Steffen Weiss

Transmission line for improved RF safety of interventional devices

A new concept is proposed to improve the safety of transmission lines with respect to heating during RF transmission. It is based on the integration of transformers into the transmission line. The concept was applied to an active tracking device. Miniature transformers were designed, and two types of tracking devices were built based on a standard line and a transformer line. Temperature measurements were performed for both devices during high specific absorption rate (SAR) scanning, and the suppression of RF heating to a physiologically non‐relevant level was demonstrated for the transformer device. The transmission properties of the transformer line were examined in simulations and RF measurements. Active tracking with the transformer device performed robustly in the phantom. Because of the favorable signal transmission properties of the tested device, it is expected that the concept can be applied to the construction of clinical devices for tracking and intravascular imaging, which are RF‐safe under clinical SAR conditions. Since the transformer line has a large bandwidth, the concept may also be applied for RF‐safe transmission of non‐MR signals. Magn Reson Med 54:182–189, 2005.

Projection reconstruction balanced fast field echo for interactive real‐time cardiac imaging

A balanced fast field echo (FFE) sequence (also referred to as true fast imaging with steady precession (true FISP)), based on projection reconstruction (PR) is evaluated in combination with real‐time reconstruction and interactive scanning capabilities for cardiac function studies. Cardiac image sequences obtained with the balanced PR‐FFE method are compared with images obtained with a spin‐warp (2D Fourier transform (2DFT)) technique. In particular, the representation of motion artifacts in both techniques is investigated. Balanced PR‐FFE provides a similar contrast to spin‐warp‐related techniques, but is less sensitive to motion artifacts. The use of angular undersampling within balanced PR‐FFE is examined as a means to increase temporal resolution while causing only minor artifacts. Furthermore, a modification of the profile order allows the reconstruction of PR images at different spatial and temporal resolution levels from the same data. This study shows that balanced PR‐FFE is a robust tool for cardiac function studies. Magn Reson Med 46:1238–1241, 2001.

In vivo safe catheter visualization and slice tracking using an optically detunable resonant marker

The purpose of this study was to test the in vivo feasibility of safe automatic catheter tracking based on an optically detunable resonant marker installed on the catheter tip, and also to test the compatibility of this approach with guidewire materials. The design of the resonant marker and the integration into the real‐time MR environment is described. The catheter was used for real‐time MR‐guided catheterization of the aorta, left ventricle, and carotid in two swine. For in‐plane visualization, the marker was repeatedly detuned. For automatic slice tracking, a projection difference measurement including detuning was interleaved with the imaging sequence. In vitro experiments were conducted to investigate the RF‐safety of the marker and the effect of the guidewires on the signal intensity. For all orientations the marker provided excellent in vivo contrast using a radial steady‐state free‐precession sequence. Flashing of the marker by repetitive tuning/detuning further improved the in‐plane visualization. Automatic slice tracking during real‐time imaging was successfully performed. The plastic guidewires did not interfere with the marker, and detuning by guidewires containing nitinol could be compensated. In conclusion, automatic slice tracking as well as excellent in‐plane visualization can be achieved with this approach and it is safe with respect to RF transmission. Magn Reson Med 52:860–868, 2004.

A safe transmission line for MRI

Magnetic resonance imaging (MRI) has been established as a reliable and safe imaging method for the human body. However, electric conductors, such as cables situated near or in the human body, should be avoided because induced currents in the cables can cause hazardous heating in the surrounding tissue. In this paper, a new principle for the design of a transmission line is introduced and demonstrated, which is capable of avoiding dangerous heating of cables. The principle is based on transformers placed along the line, splitting the long line into several short not resonant and thus safe sections. A transformer design is introduced along with the theoretical aspects for both the avoidance of the undesired induced currents and the reduction of signal attenuation. Furthermore, the design fulfills the geometrical requirements of the side lumen of a standard catheter. Matching networks, whose elements are determined by power matching, are used to reduce signal attenuation by the transformers. A prototype was built to validate both theory and the simulations. As demonstrated in this work, it is possible to build safe transmission lines for MRI, making applications such as active catheter tracking possible. We expect that even new applications, such as safe intravascular imaging will be possible in a safe manner in the future.

An MR guidewire based on micropultruded fiber-reinforced material

A novel fiber‐reinforced material for the realization of MR guidewires, made using a newly‐developed production process, is presented. The MR‐safe artificial material provides a high stiffness and torque and allows the production, in a large range of sizes, of nonmetallic MR guidewires with similar mechanical properties as conventional metallic guidewires. Based on this material, a passively visualized MR guidewire has been developed, and was found to conform to existing standards on mechanical stability. Handling and steerability were evaluated in animal studies and were found to be comparable with conventional metallic guidewires. X‐ray visibility is provided by a BaSO4‐ and tungsten‐doped jacket. A hydrophilic coating improves sliding properties and hemocompatibility. Magn Reson Med 60:1190–1196, 2008.

Image‐based tracking of optically detunable parallel resonant circuits

In this work strategies for the robust localization of parallel resonant circuits are investigated. These strategies are based on the subtraction of two images, which ideally differ in signal intensity at the positions of the devices only. To modulate their signal amplification, and thereby generate the local variations, the parallel resonant circuits are alternately detuned and retuned during the acquisition. The integration of photodiodes into the devices permits their fast optical switching. Radial and spiral imaging sequences are modified to provide the data for the two images in addition to those for a conventional image in the same acquisition time. The strategies were evaluated by phantom experiments with stationary and moving catheter‐borne devices. In particular, rapid detuning and retuning during the sampling of single profiles is shown to lead to a robust localization. Moreover, this strategy eliminates most of the drawbacks usually associated with image‐based tracking, such as low temporal resolution. Image‐based tracking may thus become a competitive (if not superior) alternative to projection‐based tracking of parallel resonant circuits. Magn Reson Med 49:1163–1174, 2003.

In vivo evaluation and proof of radiofrequency safety of a novel diagnostic MR‐electrophysiology catheter

An MR‐electrophysiology (EP) catheter is presented that provides full diagnostic EP functionality and a high level of radiofrequency safety achieved by custom‐designed transmission lines. Highly resistive wires transmit intracardiac electrograms and currents for intracardiac pacing. A transformer cable transmits the localization signal of a tip coil. Specific absorption rate simulations and temperature measurements at 1.5 T demonstrate that a wire resistance > 3 kΩ/m limits dielectric heating to a physiologically irrelevant level. Additional wires do not increase tip specific absorption rate significantly, which is important because some clinical catheters require up to 20 electrodes. It is further demonstrated that radiofrequency‐induced and pacing‐induced resistive heating of the wires is negligible under clinical conditions. The MR‐EP catheters provided uncompromised recording of electrograms and cardiac pacing in combination with a standard EP recorder in MR‐guided in vivo EP studies, and the tip coil enabled fast and robust catheter localization. In vivo temperature measurements during such a study did not detect any device‐related heating, which confirms the high level of safety of the catheter, whereas unacceptable heating was found with a standard EP catheter. The presented concept for the first time enables catheters with full diagnostic EP functionality and active tracking and at the same time a sufficient level of radiofrequency safety for MRI without specific absorption rate‐related limitations. Magn Reson Med, 2011.

Real-time magnetic resonance-guided ablation of typical right atrial flutter using a combination of active catheter tracking and passive catheter visualization in man: initial results from a consecutive patient series

AIMS Recently cardiac magnetic resonance (CMR) imaging has been found feasible for the visualization of the underlying substrate for cardiac arrhythmias as well as for the visualization of cardiac catheters for diagnostic and ablation procedures. Real-time CMR-guided cavotricuspid isthmus ablation was performed in a series of six patients using a combination of active catheter tracking and catheter visualization using real-time MR imaging. METHODS AND RESULTS Cardiac magnetic resonance utilizing a 1.5 T system was performed in patients under deep propofol sedation. A three-dimensional-whole-heart sequence with navigator technique and a fast automated segmentation algorithm was used for online segmentation of all cardiac chambers, which were thereafter displayed on a dedicated image guidance platform. In three out of six patients complete isthmus block could be achieved in the MR scanner, two of these patients did not need any additional fluoroscopy. In the first patient technical issues called for a completion of the procedure in a conventional laboratory, in another two patients the isthmus was partially blocked by magnetic resonance imaging (MRI)-guided ablation. The mean procedural time for the MR procedure was 109 ± 58 min. The intubation of the CS was performed within a mean time of 2.75 ± 2.21 min. Total fluoroscopy time for completion of the isthmus block ranged from 0 to 7.5 min. CONCLUSION The combination of active catheter tracking and passive real-time visualization in CMR-guided electrophysiologic (EP) studies using advanced interventional hardware and software was safe and enabled efficient navigation, mapping, and ablation. These cases demonstrate significant progress in the development of MR-guided EP procedures.

Preclinical evaluation of a novel fiber compound MR guidewire in vivo.

Purpose:Interventional magnetic resonance imaging requires dedicated and MR-compatible devices. The guidewire is a key item for intravascular interventions. Mechanical stability, good visibility during real-time imaging, and RF safety are essential. A novel fiber-compound MR guidewire (GW) was evaluated in different MR-guided interventional scenarios. Materials and Methods:The GW (diameter 0.032”) consists of a fiber-compound produced using a micropultrusion technique doped with iron particles and a 10-cm Nitinol tip. Several iron splints are additionally attached at regular distances to visualize GW-movement. A protective polymer jacket with hydrophilic coating covers the core material. As approved by the government committee on animal investigations, the GW was evaluated in 5 pigs. Under complete MR-guidance, catheterization of the carotid and renal arteries, segmental arteries of the kidneys, the contralateral inguinal artery, and the left ventricle was performed using real-time gradient echo sequences in a 1.5 Tesla scanner. Different interventional applications including balloon dilatation, stent deployment, and embolization of small vessels were investigated. The time to probe the vessels under magnetic resonance imaging guidance and visibility of the GW are assessed. Handling and visibility under fluoroscopy were compared with a standard Nitinol guidewire as a benchmark. Results:On real-time magnetic resonance imaging, the iron-induced artifacts enabled a distinct visualization of the GW shaft and of its markings with a mean size of 2.6 mm and 5.4 mm, respectively. This facilitated fast navigation to the target vessels (averages: renal arteries 16 seconds, carotid artery 5 seconds, and contralateral inguinal artery 42 seconds.) with an exact depiction of the respective vessel. All interventional procedures were performed successfully. No GW-related side effects as kinking or breakage of the wire or GW induced blood-clotting were observed. All interventionalists assessed handling of the GW to be nearly equal in terms of stiffness, flexibility, and guidance compared with a standard Nitinol guidewire. X-ray visibility was less distinct but still diagnostically good. Conclusion:With the aid of the GW, different fully real-time MR-guided endovascular interventions become feasible.

Toward true 3D visualization of active catheters using compressed sensing

A crucial requirement in MR‐guided interventions is the visualization of catheter devices in real time. However, true 3D visualization of the full length of catheters has hitherto been impossible given scan time constraints. Compressed sensing (CS) has recently been proposed as a method to accelerate MR imaging of sparse objects. Images acquired with active interventional devices exhibit a high CNR and are inherently sparse, therefore rendering CS ideally suited for accelerating data acquisition. A framework for true visualization of active catheters in 3D is proposed employing CS to gain high undersampling factors making real‐time applications feasible. Constraints are introduced taking into account prior knowledge of catheter geometry and catheter motion over time to improve and accelerate image reconstruction. The potential of the method is demonstrated using computer simulations and phantom experiments and in vivo feasibility is demonstrated in a pig experiment. Magn Reson Med, 2009.