Ultrasound Imaging in EP
Fluoroscopy is conventionally used to provide real time imaging of catheter based interventional procedures. Fluoroscopy is simple, easy to use, and provides excellent contrast of interventional devices. However, fluoroscopy poorly delineates the soft tissue of the heart making it difficult to guide catheters to specific cardiac structures. Ultrasound is another real time imaging modality that is used to guide interventional procedures. Visualizing procedures using body surface ultrasound imaging of the heart is difficult because of the presence of the lungs, the small number of available windows, and the depth of the scan required to reach the heart chambers. Intracardiac echocardiography (ICE), in which a catheter mounted transducer is inserted into the heart, has been used for the past 5 years to augment fluoroscopy during ablation procedures29, 30. ICE is implemented using either a mechanical scanning single transducer rotating within the catheter or a phased array mounted on the side of the catheter. ICE is used to augment fluoroscopy and is frequently used to guide atrial trans-septal puncture, to aid anatomical based catheter positioning, to assure adequate catheter tip-tissue contact, and to monitor micro-bubble generation and tissue disruption during ablation30, 31. Figure 3 shows a typical ICE image of the left atrium.
ICE-based guidance has provided eyes inside the heart to visualize some of the most critical aspects of EP procedures. However, ICE imaging has not been successful in differentiating normal from ablated myocardium. The backscatter of these two tissue types is very similar. Thus, ICE has not been used to directly titrate energy to control lesion formation, and has not been used to characterize lesions or groups of lesions in terms of their geometry.
ICE has been used to titrate ablation energy in two important ways. First, as the tissue overheats, gas microbubbles form within the tissue and near the catheter tip. The presence of these bubbles has been used as an indicator that the tissue is reaching a maximum temperature above which cavitations or “pops” are likely. Similarly, as the electrode tip temperature increases, there is an increased likelihood of coagulum formation at the tip, again, indicating that ablation energy delivery should be terminated. The presence of this coagulum can be visualized with ICE.
These indicators are helpful for avoiding overheating the tissue but they are only secondary indicators of lesion formation. As of today, there is no satisfactory real time clinical method for assessing lesion extent in the myocardium.
The variability in lesion size for a given application of energy leads to two clinical problems. First, ablation in a high blood flow rate area or with poor tip contact may lead to a smaller lesions than expected. Undersized lesions cause incomplete conduction block because the lesion is not trans mural or a gap remains bet ween adjacent lesions. Electrograms recorded from the catheter tip after a lesion is made can demonstrate the “double potentials” or reduced amplitude characteristic of block but stunned tissue on the epicardium or between lesions may later become active, necessitating repair or subsequent procedures. Second, ablation in a region of low flow or with the electrode buried in the trabeculated surface my cause excess energy to be delivered beyond what is required to make a transmural lesion. Excess energy risks overheating and the resulting pops as well as risks heating of collateral tissue leading to complications. A significant need exists for a method to directly visualize lesion extent in the heart.
Electroanatomical mapping systems are used t o characterize electrical conduction and guide the placement of the tip for ablation, they can not visualize the tissue or the lesion extent. ICE can visualize tissue structures and aid catheter placement, and ICE based ARFI can be used to characterize lesion extent. Unfortunately, there is no technology available in clinical practice today to register an image made with ICE to a conduction map or lesion fiducial marks made by electro-anatomical mapping. With no registration, it is nearly impossible to align the t wo dimensional imaging plane of the ICE system onto the narrow line of lesions on the atrial wall and to position the ablation catheter in the electro-anatomical cartoon guided by the image acquired from an ICE or an ICE based ARFI image. We propose the dev elopment of a new integrated imaging system combing ICE, ICE based ARFI imaging, and electro-anatomical mapping. This integrated system will make it possible to place lesions, image lesion continuity and transmurality, and guide the placement of the ablation catheter to repair gaps.
A joint v enture between Seimens Medical and Johnson & Johnson (Biosense) will within a year produce ICE imaging catheters with CARTO™ position sensing technology in the tip (see letter of support in Appendix). For the first time, this system will allow the registration of these two imaging technologies and will provide a new and unique tool for visualizing lesions. Int egration of ICE and electroanatomical mapping will better guide lesion placement but will not enable lesion assessment and correlation of lesion continuity with electrical maps. ICE based ARFI imaging combined with CARTO™, as proposed in this application will merge lesion evaluation with catheter positioning and electrical mapping and enable complete clinical guidance of the ablation therapy.