Bare-Metal Stent Tracking with Magnetic Particle Imaging.

Purpose: Magnetic particle imaging (MPI) is an emerging medical imaging modality that is on the verge of clinical use. In recent years, cardiovascular applications have shown huge potential like, e.g., intraprocedural imaging guidance of stent placement through MPI. Due to the lack of signal generation, nano-modifications have been necessary to visualize commercial medical instruments until now. In this work, it is investigated if commercial interventional devices can be tracked with MPI without any nano-modification. Material and Methods: Potential MPI signal generation of nine endovascular metal stents was tested in a commercial MPI scanner. Two of the stents revealed sufficient MPI signal. Because one of the two stents showed relevant heating, the imaging experiments were carried out with a single stent model (Boston Scientific/Wallstent-Uni Endoprothesis, diameter: 16 mm, length: 60 mm). The nitinol stent and its delivery system were investigated in seven different scenarios. Therefore, the samples were placed at 49 defined spatial positions by a robot in a meandering pattern during MPI scans. Image reconstruction was performed, and the mean absolute errors (MAE) between the signals' centers of mass (COM) and ground truth positions were calculated. The stent material was investigated by magnetic particle spectroscopy (MPS) and vibrating sample magnetometry (VSM). To detect metallic components within the delivery system, nondestructive testing via computed tomography was performed. Results: The tracking of the stent and its delivery system was possible without any nano-modification. The MAE of the COM were 1.49 mm for the stent mounted on the delivery system, 3.70 mm for the expanded stent and 1.46 mm for the delivery system without the stent. The results of the MPS and VSM measurements indicate that besides material properties eddy currents seem to be responsible for signal generation. Conclusion: It is possible to image medical instruments with dedicated designs without modifications by means of MPI. This enables a variety of applications without compromising the mechanical and biocompatible properties of the instruments.

First Dedicated Balloon Catheter for Magnetic Particle Imaging.

Vascular interventions are a promising application of Magnetic Particle Imaging enabling a high spatial and temporal resolution without using ionizing radiation. The possibility to visualize the vessels as well as the devices, especially at the same time using multi-contrast approaches, enables a higher accuracy for diagnosis and treatment of vascular diseases. Different techniques to make devices MPI visible have been introduced so far, such as varnish markings or filling of balloons. However, all approaches include challenges for in vivo applications, such as the stability of the varnishing or the visibility of tracer filled balloons in deflated state. In this contribution, we present for the first time a balloon catheter that is molded from a granulate incorporating nanoparticles and can be visualized sufficiently in MPI. Computed tomography is used to show the homogeneous distribution of particles within the material. Safety measurements confirm that the incorporation of nanoparticles has no negative effect on the balloon. A dynamic experiment is performed to show that the inflation as well as deflation of the balloon can be imaged with MPI.

Impact of different respiratory monitoring techniques on respiration-dependent stroke-volume measurements assessed by real-time magnetic resonance imaging.

PURPOSE: Quantitative blood flow measurements in thoracic vessels are dependent on the patient's respiration due to intrathoracic pressure changes. Several registration techniques for tracing the patient's breathing curve exist exploiting various physiological characteristics. In the presented study two registration techniques were investigated to estimate the time-shift between the associated respiration curves and the impact on respiration-dependent hemodynamic measures. MATERIALS AND METHODS: Using flow-sensitive real-time magnetic resonance imaging (3.0T, temporal resolution=24-26ms) data were acquired during a 13s time-interval under normal physiological (no) and forced breathing (fo) in the ascending aorta (AAo) and inferior vena cava (IVC). Breathing curves were obtained by using (1) an abdominal placed respiratory belt (=standard, RB) and (2) by applying a self-developed edge-detection software (ED). Respiration curves were divided into four intervals (end-expiration, inspiration, end-inspiration and expiration) to generate respiration-dependent stroke volumes (SVs) and cardiac indices (CIs). Data were available from 12 healthy controls (16.2±8.8yrs) and 18 Fontan-patients (18.6±7.1yrs). RESULTS: Respiration curves acquired with RB differs from and are shifted compared to those obtained by ED (controls: average shift=207±168ms, Fontan-patients: average shift=106±235ms). This time-shift results in statistical significant differences in cardiac indices CIAAo,fo in controls (ΔCIAAo,fo: expiration: +0.16L/min/m2, p=0.018), in CIIVC,no in Fontan-patients (ΔCIIVC,no: end-expiration: +0.94L/min/m2, p=0.002 and end-inspiration: -1.04L/min/m2, p=0.017) and in CIIVC,fo in Fontan-patients (ΔCIIVC,fo: end-expiration: +1.31L/min/m2, p=0.009; inspiration: -2.26L/min/m2, p=0.008 and end-inspiration: -1.87L/min/m2, p=0.029). CONCLUSIONS: A time-shift between both applied respiratory tracking techniques was observed resulting in significant differences in respiration-dependent CIs, which could influence the clinical decision-making.