The Evolution of Tissue Stiffness at Radiofrequency Ablation Sites During Lesion Formation and in the Peri-Ablation Period.

An imaging technique that provides visual feedback of radiofrequency ablation (RFA) completeness could potentially increase the efficacy of transcatheter cardiac ablation (TCA) procedures for the treatment of cardiac arrhythmias.1 At lethal RFA temperatures, structural proteins (collagen, elastin), and myofilaments (myosin, actin) immediately and irreversibly coagulate;2 this thermocoagulation is believed to be an endpoint for permanent cell injury3, 4 and causes the ablated region to become stiffer than the surrounding myocardium.5 Elastography imaging can visualize thermocoagulated lesions due to the increase in tissue stiffness.6 Ultrasound‐based techniques (acoustic radiation force or ARF‐based imaging,7, 8, 9, 10, 11 strain imaging12) and magnetic resonance elastography (MRE)13 are both currently being investigated as possible tools for intra‐procedure lesion assessment. Two‐dimensional (2‐D) acoustic radiation‐force impulse (ARFI) imaging is an ARF‐based ultrasound imaging method.14 ARFI images are constructed by applying a sequence of short‐duration acoustic impulses to tissue over a lateral field of view and measuring the resulting tissue displacement with conventional ultrasound. The measured ARFI‐induced displacement magnitude is inversely proportional to tissue stiffness and presents a representation of the relative stiffness.14, 15 We have previously shown in vitro that ARFI imaging can visualize RFA lesions,16, 17 and that in vivo intracardiac echocardiography (ICE) based diastolic ARFI imaging can provide high‐contrast visualization of stiff atrial lesions consistent with electrical block at the imaging site.7, 18 Initial results from a clinical feasibility study demonstrated that ARFI imaging could identify stiffness changes consistent with RF ablation in human patients.8 Shear wave elasticity imaging (SWEI) is another ARF‐based elastography technique that uses acoustic impulses to displace tissue, but monitors the off‐axis propagation of the radiation‐force induced transverse shear wave.19 The velocity of propagation, or shear wave speed (SWS), is a function of the tissue material properties and is proportional to the tissue stiffness.20, 21 Pernot et al.,9 Hollender et al.,11 and Kwiecinski et al.10 have previously demonstrated the use of shear‐wave elastography methods to quantify RFA lesion stiffness in vitro and in vivo. It is important to the clinical translation of any lesion evaluation technique that relies on elastography to determine if the visualized region of stiffness increase (RoSI) is spatially and temporally stable in the presence of transient physiological responses in the peri‐ablation period. This study used high‐spatial resolution (sub‐millimeter) epicardial 2‐D ARFI image monitoring (1 image every 30–60 seconds) of 6 right ventricular (RV) endocardial RFA sites in an open‐chest canine model to characterize the spatial and temporal stability of the RoSI in the peri‐ablation period. It is important to note that while a relatively stiff lesion can be differentiated from untreated myocardium in qualitative elastography images, variability between patients and imaging conditions could make it difficult to establish a universal parametric threshold to delineate lesion from healthy tissue based on the relative stiffness; this work does not aim to determine the threshold to define RF‐treated myocardium. Rather, in this experiment a constant 2‐D imaging plane is used to monitor tissue stiffening during RFA and to characterize the evolution of the RoSI in the 30 minutes following the ablation.