Near-Field Ultrasound Imaging of Ablation Lesion Formation

ffective lesion formation with radiofrequency (RF) catheter ablation in current practice relies on the monitoring of variables which serve as surrogates to tissue destruction.Impedance drop, electrogram diminution, adequacy of contact force and power titration, and lack of pace capture after ablation all suggest effective RF delivery to targeted tissue, but none come close to a gold standard of pathological examination (Figure ).[1][2][3][4][5] Intracardiac echocardiography can improve the safety and efficacy of catheter ablation because of its ability to offer direct visualization of the catheter-tissue interface, but in most cases provides insufficient resolution to adequately assess the depth of lesions created during RF delivery.6,7 Near-field imaging to directly visualize lesion formation would offer a substantial improvement with potential to improve the safety and efficacy of catheter ablation; not only could near-field imaging confirm that targeted tissue was truly being affected by RF, but the power, contact force, and duration of RF delivery could be titrated in a manner to avoid delivery of excessive or unnecessary ablation which could otherwise put neighboring structures (phrenic nerve, periesophageal vagal nerve, or the esophagus itself) at risk.[8][9][10] The near-field ultrasound (NFUS) ablation catheter is designed to offer just that-NFUS imaging from the catheter tip to directly monitor lesion formation in hopes to ultimately improve the efficacy and safety of catheter ablation.In the current issue of Circulation: Arrhythmia and Electrophysiology, Haines et al 11 investigate the application of the novel NFUS catheter in a series of ex vivo and in vivo canine studies.The NFUS system is configured with an 8F open irrigation catheter using a 5-mm platinum-iridium electrode tip platform.The NFUS ablation catheter has a series of 4 ultrasound transducers incorporated into the catheter tip, which are positioned in a fashion (1 at the catheter tip with axial imaging, and 3 mounted along the side of catheter tip at 120° intervals with radial imaging) such that at least 1 transducer will be in contact with tissue regardless of catheter orientation.Ultrasound imaging from the catheter tip was displayed in simultaneous Mmode, tissue Doppler, and strain rate formats with overlying bipolar electrograms using custom-built software for real-time display.The authors first evaluated the NFUS catheter's ability to measure tissue thickness before delivery of RF energy, and the NFUS catheter proved capable in its ability to accurately measure tissue thicknesses in ex vivo phantom and tissue models throughout a wide range of contact and rotational angles.There was somewhat less accuracy, but still with reasonable correlation, in canine in vivo measurements of myocardial tissue thickness when compared with intracardiac echocardiography measurements as a gold standard, although left ventricular sites were excluded from analysis as the average left ventricle wall thickness exceeded the depth of view for NFUS.Overestimates of wall thickness were common when the catheter Near-Field Ultrasound Imaging of Ablation Lesion FormationMore Than Meets the Eye?