Marcus Warmuth

Measuring RF-induced currents inside implants: Impact of device configuration on MRI safety of cardiac pacemaker leads

Radiofrequency (RF)‐related heating of cardiac pacemaker leads is a serious concern in magnetic resonance imaging (MRI). Recent investigations suggest such heating to be strongly dependent on an implants position within the surrounding medium, but this issue is currently poorly understood. In this study, phantom measurements of the RF‐induced electric currents inside a pacemaker lead were performed to investigate the impact of the device position and lead configuration on the amount of MRI‐related heating at the lead tip. Seven hundred twenty device position/lead path configurations were investigated. The results show that certain configurations are associated with a highly increased risk to develop MRI‐induced heating, whereas various configurations do not show any significant heating. It was possible to precisely infer implant heating on the basis of current intensity values measured inside a pacemaker lead. Device position and lead configuration relative to the surrounding medium are crucial to the amount of RF‐induced heating in MRI. This indicates that a considerable number of implanted devices may incidentally not develop severe heating in MRI because of their specific configuration in the body. Small variations in configuration can, however, strongly increase the risk for such heating effects, meaning that hazardous situations might appear during MRI. Magn Reson Med, 2009.

Spatial distribution of RF-induced E-fields and implant heating in MRI

The purpose of this study was to assess the distribution of RF‐induced E‐fields inside a gel‐filled phantom of the human head and torso and compare the results with the RF‐induced temperature rise at the tip of a straight conductive implant, specifically examining the dependence of the temperature rise on the position of the implant inside the gel. MRI experiments were performed in two different 1.5T MR systems of the same manufacturer. E‐field distribution inside the liquid was assessed using a custom measurement system. The temperature rise at the implant tip was measured in various implant positions and orientations using fluoroptic thermometry. The results show that local E‐field strength in the direction of the implant is a critical factor in RF‐related tissue heating. The actual E‐field distribution, which is dependent on phantom/body properties and the MR‐system employed, must be considered when assessing the effects of RF power deposition in implant safety investigations. Magn Reson Med 60:312–319, 2008.

Feasibility of Real-Time MRI With a Novel Carbon Catheter for Interventional Electrophysiology

Background—Cardiac MRI offers 3D real-time imaging with unsurpassed soft tissue contrast without x-ray exposure. To minimize safety concerns and imaging artifacts in MR-guided interventional electrophysiology (EP), we aimed at developing a setup including catheters for ablation therapy based on carbon technology. Methods and Results—The setup, including a steerable carbon catheter, was tested for safety, image distortion, and feasibility of diagnostic EP studies and radiofrequency ablation at 1.5 T. MRI was performed in 3 different 1.5-T whole-body scanners using various receive coils and pulse sequences. To assess unintentional heating of the catheters by radiofrequency pulses of the MR scanner in vitro, a fluoroptic thermometry system was used to record heating at the catheter tip. Programmed stimulation and ablation therapy was performed in 8 pigs. There was no significant heating of the carbon catheters while using short, repetitive radiofrequency pulses from the MR system. Because there was no image distortion when using the carbon catheters, exact targeting of the lesion sites was possible. Both atrial and ventricular radiofrequency ablation procedures including atrioventricular node modulation were performed successfully in the scanner. Potential complications such as pericardial effusion after intentional perforation of the right ventricular free wall during ablation could be monitored in real time as well. Conclusion—We describe a newly developed EP technology for interventional electrophysiology based on carbon catheters. The feasibility of this approach was demonstrated by safety testing and performing EP studies and ablation therapy with carbon catheters in the MRI environment.

Feasibility of Real Time Magnetic Resonance Imaging with a Novel Carbon Catheter for Interventional Electrophysiology

Background—Cardiac MRI offers 3D real-time imaging with unsurpassed soft tissue contrast without x-ray exposure. To minimize safety concerns and imaging artifacts in MR-guided interventional electrophysiology (EP), we aimed at developing a setup including catheters for ablation therapy based on carbon technology. Methods and Results—The setup, including a steerable carbon catheter, was tested for safety, image distortion, and feasibility of diagnostic EP studies and radiofrequency ablation at 1.5 T. MRI was performed in 3 different 1.5-T whole-body scanners using various receive coils and pulse sequences. To assess unintentional heating of the catheters by radiofrequency pulses of the MR scanner in vitro, a fluoroptic thermometry system was used to record heating at the catheter tip. Programmed stimulation and ablation therapy was performed in 8 pigs. There was no significant heating of the carbon catheters while using short, repetitive radiofrequency pulses from the MR system. Because there was no image distortion when using the carbon catheters, exact targeting of the lesion sites was possible. Both atrial and ventricular radiofrequency ablation procedures including atrioventricular node modulation were performed successfully in the scanner. Potential complications such as pericardial effusion after intentional perforation of the right ventricular free wall during ablation could be monitored in real time as well. Conclusion—We describe a newly developed EP technology for interventional electrophysiology based on carbon catheters. The feasibility of this approach was demonstrated by safety testing and performing EP studies and ablation therapy with carbon catheters in the MRI environment.

Impact of imaging landmark on the risk of MRI‐related heating near implanted medical devices like cardiac pacemaker leads

Implanted medical devices such as cardiac pacemakers pose a potential hazard in magnetic resonance imaging. Electromagnetic fields have been shown to cause severe radio frequency‐induced tissue heating in some cases. Imaging exclusion zones have been proposed as an instrument to reduce patient risk. The purpose of this study was to further assess the impact of the imaging landmark on the risk for unintended implant heating by measuring the radio frequency‐induced electric fields in a body phantom under several imaging conditions at 1.5T. The results show that global radio frequency‐induced coupling is highest with the torso centered along the superior–inferior direction of the transmit coil. The induced E‐fields inside the body shift when changing body positioning, reducing both global and local radio frequency coupling if body and/or conductive implant are moved out from the transmit coil center along the z‐direction. Adequate selection of magnetic resonance imaging landmark can significantly reduce potential hazards in patients with implanted medical devices. Magn Reson Med, 2010.

Feasibility of Contrast-Enhanced and Nonenhanced MRI for Intraprocedural and Postprocedural Lesion Visualization in Interventional Electrophysiology Animal Studies and Early Delineation of Isthmus Ablation Lesions in Patients With Typical Atrial Flutter

Background— Imaging of myocardial ablation lesions during electrophysiology procedures would enable superior guidance of interventions and immediate identification of potential complications. The aim of this study was to establish clinically suitable MRI-based imaging techniques for intraprocedural lesion visualization in interventional electrophysiology. Methods and Results— Interventional electrophysiology was performed under magnetic resonance guidance in an animal model, using a custom setup including magnetic resonance–conditional catheters. Various pulse sequences were explored for intraprocedural lesion visualization after radiofrequency ablation. The developed visualization techniques were then used to investigate lesion formation in patients immediately after ablation of atrial flutter. The animal studies in 9 minipigs showed that gadolinium-DTPA–enhanced T1-weighted and nonenhanced T2-weighted pulse sequences are particularly suitable for lesion visualization immediately after radiofrequency ablation. MRI-derived lesion size correlated well with autopsy (R2=0.799/0.709 for contrast-enhanced/nonenhanced imaging). Non–contrast agent–enhanced techniques were suitable for repetitive lesion visualization during electrophysiological interventions, thus allowing for intraprocedural monitoring of ablation success. The patient studies in 24 patients with typical atrial flutter several minutes to hours after cavotricuspid isthmus ablation confirmed the results from the animal experiments. Therapeutic lesions could be visualized in all patients using contrast-enhanced and also nonenhanced MRI with high contrast-to-noise ratio (94.6±35.2/111.1±32.6 versus 48.0±29.0/68.0±37.3 for ventricular/atrial lesions and contrast-enhanced versus nonenhanced imaging). Conclusions— MRI allows for precise lesion visualization in electrophysiological interventions just minutes after radiofrequency ablation. Nonenhanced T2-weighted MRI is particularly feasible for intraprocedural delineation of lesion formation as lesions are detectable within minutes after radiofrequency delivery and imaging can be repeated during interventions.

MRI thermometry: Fast mapping of RF‐induced heating along conductive wires

Conductive implants are in most cases a strict contraindication for MRI examinations, as RF pulses applied during the MRI measurement can lead to severe heating of the surrounding tissue. Understanding and mapping of these heating effects is therefore crucial for determining the circumstances under which patient examinations are safe. The use of fluoroptic probes is the standard procedure for monitoring these heating effects. However, the observed temperature increase is highly dependent on the positioning of such a probe, as it can only determine the temperature locally. Temperature mapping with MRI after RF heating can be used, but cooling effects during imaging lead to a significant underestimation of the heating effect. In this work, an MRI thermometry method was combined with an MRI heating sequence, allowing for temperature mapping during RF heating. This technique may provide new opportunities for implant safety investigations. Magn Reson Med, 2008.

Reducing RF-related heating of cardiac pacemaker leads in MRI: implementation and experimental verification of practical design changes.

There are serious concerns regarding safety when performing magnetic resonance imaging in patients with implanted conductive medical devices, such as cardiac pacemakers, and associated leads, as severe incidents have occurred in the past. In this study, several approaches for altering an implants lead design were systematically developed and evaluated to enhance the safety of implanted medical devices in a magnetic resonance imaging environment. The individual impact of each design change on radiofrequency heating was then systematically investigated in functional lead prototypes at 1.5 T. Radiofrequency‐induced heating could be successfully reduced by three basic changes in conventional pacemaker lead design: (1) increasing the lead tip area, (2) increasing the lead conductor resistance, and (3) increasing outer lead insulation conductivity. The findings show that radiofrequency energy pickup in magnetic resonance imaging can be reduced and, therefore, patient safety can be improved with dedicated construction changes according to a “safe by design” strategy. Incorporation of the described alterations into implantable medical devices such as pacemaker leads can be used to help achieve favorable risk‐benefit‐ratios when performing magnetic resonance imaging in the respective patient group. Magn Reson Med, 2012.

MR safety: Fast T1 thermometry of the RF‐induced heating of medical devices

Determining the MR compatibility of medical implants and devices is becoming increasingly relevant. In most cases, the heating of conductive implants due to radiefrequency (RF) excitation pulses is measured by fluoroptic temperature sensors in relevant tests for approval. Another common method to determine these heating effects is MR thermometry using the proton resonance frequency. This method gives good results in homogeneous phantoms. However in many cases, technical shortcomings such as susceptibility artifacts prohibit exact proton resonance frequency thermometry near medical implants. Therefore, this work aimed at developing a fast T1‐based method which allows controlled MR‐related heating of a medical implant while simultaneously quantifying the spatial and temporal temperature distribution. To this end, an inversion recovery snapshot Fast Low‐Angle Shot (FLASH) sequence was modified with additional off‐resonant heating pulses. With an accelerated imaging method and a sliding‐window technique, every 7.6 s a new temperature map could be generated with a spatial in‐plane resolution of 2 mm. The temperature deviation from calculated temperature values to reference fluoroptic probe was found to be smaller than 1 K. Magn Reson Med, 2012.