Steve Wedan

MR Imaging–guided Electrophysiological Ablation Studies in Humans with Passive Catheter Tracking: Initial Results

PURPOSE To assess if real-time magnetic resonance (MR) imaging-guided radiofrequency (RF) ablation for atrial flutter is feasible in patients. MATERIALS AND METHODS The study complied with the Declaration of Helsinki and was approved by the local ethics committee. All patients were informed about the investigational nature of the procedures and provided written informed consent. Ten patients (six men; mean age ± standard deviation, 68 years ± 10) with symptomatic atrial flutter underwent isthmus ablation. In all patients, two MR imaging conditional steerable diagnostic and ablation catheters were inserted into the coronary sinus via femoral sheaths and into the right atrium with fluoroscopic guidance. The patients were then transferred to a 1.5-T whole-body MR imager for an ablation procedure, in which the catheters were manipulated by an electrophysiologist by using a commercially available interactive real-time steady-state free precession MR imaging sequence. RESULTS All catheters were placed in standard positions successfully. Furthermore, simple programmed stimulation maneuvers were performed. In one of 10 patients, a complete conduction block was performed with MR imaging guidance. In nine of 10 patients, creating only a small number of additional touch-up lesions was necessary to complete the isthmus block with conventional fluoroscopy (median, three lesions; interquartile range, two to four lesions). CONCLUSION Real-time MR imaging-guided placement of multiple catheters is feasible in patients, with subsequent performance of stimulation maneuvers and occasional complete isthmus ablation.

Assessing the Electromagnetic Fields Generated By a Radiofrequency MRI Body Coil at 64 MHz: Defeaturing Versus Accuracy

Goal: This study aims at a systematic assessment of five computational models of a birdcage coil for magnetic resonance imaging (MRI) with respect to accuracy and computational cost. Methods: The models were implemented using the same geometrical model and numerical algorithm, but different driving methods (i.e., coil “defeaturing”). The defeatured models were labeled as: specific (S2), generic (G32, G16), and hybrid (H16, H16fr-forced). The accuracy of the models was evaluated using the “symmetric mean absolute percentage error” (“SMAPE”), by comparison with measurements in terms of frequency response, as well as electric (||E⃗||) and magnetic (||B⃗||) field magnitude. Results: All the models computed the ||B⃗|| within 35% of the measurements, only the S2, G32, and H16 were able to accurately model the ||E⃗|| inside the phantom with a maximum SMAPE of 16%. Outside the phantom, only the S2 showed a SMAPE lower than 11%. Conclusions: Results showed that assessing the accuracy of ||B⃗|| based only on comparison along the central longitudinal line of the coil can be misleading. Generic or hybrid coils - when properly modeling the currents along the rings/rungs - were sufficient to accurately reproduce the fields inside a phantom while a specific model was needed to accurately model ||E⃗|| in the space between coil and phantom. Significance: Computational modeling of birdcage body coils is extensively used in the evaluation of radiofrequency-induced heating during MRI. Experimental validation of numerical models is needed to determine if a model is an accurate representation of a physical coil.

A numerical investigation on the effect of RF coil feed variability on global and local electromagnetic field exposure in human body models at 64 MHz

This study aims to investigate how the positions of the feeding sources of the transmit radiofrequency (RF) coil, field orientation direction with respect to the patient, and patient dimensions affect the global and local electromagnetic exposure in human body models.

MRI safety assessment of a generic deep brain stimulator

The radio frequency (RF) electromagnetic field of magnetic resonance (MR) scanners can result in significant tissue heating due to the RF coupling with the conducting parts of medical implants. The objective of this paper is to assess the safety of a generic deep brain stimulator (DBS) during MRI scans based on a combined numerical and experimental procedure described in [1]. The evaluation is performed for 1.5 T MR scanners using a generic model of a deep brain stimulator with a helical lead. The results show that the approach is technically feasible and provides sound and conservative information on the potential heating of implants.

RF induced energy for partially implanted catheters: A computational study

Magnetic Resonance Imaging (MRI) is a radiological imaging technique widely used in clinical practice. MRI has been proposed to guide the catheters for interventional procedures, such as cardiac ablation. However, there are risks associated with this procedure, such as RF-induced heating of tissue near the catheters. The aim of this study is to develop a quantitative RF-safety method for patients with partially implanted leads at 64 MHz. RF-induced heating is related to the electric field incident along the catheter, which in turns depends on several variables, including the position of the RF feeding sources and the orientation of the polarization, which are however often unknown. This study evaluates the electric field profile along the lead trajectory using simulations with an anatomical human model landmarked at the heart. The energy absorbed in the volume near the tip of ageneric partially implanted lead was computed for all source positions and field orientation. The results showed that varying source positions and field orientation may result in changes of up to 18% for the E-field magnitude and up to 60% for the 10g-averaged specific absorption rate (SAR) in the volume surrounding the tip of the lead.