Conductance catheter measurements of lumen area of stenotic coronary arteries: theory and experiment

Extending bioelectric navigation for displacement and direction detection

PURPOSE

Bioelectric navigation is a navigation modality for minimally invasive endovascular procedures promising non-fluoroscopic navigation. However, the method offers only limited navigation accuracy between anatomical features and expects the tracked catheter to move only in one direction at all times. We propose to extend bioelectric navigation with additional sensing capabilities, allowing for the estimation of the distance traveled by the catheter, thereby improving accuracy between feature locations and allowing to track also under alternating forward- and backward motion.

METHODS

We perform experiments in finite element method (FEM) simulations and in a 3D printed phantom. A solution for estimating the traveled distance using a stationary electrode is proposed, together with an approach on how to evaluate the signals obtained with this additional electrode. We investigate the effects of surrounding tissue conductance on this approach. Finally, the approach is refined in order to mitigate the effects of parallel conductance on the navigation accuracy.

RESULTS

The approach allows to estimate the catheter movement direction and the distance traveled. Simulations show absolute errors below 0.89 mm for non-conducting surrounding tissue, but errors up to 60.27 mm when the tissue is electrically conductive. This effect can be mitigated by a more sophisticated modeling (errors up to 33.96 mm). In experiments in a 3D printed phantom, the mean absolute error over 6 catheter paths is 6.3 mm, with standard deviations smaller than or equal to 1.1 mm.

CONCLUSIONS

Extending the setup of bioelectric navigation with an additional stationary electrode allows to estimate the distance traveled by the catheter, as well as the movement direction. The effects of parallel conductive tissue could be partially mitigated in simulations, but further research is needed to investigate these effects in real biological tissue, and to bring the introduced errors down to a clinically acceptable level.

Optimization of Peripheral Vascular Sizing with Conductance Guidewire: Theory and Experiment

Although the clinical range of interventions for coronary arteries is about 2 to 5 mm, the range of diameters of peripheral vasculature is significantly larger (about 10 mm for human iliac artery). When the vessel diameter is increased, the spacing between excitation electrodes on a conductance sizing device must also increase to accommodate the greater range of vessel diameters. The increase in the excitation electrodes distance, however, causes higher parallel conductance or current losses outside of artery lumen. We have previously shown that the conductance catheter/guidewire excitation electrode distances affects the measurement accuracy for the peripheral artery lumen sizing. Here, we propose a simple solution that varies the detection electrode distances to compensate for parallel conductance losses. Computational models were constructed to simulate the conductance guidewire with various electrodes spacing combinations over a range of peripheral artery lumen diameters and surrounding tissue electrical conductivities. The results demonstrate that the measurement accuracy may be significantly improved by increased detection spacing. Specifically, an optimally configured detection/excitation spacing (i.e., 5-5-5 or an equidistant electrode interval with a detection-to-excitation spacing ratio of 0.3) was shown to accurately predict the lumen diameter (i.e., -10% < error < 10%) over a broad range of peripheral artery dimensions (4 mm < diameter < 10 mm). The computational results were substantiated with both ex-vivo and in-vivo measurements of peripheral arteries. The present results support the accuracy of the conductance technique for measurement of peripheral reference vessel diameter.

Computer modeling and ex vivo experiments with a (saline-linked) irrigated electrode for RF-assisted heating

BACKGROUND

Externally irrigated radiofrequency (RF) electrodes have been widely used to thermally ablate tumors in surface tissue and to thermally coagulate the transection plane during a surgical resection. As far as we know, no mathematical model has yet been developed to study the electrical and thermal performance of these electrodes, especially the role of the saline layer that forms around the electrode.

METHODS

Numerical models of a TissueLink device model DS3.0 (Salient Surgical Technologies, Portsmouth, NH, USA) were developed. Irrigation was modeled including a saline layer and a heat convection term in the governing equation. Ex vivo experiments based on fragments of bovine hepatic tissue were conducted to obtain information which was used in building the numerical model. We compared the 60°C isotherm of the computer results with the whitening contour in the heated samples.

RESULTS

Computer and experimental results were in fine agreement in terms of lesion depth (2.4 mm in the simulations and 2.4 ± 0.6 mm in the experiments). In contrast, the lesion width was greater in the simulation (9.6 mm vs. 7.8 ± 1.8 mm). The computer simulations allowed us to explain the role of the saline layer in creating the thermal lesion. Impedance gradually decreased as heating proceeded. The saline was not observed to boil. In the proximity of the electrode (around 1 mm) the thermal lesion was mainly created by the RF power in this zone, while at a further distance the thermal lesion was created by the hot saline on the tissue surface by simple thermal conduction. Including the heat convection term associated with the saline velocity in the governing equation was crucial to verifying that the saline layer had not reached boiling temperature.

CONCLUSIONS

The model reproduced thermal performance during heating in terms of lesion depth, and provided an explanation for: 1) the relationship between impedance, electrode insertion depth, and saline layer, and 2) the process of creating thermal lesions in the tissue with this type of electrode.

Effect of saline injection mixing on accuracy of conductance lumen sizing of peripheral vessels

Transient displacement of blood in vessel lumen with saline injection is necessary in the conductance method for measurement of arterial cross-sectional area (CSA). The displacement of blood is dictated by the interactions between arterial flow hemodynamics and saline injection dynamics. The objective of the present study is to understand how the accuracy of conductance measurements is affected by the saline injection. Computational simulations were performed to assess the error in predictions of arterial CSA using conductance measurements over a range of peripheral artery diameters (i.e., 4, 7, and 10 mm) with an introducing catheter (6 Fr.) for various blood flow and saline injection rates. The simulation results were validated using the conductance measurements of the phantoms with known diameters (i.e., 7 and 10 mm). The results demonstrated that a minimum ratio of saline injection rate to blood flow rate of 3 is needed to fully displace the blood and result in accurate measurement of CSA for the peripheral artery sizes considered. Furthermore, the error was shown to be minimized as the detection electrodes are positioned between the distal to the mixing zone induced by saline injection and far downstream (4–8 cm from the injection catheter tip). The present study shows that even for the large peripheral arteries (7–10 mm) where mixing can occur, an appropriate injection rate and detection position can produce accurate measurement of lumen size.

Implications of complex anatomical junctions on conductance catheter measurements of coronary arteries

In vivo, the position of the conductance catheter to measure vessel lumen cross-sectional area may vary depending on where the conductance catheter is deployed in the complex anatomical geometry of arteries, including branches, bifurcations, or curvatures. The objective here is to determine how such geometric variations affect the cross-sectional area (CSA) estimates obtained using the cylindrical model. Computer simulations and in vitro and in vivo experiments were used to assess how the electric field and associated CSA measurement accuracy are affected by three typical in vivo conditions: 1) a vessel with abrupt change in lumen diameter (e.g., transition from aorta to coronary ostia); 2) a vessel with a T-bifurcation or a Y-bifurcation; and 3) a vessel curvature, such as in the right coronary artery, aorta, or pulmonary artery. The error in diameter from simulation results was shown to be relatively small (<7%), unless the detection electrodes were placed near the junction between two different lumen diameters or at a bifurcation junction. Furthermore, the present findings show that the effect of misaligned catheter-vessel geometrical configuration and vessel curvature on measurement accuracy is negligible. Collectively, the findings support the accuracy of the conductance method for sizing blood vessels, despite the geometric complexities of the cardiovascular system, as long as the detection electrodes are not placed at a large discontinuity in diameter or at bifurcation junctions.