PURPOSE

Catheter-based radiofrequency (RF) ablation is widely used clinically for the treatment of cardiac electrical disorders. However, a significant rate of recurrence (~30%) has been reported due to suboptimal RF energy deposition into the myocardium¹ and lack of real-time lesion visualization. Fluoroscopic guidance provides limited information on the actual lesion during the procedure due to a poor soft-tissue contrast and restrictions in X-ray exposure time. Thus, cardiac MR thermometry using the proton resonance frequency (PRF) shift technique has been suggested to map the temperature distribution and to compute the accumulated thermal dose during RF ablation. Recent studies proposed to compensate for cardiac motion (myocardial contraction and breathing) by positioning the imaging slices in the main direction of displacement² or by updating the slices position according to the respiratory trace recorded at the liver-lung interface using an echo-navigator³. In this study, we propose to actively track the catheter position and to update the MR-thermometry slice position accordingly. Temperature stability of the proposed method was evaluated in vitro on a moving agar gel (without heating). Then during a RF ablation with the same experimental parameters.

METHODS

A MR-compatible catheter (MRI Interventions, Irvine, CA, U.S.A.) equipped with four MR receivers (Figure 1-A) was interfaced with a 1.5 T MR scanner (Magnetom Aera, Siemens Healthineers). A multi-slice single-shot echo-planar imaging (EPI) pulse sequence was interleaved with a catheter tracking scheme. It consisted in a non-selective RF pulse applied before a linear gradient was played along one direction during signal reception. This scheme was repeated 3 times with gradients applied along 3 orthogonal directions (Figure 1-B). The signal collected by each micro-coil of the catheter showed a sharp peak in frequency, whose position is proportional to the spatial location of the micro-coil along the applied gradient direction (Figure 1-C). The detected 3D position of the coil at the catheter tip was used to update the following stack of EPI slices on the fly. Despite the compensation of the in-plane (for coronal and sagittal orientations) and out-of-plane (for transverse orientation) motion using catheter tracking, phase images remained corrupted by local magnetic field changes associated with motion. These artifacts were corrected on-line by performing phase modeling based on the updated slice position, as reported recently by Wang et al.⁴. A periodic motion in the z-axis direction (head-feet) of an agar gel was induced by a home-made rocker, with an amplitude of 10 mm, higher than the slice thickness (3 mm). Temperature mapping was first evaluated without heating to calculate the temporal standard deviation of temperature σ_T over three min of acquisition with different slices orientation (parallel or perpendicular to the direction of motion). The same protocol was then repeated during RF ablation at 4 W for 20 s.

RESULTS

This new cardiac thermometry pipeline was operated on the fly with a temporal resolution of ~100 ms per image and a voxel size of 2x2x3 mm³. The measurement of the temperature quality without heating showed similar results whatever the slice orientation, with a temporal standard deviation σ_T below 2°C (Figure 2, A-C). Monitoring of temperature during RF ablation was successfully performed using the proposed method. Figure 3 shows the maximal temperature changes at the end of the RF delivery during rocker-driven motion.

DISCUSSION

This new MR thermometry method using active catheter tracking shows good performances in presence of in-plane and through-plane motions. This method allows automatic positioning of the imaging slices on the catheter in real-time to continuously visualize the temperature distribution. In the current setup, residual electromagnetic interferences (harmonics of the RF ablation signal) were observed on catheter’s coil signal at high RF power that disturbed the catheter tracking performances. Further in vivo evaluation on a large animal model are envisioned once this technological issue is solved.

CONCLUSION

The combination of catheter tracking and MR thermometry allows accurate thermometry in presence of motion. This method is promising for further monitoring of cardiac RF ablation, avoiding any constraint on image orientations nor assumption on scaling factor between the liver motion followed by an echo navigator and the heart motion.

Acknowledgements

No acknowledgement found.