Shape accuracy of fiber optic sensing for medical devices characterized in bench experiments.

PURPOSE: Fiber Optic RealShape (FORS) is a new technology that visualizes the full three-dimensional shape of medical devices, such as catheters and guidewires, using an optical fiber embedded in the device. This three-dimensional shape provides guidance to clinicians during minimally invasive procedures, and enables intuitive navigation. The objective of this paper is to assess the accuracy of the FORS technology, as implemented in the current state-of-the-art Philips FORS system. The FORS system provides the shape of the entire device, including tip location and orientation. We consider all three aspects. METHODS: In bench experiments, we determined the accuracy of the location and orientation of the tip by displacing and rotating the fiber end, while allowing the rest of the fiber to change shape freely. To test the accuracy of the full shape, we have placed the fiber in a groove, which was accurately machined in a thick, stiff metal "path plate." We then compared the reconstructed shape with the known shape of the groove. RESULTS: The tip location is found with submillimeter accuracy, and the orientation is sensed with milliradian accuracy. The shape of a fiber in the path plate faithfully follows the known shape of the groove, with typical deviation less than 0.5 mm in the plane of the plate. Out of plane accuracy, perhaps slightly less relevant clinically, is more challenging, due to the influence of twist; yet even out of the plane, the deviation is only submillimeter. CONCLUSION: The technology achieves submillimeter precision and provides full three-dimensional shape, surpassing the reported precision of other navigation and tracking technologies, and therefore may potentially alleviate the need for fluoroscopy.

Visualizing intramyocardial steam formation with a radiofrequency ablation catheter incorporating near-field ultrasound.

INTRODUCTION: Steam pops are a risk of irrigated RF ablation even when limiting power delivery. There is currently no way to predict gas formation during ablation. It would be useful to visualize intramyocardial gas formation prior to a steam pop occurring using near-field ultrasound integrated into a RF ablation catheter. METHODS AND RESULTS: In an in vivo open-chest ovine model (n = 9), 86 lesions were delivered to the epicardial surface of the ventricles. Energy was delivered for 15-60 seconds, to achieve lesions with and without steam pops, based on modeling data. The ultrasound image was compared to a digital audio recording from within the pericardium by a blinded observer. Of 86 lesions, 28 resulted in an audible steam pop. For lesions that resulted in a steam pop compared to those that did not (n = 58), the mean power delivered was 8.0 ± 1.8 W versus 6.7 ± 2.0 W, P = 0.006. A change in US contrast due to gas formation in the tissue occurred in all lesions that resulted in a steam pop. In 4 ablations, a similar change in US contrast was observed in the tissue and RF delivery was stopped; in these cases, no pop occurred. The mean depth of gas formation was 0.9 ± 0.8 mm, which correlated with maximal temperature predicted by modeling. Changes in US contrast occurred 7.6 ± 7.2 seconds before the impedance rise and 7.9 ± 6.2 seconds (0.1-17.0) before an audible pop. CONCLUSION: Integrated US in an RF ablation catheter is able to visualize gas formation intramyocardially several seconds prior to a steam pop occurring. This technology may help prevent complications arising from steam pops.

Real-time lesion assessment using a novel combined ultrasound and radiofrequency ablation catheter.

BACKGROUND: Assessment of lesion size and transmurality is currently via indirect measures. Real-time image assessment may allow ablation parameters to be titrated to achieve transmurality and reduce recurrences due to incomplete lesions. OBJECTIVE: The purpose of this study was to visualize lesion formation in real time using a novel combined ultrasound and externally irrigated ablation catheter. METHODS: In an in vivo open-chest sheep model, 144 lesions were delivered in 11 sheep to both the atria and the ventricles, while lesion development was monitored in real time. Energy was delivered for a minimum of 15 seconds and a maximum of 60 seconds, with a range of powers, to achieve different lesion depths. Twenty-two lesions were also delivered endocardially. The ultrasound appearance was assessed and compared with the pathological appearance by four independent blinded observers. RESULTS: For the ventricular lesions (n = 126), the mean power delivered was 6.1 ± 2.0 W, with a mean impedance of 394.7 ± 152.4 Ω and with an impedance drop of 136.4 ± 100.1 Ω. Lesion depths varied from 0 to 10 mm, with a median depth of 3.5 mm. At tissue depths up to 5 mm, changes in ultrasound contrast correlated well (r = 0.79, R(2) = 0.62) with tissue necrosis. The depth of ultrasound contrast correlated poorly with the depth of the zone of hemorrhage (r = 0.33, R(2) = 0.11), and impedance change correlated poorly with lesion depth (r = 0.29, R(2) = 0.08). CONCLUSION: Real-time lesion assessment using high-frequency ultrasound integrated into an ablation catheter is feasible and allows differentiation between true necrosis and hemorrhage. This may lead to safer and more efficient power delivery, allowing more effective lesion formation.