller anisotropies than at 25 kHz. Furthermore, the simulations show that experimental spectroscopic results for Resovist® can only be explained quantitatively by particles with a magnetic core size of at least 25 nm.

Magnetic particle imaging: visualization of instruments for cardiovascular intervention.

PURPOSE: To evaluate the feasibility of different approaches of instrument visualization for cardiovascular interventions guided by using magnetic particle imaging (MPI). MATERIALS AND METHODS: Two balloon (percutaneous transluminal angioplasty) catheters were used. The balloon was filled either with diluted superparamagnetic iron oxide (SPIO) ferucarbotran (25 mmol of iron per liter) or with sodium chloride. Both catheters were inserted into a vessel phantom that was filled oppositional to the balloon content with sodium chloride or diluted SPIO (25 mmol of iron per liter). In addition, the administration of a 1.4-mL bolus of pure SPIO (500 mmol of iron per liter) followed by 5 mL of sodium chloride through a SPIO-labeled balloon catheter into the sodium chloride-filled vessel phantom was recorded. Images were recorded by using a preclinical MPI demonstrator. All images were acquired by using a field of view of 3.6 × 3.6 × 2.0 cm. RESULTS: By using MPI, both balloon catheters could be visualized with high temporal (21.54 msec per image) and sufficient spatial (≤ 3 mm) resolution without any motion artifacts. The movement through the field of view, the inflation and deflation of the balloon, and the application of the SPIO bolus were visualized at a rate of 46 three-dimensional data sets per second. CONCLUSION: Visualization of SPIO-labeled instruments for cardiovascular intervention at high temporal resolution as well as monitoring the application of a SPIO-based tracer by using labeled instruments is feasible. Further work is necessary to evaluate different labeling approaches for diagnostic catheters and guidewires and to demonstrate their navigation in the vascular system after administration of contrast material. SUPPLEMENTAL MATERIAL: http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.12120424/-/DC1.

Fundamentals and applications of magnetic particle imaging.

Magnetic particle imaging (MPI) is a new medical imaging technique which performs a direct measurement of magnetic nanoparticles, also known as superparamagnetic iron oxide. MPI can acquire quantitative images of the local distribution of the magnetic material with high spatial and temporal resolution. Its sensitivity is well above that of other methods used for the detection and quantification of magnetic materials, for example, magnetic resonance imaging. On the basis of an intravenous injection of magnetic particles, MPI has the potential to play an important role in medical application areas such as cardiovascular, oncology, and also in exploratory fields such as cell labeling and tracking. Here, we present an introduction to the basic function principle of MPI, together with an estimation of the spatial resolution and the detection limit. Furthermore, the above-mentioned medical applications are discussed with respect to an applicability of MPI.

Analysis of a 3-D system function measured for magnetic particle imaging.

Magnetic particle imaging (MPI) is a new tomographic imaging approach that can quantitatively map magnetic nanoparticle distributions in vivo. It is capable of volumetric real-time imaging at particle concentrations low enough to enable clinical applications. For image reconstruction in 3-D MPI, a system function (SF) is used, which describes the relation between the acquired MPI signal and the spatial origin of the signal. The SF depends on the instrumental configuration, the applied field sequence, and the magnetic particle characteristics. Its properties reflect the quality of the spatial encoding process. This work presents a detailed analysis of a measured SF to give experimental evidence that 3-D MPI encodes information using a set of 3-D spatial patterns or basis functions that is stored in the SF. This resembles filling 3-D k-space in magnetic resonance imaging, but is faster since all information is gathered simultaneously over a broad acquisition bandwidth. A frequency domain analysis shows that the finest structures that can be encoded with the presented SF are as small as 0.6 mm. SF simulations are performed to demonstrate that larger particle cores extend the set of basis functions towards higher resolution and that the experimentally observed spatial patterns require the existence of particles with core sizes of about 30 nm in the calibration sample. A simple formula is presented that qualitatively describes the basis functions to be expected at a certain frequency.

Theory, simulation and experimental results of the acoustic detection of magnetization changes in superparamagnetic iron oxide.

BACKGROUND: Magnetic Particle Imaging is a novel method for medical imaging. It can be used to measure the local concentration of a tracer material based on iron oxide nanoparticles. While the resulting images show the distribution of the tracer material in phantoms or anatomic structures of subjects under examination, no information about the tissue is being acquired. To expand Magnetic Particle Imaging into the detection of soft tissue properties, a new method is proposed, which detects acoustic emissions caused by magnetization changes in superparamagnetic iron oxide. METHODS: Starting from an introduction to the theory of acoustically detected Magnetic Particle Imaging, a comparison to magnetically detected Magnetic Particle Imaging is presented. Furthermore, an experimental setup for the detection of acoustic emissions is described, which consists of the necessary field generating components, i.e. coils and permanent magnets, as well as a calibrated microphone to perform the detection. RESULTS: The estimated detection limit of acoustic Magnetic Particle Imaging is comparable to the detection limit of magnetic resonance imaging for iron oxide nanoparticles, whereas both are inferior to the theoretical detection limit for magnetically detected Magnetic Particle Imaging. Sufficient data was acquired to perform a comparison to the simulated data. The experimental results are in agreement with the simulations. The remaining differences can be well explained. CONCLUSIONS: It was possible to demonstrate the detection of acoustic emissions of magnetic tracer materials in Magnetic Particle Imaging. The processing of acoustic emission in addition to the tracer distribution acquired by magnetic detection might allow for the extraction of mechanical tissue parameters. Such parameters, like for example the velocity of sound and the attenuation caused by the tissue, might also be used to support and improve ultrasound imaging. However, the method can also be used to perform imaging on its own.

MPI Cell Tracking: What Can We Learn from MRI?

Magnetic resonance imaging (MRI) cell tracking has become an important non-invasive technique to interrogate the fate of cells upon transplantation. At least 6 clinical trials have been published at the end of 2010, all of which have shown that real-time monitoring of the injection procedure, initial engraftment, and short-term biodistribution of cells is critical to further advance the field of cellular therapeutics. In MRI cell tracking, cells are loaded with superparamagnetic iron oxide (SPIO) particles that provide an MRI contrast effect through microscopic magnetic field disturbances and dephasing of protons. Magnetic particle imaging (MPI) has recently emerged as a potential cellular imaging technique that promises to have several advantages over MRI, primarily linear quantification of cells, a higher sensitivity, and "hot spot" tracer identification without confounding background signal. Although probably not fully optimized, SPIO particles that are currently used as MRI contrast agent can be employed as MPI tracer. Initial studies have shown that cells loaded with SPIO particles can give a detectable MPI signal, encouraging further development of MPI cell tracking.

2D model-based reconstruction for magnetic particle imaging.

PURPOSE: Magnetic particle imaging (MPI) is a new quantitative imaging technique capable of determining the spatial distribution of superparamagnetic nanoparticles at high temporal and spatial resolution. For reconstructing this spatial distribution, the particle dynamics and the scanner properties have to be known. To date, they are obtained in a tedious calibration procedure by measuring the magnetization response of a small delta sample shifted through the measuring field. Recently, first reconstruction results using a 1D model-based system function were published, showing comparable image quality as obtained with a measured system function. In this work, first 2D model-based reconstruction results of measured MPI data are presented. METHODS: To simulate the system function, various parameters have to be modeled, namely, the magnetic field, the particle magnetization, the voltage induced in the receive coils, and the transfer function of the receive chain. To study the accuracy of the model-based approach, 2D MPI data are measured and reconstructed with modeled and measured system functions. RESULTS: It is found that the model-based system function is sufficiently accurate to allow for reconstructing experimental data. The resulting image quality is close to that obtained with a measurement-based reconstruction. CONCLUSIONS: The model-based system function approach addresses a major drawback of the measurement-based procedure, namely, the long acquisition time. In this work, the acquisition of the measurement-based system function took 45 min, while the model-based system function was obtained in only 15 s. For 3D data, where the acquisition of the measurement-based system function takes more than 6 h, the need for an efficient system function generation is even more obvious.

Model-based reconstruction for magnetic particle imaging.

Magnetic particle imaging (MPI) is a new imaging modality capable of imaging distributions of superparamagnetic nanoparticles with high sensitivity, high spatial resolution and, in particular, high imaging speed. The image reconstruction process requires a system function, describing the mapping between particle distribution and acquired signal. To date, the system function is acquired in a tedious calibration procedure by sequentially measuring the signal of a delta sample at the positions of a grid that covers the field of view. In this work, for the first time, the system function is calculated using a model of the signal chain. The modeled system function allows for reconstruction of the particle distribution in a 1-D MPI experiment. The approach thus enables fast generation of system functions on arbitrarily dense grids. Furthermore, reduction in memory requirements may be feasible by generating parts of the system function on the fly during reconstruction instead of keeping the complete matrix in memory.

Signal encoding in magnetic particle imaging: properties of the system function.

BACKGROUND: Magnetic particle imaging (MPI) is a new tomographic imaging technique capable of imaging magnetic tracer material at high temporal and spatial resolution. Image reconstruction requires solving a system of linear equations, which is characterized by a "system function" that establishes the relation between spatial tracer position and frequency response. This paper for the first time reports on the structure and properties of the MPI system function. METHODS: An analytical derivation of the 1D MPI system function exhibits its explicit dependence on encoding field parameters and tracer properties. Simulations are used to derive properties of the 2D and 3D system function. RESULTS: It is found that for ideal tracer particles in a harmonic excitation field and constant selection field gradient, the 1D system function can be represented by Chebyshev polynomials of the second kind. Exact 1D image reconstruction can thus be performed using the Chebyshev transform. More realistic particle magnetization curves can be treated as a convolution of the derivative of the magnetization curve with the Chebyshev functions. For 2D and 3D imaging, it is found that Lissajous excitation trajectories lead to system functions that are closely related to tensor products of Chebyshev functions. CONCLUSION: Since to date, the MPI system function has to be measured in time-consuming calibration scans, the additional information derived here can be used to reduce the amount of information to be acquired experimentally and can hence speed up system function acquisition. Furthermore, redundancies found in the system function can be removed to arrive at sparser representations that reduce memory load and allow faster image reconstruction.

Electromagnetic tracking for thermal ablation and biopsy guidance: clinical evaluation of spatial accuracy.

PURPOSE: To evaluate the spatial accuracy of electromagnetic needle tracking and demonstrate the feasibility of ultrasonography (US)-computed tomography (CT) fusion during CT- and US-guided biopsy and radiofrequency ablation procedures. MATERIALS AND METHODS: The authors performed a 20-patient clinical trial to investigate electromagnetic needle tracking during interventional procedures. The study was approved by the institutional investigational review board, and written informed consent was obtained from all patients. Needles were positioned by using CT and US guidance. A commercial electromagnetic tracking device was used in combination with prototype internally tracked needles and custom software to record needle positions relative to previously obtained CT scans. Position tracking data were acquired to evaluate the tracking error, defined as the difference between tracked needle position and reference standard needle position on verification CT scans. Registration between tracking space and image space was obtained by using reference markers attached to the skin ("fiducials"), and different registration methods were compared. The US transducer was tracked to demonstrate the potential use of real-time US-CT fusion for imaging guidance. RESULTS: One patient was excluded from analysis because he was unable to follow breathing instructions during the acquisition of CT scans. Nineteen of the 20 patients were evaluable, demonstrating a basic tracking error of 5.8 mm +/- 2.6, which improved to 3.5 mm +/- 1.9 with use of nonrigid registrations that used previous internal needle positions as additional fiducials. Fusion of tracked US with CT was successful. Patient motion and distortion of the tracking system by the CT table and gantry were identified as sources of error. CONCLUSIONS: The demonstrated spatial tracking accuracy is sufficient to display clinically relevant preprocedural imaging information during needle-based procedures. Virtual needles displayed within preprocedural images may be helpful for clandestine targets such as arterial phase enhancing liver lesions or during thermal ablations when obscuring gas is released. Electromagnetic tracking may help improve imaging guidance for interventional procedures and warrants further investigation, especially for procedures in which the outcomes are dependent on accuracy.