Interventional CMR Course
SCMR 22nd Annual Scientific Sessions
Background: Catheter ablation as the first line treatment for many cardiac arrhythmias is generally performed under X-ray fluoroscopy guidance. However, the physician performing the procedure and the patient are at risk from exposure to radiation. When compared to X-ray fluoroscopy, the magnetic fields used in MRI do not expose the patient and physician to harmful radiation. MRI is also capable of producing more detailed scans and delivers better therapy . On the other hand, RF coil induced heating around the catheter, specifically around the tip that is exposed to the tissue, can be harmful. A careful design and evaluation is required for the catheters to be labeled as MRI conditionally safe.
An MRI-compatible ablation circuit (Figure 1a) was designed by increasing the impedance of the device circuitry at the 64MHz MRI operating frequency (1.5Tesla) while maintaining low impedance at the 500KHz ablation frequency. The circuit was designed to fit a 1350mm catheter body with a 1400mm conductor and a gold ablation tip. The full circuit has 13 inductors every 76mm along the conductor and an LC tank between the ablation tip and first inductor. Each inductor has an impedance of 200Ω at 64MHz. The LC tank circuit is tuned to resonate at 64MHz with an impedance of ~2kΩ. Three comparison circuits were also designed by eliminating just the LC tank, the tank and inductors, and replacing the circuit with a straight wire (Figure 1c, 1b). A physical version of each circuit and a fully functional ablation catheter were built.
The specific absorption rate (SAR) around each circuit was determined using an FDTD model (Sim4Life) (Figure 1d, 2a) to compare to experimental measurements . Experimental heat testing was conducted per ASTM F2182 using fiber optic temperature probes in a saline torso phantom. The full ablation catheter was used to perform ablations in swine using MRI guidance.
Figure 1e and Table 1 show the simulation and experimental results in stages ranging from no heat-mitigation to the full heat-mitigation circuitry. SAR is dependent on tissue specific heat capacity and the slope of the time-dependent heating curve. SAR was seen to decrease by changing the shape of ablation tip and by adding the inductors and LC tank. The size and consistency of lesions created with the new catheter was comparable to benchmarked catheters (Figure 2b, 2c).
Conclusion: We have designed an MRI-compatible ablation catheter using FDTD simulation and we have demonstrated the effectiveness of this design through experimental measurements and swine models.