Experience with magnetic resonance imaging of human subjects with passive implants and tattoos at 7 T: a retrospective study.

OBJECT: Over the last decade, the number of clinical MRI studies at 7 T has increased dramatically. Since only limited information about the safety of implants/tattoos is available at 7 T, many centers either conservatively exclude all subjects with implants/tattoos or have started to perform dedicated tests for selected implants. This work presents our experience in imaging volunteers with implants/tattoos at 7 T over the last seven and a half years. MATERIALS AND METHODS: 1796 questionnaires were analyzed retrospectively to identify subjects with implants/tattoos imaged at 7 T. For a total of 230 subjects, the type of local transmit/receive RF coil used for examination, imaging sequences, acquisition time, and the type of implants/tattoos and their location with respect to the field of view were documented. These subjects had undergone examination after careful consideration by an internal safety panel consisting of three experts in MR safety and physics. RESULTS: None of the subjects reported sensations of heat or force before, during, or after the examination. None expressed any discomfort related to implants/tattoos. Artifacts were reported in 52% of subjects with dental implants; all artifacts were restricted to the mouth area and did not affect image quality in the brain parenchyma. CONCLUSION: Our initial experience at 7 T indicates that a strict rejection of subjects with tattoos and/or implants is not justified. Imaging can be conditionally performed in carefully selected subjects after collection of substantial safety information and evaluation of the detailed exposure scenario (RF coil/type and position of implant). Among the assessed subjects with tattoos, no side effects from the exposure to 7 T MRI were reported.

Towards real-time cardiovascular magnetic resonance-guided transarterial aortic valve implantation: in vitro evaluation and modification of existing devices.

BACKGROUND: Cardiovascular magnetic resonance (CMR) is considered an attractive alternative for guiding transarterial aortic valve implantation (TAVI) featuring unlimited scan plane orientation and unsurpassed soft-tissue contrast with simultaneous device visualization. We sought to evaluate the CMR characteristics of both currently commercially available transcatheter heart valves (Edwards SAPIEN™, Medtronic CoreValve®) including their dedicated delivery devices and of a custom-built, CMR-compatible delivery device for the Medtronic CoreValve® prosthesis as an initial step towards real-time CMR-guided TAVI. METHODS: The devices were systematically examined in phantom models on a 1.5-Tesla scanner using high-resolution T1-weighted 3D FLASH, real-time TrueFISP and flow-sensitive phase-contrast sequences. Images were analyzed for device visualization quality, device-related susceptibility artifacts, and radiofrequency signal shielding. RESULTS: CMR revealed major susceptibility artifacts for the two commercial delivery devices caused by considerable metal braiding and precluding in vivo application. The stainless steel-based Edwards SAPIEN™ prosthesis was also regarded not suitable for CMR-guided TAVI due to susceptibility artifacts exceeding the valve's dimensions and hindering an exact placement. In contrast, the nitinol-based Medtronic CoreValve® prosthesis was excellently visualized with delineation even of small details and, thus, regarded suitable for CMR-guided TAVI, particularly since reengineering of its delivery device toward CMR-compatibility resulted in artifact elimination and excellent visualization during catheter movement and valve deployment on real-time TrueFISP imaging. Reliable flow measurements could be performed for both stent-valves after deployment using phase-contrast sequences. CONCLUSIONS: The present study shows that the Medtronic CoreValve® prosthesis is potentially suited for real-time CMR-guided placement in vivo after suggested design modifications of the delivery system.

Initial in vivo studies with a polymer-based MR-compatible guide wire.

The purpose of this study was to evaluate a new polymer-based and magnetic resonance (MR) imaging-compatible guide wire for its application in MR-guided endovascular interventions, particularly for interventional peripheral MR angiography in swine experiments in vivo. A passive device tracking method tailored to the specific conditions of peripheral MR angiography was developed. Near-real-time visualization of the guide wire was accomplished in vivo in the carotid artery, aorta, heart, and iliac arteries of two domestic pigs. Results show great potential for this guide wire in aiding the realization of interventional peripheral MR angiography in humans.

Temperature and concentration calibration of aqueous polyvinylpyrrolidone (PVP) solutions for isotropic diffusion MRI phantoms.

To use the "apparent diffusion coefficient" (Dapp) as a quantitative imaging parameter, well-suited test fluids are essential. In this study, the previously proposed aqueous solutions of polyvinylpyrrolidone (PVP) were examined and temperature calibrations were obtained. For example, at a temperature of 20°C, Dapp ranged from 1.594 (95% CI: 1.593, 1.595) µm2/ms to 0.3326 (95% CI: 0. 3304, 0.3348) µm2/ms for PVP-concentrations ranging from 10% (w/w) to 50% (w/w) using K30 polymer lengths. The temperature dependence of Dapp was found to be so strong that a negligence seems not advisable. The temperature dependence is descriptively modelled by an exponential function exp(c2 (T - 20°C)) and the determined c2 values are reported, which can be used for temperature calibration. For example, we find the value 0.02952 K-1 for 30% (w/w) PVP-concentration and K30 polymer length. In general, aqueous PVP solutions were found to be suitable to produce easily applicable and reliable Dapp-phantoms.

In vitro evaluation of current thoracic aortic stent-grafts for real-time MR-guided placement.

PURPOSE: To systematically evaluate the magnetic resonance imaging (MRI) characteristics of current thoracic aortic stent-graft devices before, during, and after in vitro deployment as a step toward real-time MRI-guided stent placement. METHODS: Six stent-graft devices used for thoracic aortic repair were examined in a dedicated phantom model using a 1.5-T MRI scanner. First, the delivery systems with the mounted stent-graft were examined using real-time fast imaging with steady-state precession (TrueFISP) with Cartesian and radial k-space filling. TrueFISP imaging was subsequently used for real-time monitoring of stent-graft expansion. The deployed stent-grafts were then examined in a water bath containing gadolinium (1:40) with high-resolution T1-weighted 3D fast low-angle shot (FLASH) sequences. The images were analyzed for artifacts, radiofrequency caging effects, and device visualization quality. RESULTS: Three delivery systems with mounted stent-grafts did not contain ferromagnetic elements and were well visualized. Imaging with radial k-space filling showed fewer artifacts than Cartesian imaging. Movement of the delivery system and stent-graft expansion of these devices were successfully demonstrated at a rate of up to 6 frames per second. Evaluation of the expanded stent-grafts revealed only minor susceptibility artifacts without relevant signal attenuation in the stent-graft lumen for 5 nitinol-based stent-grafts. Only a stainless steel-based stent-graft was associated with severe artifacts, thwarting visualization of its lumen or surroundings. CONCLUSION: The present study shows that 3 nitinol-based thoracic stent-graft devices are potentially suited for real-time MRI-guided placement with respect to both the delivery system and the stent-graft itself. These observations provide the basis for the evaluation of MRI-guided stent-graft placement in vivo.