Jochen Franke

MRI-Based Attenuation Correction for Hybrid PET/MRI Systems: A 4-Class Tissue Segmentation Technique Using a Combined Ultrashort-Echo-Time/Dixon MRI Sequence

Accurate γ-photon attenuation correction (AC) is essential for quantitative PET/MRI as there is no simple relation between MR image intensity and attenuation coefficients. Attenuation maps (μ-maps) can be derived by segmenting MR images and assigning attenuation coefficients to the compartments. Ultrashort-echo-time (UTE) sequences have been used to separate cortical bone and air, and the Dixon technique has enabled differentiation between soft and adipose tissues. Unfortunately, sequential application of these sequences is time-consuming and complicates image registration. Methods: A UTE triple-echo (UTILE) MRI sequence is proposed, combining UTE sampling for bone detection and gradient echoes for Dixon water–fat separation in a radial 3-dimensional acquisition (repetition time, 4.1 ms; echo times, 0.09/1.09/2.09 ms; field strength, 3 T). Air masks are derived mainly from the phase information of the first echo; cortical bone is segmented using a dual-echo technique. Soft-tissue and adipose-tissue decomposition is achieved using a 3-point Dixon-like decomposition. Predefined linear attenuation coefficients are assigned to classified voxels to generate MRI-based μ-maps. The results of 6 patients are obtained by comparing μ-maps, reciprocal sensitivity maps, reconstructed PET images, and brain region PET activities based on either CT AC, two 3-class MRI AC techniques, or the proposed 4-class UTILE AC. Results: Using the UTILE MRI sequence, an acquisition time of 214 s was achieved for the head-and-neck region with 1.75-mm isotropic resolution, compared with 164 s for a single-echo UTE scan. MRI-based reciprocal sensitivity maps show a high correlation with those derived from CT scans (R2 = 0.9920). The same is true for PET activities (R2 = 0.9958). An overall voxel classification accuracy (compared with CT) of 81.1% was reached. Bone segmentation is inaccurate in complex regions such as the paranasal sinuses, but brain region activities in 48 regions across 6 patients show a high correlation after MRI-based and CT-based correction (R2 = 0.9956), with a regression line slope of 0.960. All overall correlations are higher and brain region PET activities more accurate in terms of mean and maximum deviations for the 4-class technique than for 3-class techniques. Conclusion: The UTILE MRI sequence enables the generation of MRI-based 4-class μ-maps without anatomic priors, yielding results more similar to CT-based results than can be obtained with 3-class segmentation only.

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

Abstract 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.

First hybrid MPI-MRI imaging system as integrated design for mice and rats: Description of the instrumentation setup

This hybrid MPI-MRI imaging system (see Fig. 1 RIGHT) is currently under assembly at the research labs of Bruker BioSpin MRI GmbH, whereas single components were already tested. The magnetic field strength and the magnetic field gradient were measured to be up to _Bz=0.495 T and up to G_z=2.2 T/m for the PF and the SF, respectively (see Fig. 1 LEFT). The MRI gradient strengths were determined to be G_x=0.84 mT/A/m, G_y=0.9 mT/A/m, G_z=0.87 mT/A/m (I_max=300 A) and the MRI RF coil was tuned to the Larmor frequency of protons at 0.495 T. The DF resonant circuits were tuned to excitation frequencies of around 25 kHz. With expected maximal DF amplitude of 20 mT and the strongest SF gradient strength a field of view of 18×36×36 mm³ could be reached.

System Characterization of a Highly Integrated Preclinical Hybrid MPI-MRI Scanner

Magnetic particle imaging (MPI) is a novel tracer-based in vivo imaging modality allowing quantitative measurements of the spatial distributions of superparamagnetic iron oxide (SPIO) nanoparticles in three dimensions (3D) and in real time using electromagnetic fields. However, MPI lacks the detection of morphological information which makes it difficult to unambiguously assign spatial SPIO distributions to actual organ structures. To compensate for this, a preclinical highly integrated hybrid system combining MPI and Magnetic Resonance Imaging (MRI) has been designed and gets characterized in this work. This hybrid MPI-MRI system offers a high grade of integration with respect to its hard- and software and enables sequential measurements of MPI and MRI within one seamless study and without the need for object repositioning. Therefore, time-resolved measurements of SPIO distributions acquired with MPI as well as morphological and functional information acquired with MRI can be combined with high spatial co-registration accuracy. With this initial phantom study, the feasibility of a highly integrated MPI-MRI hybrid systems has been proven successfully. This will enable dual-modal in vivo preclinical investigations of mice and rats with high confidence of success, offering the unique feature of precise MPI FOV planning on the basis of MRI data and vice versa.

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 hightemporal resolution and a simple catheter/guide wire maneuver was demonstrated.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.

First 3D dual modality phantom measurements of a hybrid MPI-MRI system using a resistive 12 channel MPI-MRI magnet design

Despite the capability of Magnetic Particle Imaging (MPI) systems [1] to measure spatial distributions of superparamagnetic iron-oxide (SPIO) tracer particles quantitatively with very high temporal resolution, MPI data lack morphological and functional information. To compensate for this, in 2013 combinations of MPI and Magnetic Resonance Imaging (MRI) were proposed as preclinical hybrid MPI-MRI system with integrated dual-operation magnet coils [2] [3]. An alternative approach uses an Travelling Wave MPI insert for a stand-alone pre-polarized low field MRI system [4]. Image fusion of morphological/functional MRI data with complementary MPI data facilitates distinct insight in the pathways and distribution of the administered SPIOs. Hybrid data fusion with high spatial confidence is a basic prerequisite for accurate analysis of in vivo studies. This precision can be achieved optimally only with a hybrid scanner approach featuring integrated dual-operation magnet coils which omits any subject repositioning between MPI and MRI scans.

ECG signal acquisition within a continuous LF powered region

Neither unwanted MPI signals due to the presence of ECG electrode material within the sensitive Tx/Rx DF nor ECG signal perturbations caused by the 25 kHz DF excitation field have been detected for either electrode type. However, in case of electrode type [A] the maximum observed electrode temperature was 48.8 °C (c.f. Fig 1 LEFT) whereas a temperature of more than 115 °C (c.f. Fig. 1 RIGHT) has been detected in case of electrode type [B].

Alternative hybrid MPI-MRI imaging system design: Superconductive field generator topology

It was shown that a hybrid MPI-MRI MFG can be realized as superconductive topology which renders the possibility of acquiring sequentially high resolution MPI and high quality MRI data with versatile soft tissue contrast of mice and rats in a single device. Furthermore, depending on the technically feasible and biologically tolerable drive field strength and/or desired field of view, the superconductive MFG can be realized in a broad range in terms of the SF gradient strength independently to the magnetic field strength of the MRI region. This can be realized by selecting a dedicated gradient enhancing coil current density. As this MFG topology depicts two separated imaging regions for MPI and MRI, respectively, the subject hast to be transported by a simple translational movement whereas the signal detector of each modality can be located closely to the imaging volume which allows in both modalities high sensitivity receiving coils.

Generic multi-purpose multi-modality phantom kit design

Phantoms are well-established tools for the characterization of imaging systems with regard to resolution, sensitivity, geometric distortions, or flow detection [1]. For the MPI community, the availability of standard phantoms is highly desirable to facilitate direct comparisons between the different MPI scanner designs that have emerged since the inception of this novel imaging modality [2-7]. To be stable over longer times, such phantoms require a sealed containment for liquid contrast agents to prevent leakage or solvent evaporation. As MPI typically requires a second modality for providing morphological reference, it is desirable to use phantoms that also exhibit a good contrast in other modalities such as MRI or μCT. As the spatial resolution of MPI scanners is typically anisotropic, it is desirable to allow different phantom alignments. In this study, we evaluate our new generic kit for building compact phantoms that meet the above requirements.

SPIO-coating of devices for MPI-guided cardiovascular interventions: Proof of principle

Initial studies have demonstrated that cardiovascular interventions should be feasible under MPI guidance with high temporal and sufficient spatial resolution. However, the interventional devices need to be visualized and must be safe. It is possible to visualize balloon-catheters for dilatation of vascular stenosis by filling their second lumen with a Resovist suspension[1], but this is not possible for diagnostic catheters and guide wires, as they possess only one or no lumen, respectively. The aim of this study was to develop a SPIO-based coating for visualization of Nitinol-based guide wires and diagnostic catheters, which have shown neither heating nor the occurrence of artifacts in MPI systems[2, 3].