Florian M. Vogt

MR-based attenuation correction for torso-PET/MR imaging: pitfalls in mapping MR to CT data

PurposeMR-based attenuation correction (AC) will become an integral part of combined PET/MR systems. Here, we propose a toolbox to validate MR-AC of clinical PET/MRI data sets.MethodsTorso scans of ten patients were acquired on a combined PET/CT and on a 1.5-T MRI system. MR-based attenuation data were derived from the CT following MR–CT image co-registration and subsequent histogram matching. PET images were reconstructed after CT- (PETCT) and MR-based AC (PETMRI). Lesion-to-background (L/B) ratios were estimated on PETCT and PETMRI.ResultsMR–CT histogram matching leads to a mean voxel intensity difference in the CT- and MR-based attenuation images of 12% (max). Mean differences between PETMRI and PETCT were 19% (max). L/B ratios were similar except for the lung where local misregistration and intensity transformation leads to a biased PETMRI.ConclusionOur toolbox can be used to study pitfalls in MR-AC. We found that co-registration accuracy and pixel value transformation determine the accuracy of PETMRI.

Parallel acquisition techniques for accelerated volumetric interpolated breath-hold examination magnetic resonance imaging of the upper abdomen: assessment of image quality and lesion conspicuity.

To evaluate the impact of parallel acquisition techniques (PATs) on image quality and detection of liver metastases using three‐dimensional volumetric interpolated breath‐hold examination (VIBE) for clinical liver imaging.

Magnetic particle imaging: current developments and future directions

Magnetic particle imaging (MPI) is a novel imaging method that was first proposed by Gleich and Weizenecker in 2005. Applying static and dynamic magnetic fields, MPI exploits the unique characteristics of superparamagnetic iron oxide nanoparticles (SPIONs). The SPIONs’ response allows a three-dimensional visualization of their distribution in space with a superb contrast, a very high temporal and good spatial resolution. Essentially, it is the SPIONs’ superparamagnetic characteristics, the fact that they are magnetically saturable, and the harmonic composition of the SPIONs’ response that make MPI possible at all. As SPIONs are the essential element of MPI, the development of customized nanoparticles is pursued with the greatest effort by many groups. Their objective is the creation of a SPION or a conglomerate of particles that will feature a much higher MPI performance than nanoparticles currently available commercially. A particle’s MPI performance and suitability is characterized by parameters such as the strength of its MPI signal, its biocompatibility, or its pharmacokinetics. Some of the most important adjuster bolts to tune them are the particles’ iron core and hydrodynamic diameter, their anisotropy, the composition of the particles’ suspension, and their coating. As a three-dimensional, real-time imaging modality that is free of ionizing radiation, MPI appears ideally suited for applications such as vascular imaging and interventions as well as cellular and targeted imaging. A number of different theories and technical approaches on the way to the actual implementation of the basic concept of MPI have been seen in the last few years. Research groups around the world are working on different scanner geometries, from closed bore systems to single-sided scanners, and use reconstruction methods that are either based on actual calibration measurements or on theoretical models. This review aims at giving an overview of current developments and future directions in MPI about a decade after its first appearance.

Magnetic resonance imaging of experimental atherosclerotic plaque: Comparison of two ultrasmall superparamagnetic particles of iron oxide

To evaluate a new ultrasmall superparamagnetic particles of iron oxide (USPIO) compound, ferumoxytol, as a marker of macrophage activity in atherosclerotic plaques and to compare it to ferumoxtran‐10.

Using a 1 M Gd-chelate (gadobutrol) for total-body three-dimensional MR angiography: Preliminary experience

To determine whether higher concentrated gadolinium chelates are advantageous for the recently introduced concept of total‐body magnetic resonance angiography (MRA), allowing whole‐body coverage, extending from the carotid arteries to the runoff vessels, in merely 72 seconds.

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.

High‐resolution continuously acquired peripheral MR angiography featuring partial parallel imaging GRAPPA

Continuously‐moving‐table MRI, in contrast to traditional multistation techniques, potentially can improve the scan time efficiency of whole‐body applications and provide seamless images of an extended field of view (FOV). Contrast‐enhanced MR angiography (CE‐MRA) in particular requires high spatial resolution and at the same time has rigid scan time constraints due to the limited arterial contrast window. In this study a reconstruction method for continuously acquired 3D data sets during table movement was combined with a self‐calibrated partial parallel imaging algorithm (generalized autocalibrating partially parallel acquisitions (GRAPPA)). The method was applied to peripheral CE‐MRA and compared with a standard continuously‐moving‐table MRA protocol. The gain in scan time was used to increase the data acquisition matrix and decrease the slice thickness. The method was evaluated in five healthy volunteers and applied to one patient with peripheral arterial occlusive disease (PAOD). The protocols were intraindividually compared with respect to the signal‐to‐noise ratio (SNR) and contrast‐to‐noise ratio (CNR) in selected vessel segments, as well as overall vessel depiction. The combination of the continuously‐moving‐table technique with parallel imaging enabled the acquisition of seamless peripheral 3D MRA with increased resolution and an overall crisper appearance. Magn Reson Med, 2006.

Myocardial Mass and Volume Measurement of Hypertrophic Left Ventricles by MRI—Study in Dialysis Patients Examined Before and After Dialysis

Techniques to reliably quantify left ventricular myocardial mass (LVMM) are mandatory for monitoring therapy in patients with left ventricular hypertrophy (LVH). The purpose of this study was to measure LVMM and volumes by cine magnetic resonance imaging (MRI), and to assess acute changes through hemodialysis as a model for different loading states. Seven dialysis patients with LVH were examined before and immediately after hemodialysis. All MR imaging was done with a steady-state free precession (SSFP) cine sequence (TrueFISP; TR, 3.2 ms; TE, 1.6 ms; flip angle, 60 degrees; slice thickness, 8 mm). LV volumes, ejection fraction (EF), and LVMM were determined by slice summation after manual planimetry in short axes. A significant reduction of end-diastolic volume (EDV) (mean pre, 140 mL; post, 109 mL; p < 0.01), end-systolic volume (ESV) (49 mL-->42 mL; p < 0.05), and stroke volume (91 mL-->66 mL; p < 0.01) through dialysis was revealed by MRI. Ejection fraction did not change significantly. A slight decrease in LVMM was detected in all patients (mean pre, 184 g; post, 177 g; p < 0.05). Intra- and interobserver variability for EDV, ESV, and LVMM were 1.3 +/- 6.2 mL, -0.9 +/- 4.1 mL, -1.4 +/- 3.9 g, and 3.3 +/- 7.5 mL, 2.6 +/- 5.0 mL, -2.4 +/- 4.6 g, respectively. Standard error of estimation (SEE) was +/- 2.3 mL, +/- 2.0 mL, +/- 1.6 g, and +/- 2.6 mL, +/- 2.1 mL, and +/- 2.0 g for intra- and interobserver variability. In conclusion, cine MRI is a reliable technique for LVMM measurement that is independent of LV loading status. This method allows for detection of small changes, which is crucial for accurate therapy monitoring in LVH. Left ventricular myocardial mass and volumes decrease significantly during hemodialysis.

Magnetic particle imaging: Introduction to imaging and hardware realization

Magnetic Particle Imaging (MPI) is a recently invented tomographic imaging method that quantitatively measures the spatial distribution of a tracer based on magnetic nanoparticles. The new modality promises a high sensitivity and high spatial as well as temporal resolution. There is a high potential of MPI to improve interventional and image-guided surgical procedures because, today, established medical imaging modalities typically excel in only one or two of these important imaging properties. MPI makes use of the non-linear magnetization characteristics of the magnetic nanoparticles. For this purpose, two magnetic fields are created and superimposed, a static selection field and an oscillatory drive field. If superparamagnetic iron-oxide nanoparticles (SPIOs) are subjected to the oscillatory magnetic field, the particles will react with a non-linear magnetization response, which can be measured with an appropriate pick-up coil arrangement. Due to the non-linearity of the particle magnetization, the received signal consists of the fundamental excitation frequency as well as of harmonics. After separation of the fundamental signal, the nanoparticle concentration can be reconstructed quantitatively based on the harmonics. The spatial coding is realized with the static selection field that produces a field-free point, which is moved through the field of view by the drive fields. This article focuses on the frequency-based image reconstruction approach and the corresponding imaging devices while alternative concepts like x-space MPI and field-free line imaging are described as well. The status quo in hardware realization is summarized in an overview of MPI scanners.

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.