Simulated characterization of atherosclerotic lesions in the coronary arteries by measurement of bioimpedance

A coupled thermal-electrical-structural model for balloon-based thermoplasty treatment of atherosclerosis

OBJECTIVES

The outcome of balloon-based atherosclerosis thermoplasty is closely related to the temperature/stress distribution during the treatment. For precise prediction of a required thermal lesion in the heterogeneous and thin atherosclerotic vessel, a numerical model incorporating heat-induced tissue expansion or shrinkage and the strain caused by balloon dilation is necessary.

METHODS

A fully coupled thermal-electrical-structural new model was established. The model features a heterogeneous structure including eccentric plaque, healthy artery and surrounding tissue. Tissue expansion/shrinkage and hyperelasticity material model were taken into consideration. Different heating strategies and plaque mechanical properties were investigated. The temperature distribution was compared with the traditional thermal-electrical coupled model. The possibility of thermoplasty treatment using balloons with different sizes was also explored.

RESULTS

The temperature, the electrical intensity and the stress during the thermoplasty were obtained. Lower stress was found in the heating region where tissue shrinkage occurred. The ablation depth was predicted to be ∼0.42 mm larger without coupling the biomechanical influence. The mechanical properties and input condition significantly affect the temperature and stress distribution considering the small dimensions of the tissue. Besides, with a 12.5% reduction of balloon diameter, the largest Von Mises stress decreases by 25.4%.

CONCLUSIONS

It is confirmed that a coupled thermal-electrical-structural model is needed for precise temperature prediction in the balloon-based thermoplasty of the heterogeneous and thin tissue. The model presented may help with future development of optimized treatment planning considering both ablation depth and minimum stress.

Biologically Inspired Catheter for Endovascular Sensing and Navigation

Minimally invasive treatment of vascular disease demands dynamic navigation through complex blood vessel pathways and accurate placement of an interventional device, which has resulted in increased reliance on fluoroscopic guidance and commensurate radiation exposure to the patient and staff. Here we introduce a guidance system inspired by electric fish that incorporates measurements from a newly designed electrogenic sensory catheter with preoperative imaging to provide continuous feedback to guide vascular procedures without additional contrast injection, radiation, image registration, or external tracking. Electrodes near the catheter tip simultaneously create a weak electric field and measure the impedance, which changes with the internal geometry of the vessel as the catheter advances through the vasculature. The impedance time series is then mapped to a preoperative vessel model to determine the relative position of the catheter within the vessel tree. We present navigation in a synthetic vessel tree based on our mapping technique. Experiments in a porcine model demonstrated the sensor's ability to detect cross-sectional area variation in vivo. These initial results demonstrate the capability and potential of this novel bioimpedance-based navigation technology as a non-fluoroscopic technique to augment existing imaging methods.

An active electrolocation catheter system for imaging and analysis of coronary plaques

Coronary artery disease-currently one of the most frequent causes of death-is characterized by atherosclerotic plaques grown in the wall of blood vessels and inhibiting blood flow. Preventive assessment focusses on critical sizes of structural plaque parameters like relative lipid core area and cap thickness to identify high-risk plaques called thin cap fibroatheromas. Although state-of-the-art catheter systems were successfully applied in invasive plaque diagnostics, the high costs induced by these devices inhibit usage in daily clinical practice. To overcome this shortcoming, we follow a biomimetic approach to construct a prospective low-cost catheter system that adapts the active electrolocation principles of weakly electric fish Gnathonemus petersii. Only a few and simple parameters relevant for plaque detection and characterization are estimated from plaque-evoked electric images which are projected on the surface of the catheter. Two prototypical electrolocation catheter systems were tested. The first catheter system featured a ring electrode catheter and was used to obtain dynamic 1D electric images of synthetic plaques in an agarose atherosclerosis model. Our proof of concept showed that synthetic plaques could be reliably detected from 1D electric images. Based on a cluster analysis of selected key image features, synthetic plaques could be categorized into four plaque conditions, predefined from thresholds for critical structural parameters, representing high to low risk plaques. In the second recording approach, plaque-evoked dynamic and static spatial electric images were obtained by a multi-electrode catheter system. Based on these recordings, a synthetic plaque with a critical cap thickness could be detected and localized in a pig coronary artery.

Cellular imaging of human atherosclerotic lesions by intravascular electric impedance spectroscopy

Background

Newer techniques are required to identify atherosclerotic lesions that are prone to rupture. Electric impedance spectroscopy (EIS) is able to provide information about the cellular composition of biological tissue. The present study was performed to determine the influence of inflammatory processes in type Va (lipid core, thick fibrous cap) and Vc (abundant fibrous connective tissue while lipid is minimal or even absent) human atherosclerotic lesions on the electrical impedance of these lesions measured by EIS.

Methods and Results

EIS was performed on 1 aortic and 3 femoral human arteries at 25 spots with visually heavy plaque burden. Severely calcified lesions were excluded from analysis. A highly flexible micro-electrode mounted onto a balloon catheter was placed on marked regions to measure impedance values at 100 kHz. After paraffin embedding, visible marked cross sections (n = 21) were processed. Assessment of lesion types was performed by Movats staining. Immunostaining for CD31 (marker of neovascularisation), CD36 (scavenger cells) and MMP-3 (matrix metalloproteinase-3) was performed. The amount of positive cells was assessed semi-quantitatively. 15 type Va lesions and 6 type Vc lesions were identified. Lesions containing abundant CD36-, CD31- and MMP-3-positive staining revealed significantly higher impedance values compared to lesions with marginal or without positive staining (CD36+455±50 Ω vs. CD36- 346±53 Ω, p = 0.001; CD31+436±43 Ω vs. CD31- 340±55 Ω, p = 0.001; MMP-3+ 400±68 Ω vs. MMP-3- 323±33 Ω, p = 0.03).

Conclusions

Atherosclerotic lesions with abundant neovascularisation (CD31), many scavenger receptor class B expressing cells (CD36) or high amount of MMP-3 immunoreactivity reveal significantly higher impedance values compared to lesions with marginal or no detection of immunoreactivity. Findings suggest that inflammatory processes in vulnerable plaques affect the impedance of atherosclerotic lesions and might therefore be detected by EIS.

Electrochemical impedance spectroscopy to characterize inflammatory atherosclerotic plaques

Despite advances in diagnosis and therapy, atherosclerotic cardiovascular disease remains the leading cause of morbidity and mortality in the Western world. Predicting metabolically active atherosclerotic lesions has remained an unmet clinical need. We hereby developed an electrochemical strategy to characterize the inflammatory states of high-risk atherosclerotic plaques. Using the concentric bipolar microelectrodes, we sought to demonstrate distinct Electrochemical Impedance Spectroscopic (EIS) measurements for unstable atherosclerotic plaques that harbored active lipids and inflammatory cells. Using equivalent circuits to simulate vessel impedance at the electrode-endoluminal tissue interface, we demonstrated specific electric elements to model working and counter electrode interfaces as well as the tissue impedance. Using explants of human coronary, carotid, and femoral arteries at various Stary stages of atherosclerotic lesions (n=15), we performed endoluminal EIS measurements (n=147) and validated with histology and immunohistochemistry. We computed the vascular tissue resistance using the equivalent circuit model and normalized the resistance to the lesion-free regions. Tissue resistance was significantly elevated in the oxLDL-rich thin-cap atheromas (1.57±0.40, n=14, p<0.001) and fatty streaks (1.36±0.28, n=33, p<0.001) as compared with lesion-free region (1.00±0.18, n=82) or oxLDL-absent fibrous atheromas (0.86±0.30, n=12). Tissue resistance was also elevated in the calcified core of fibrous atheroma (2.37±0.60, n=6, p<0.001). Despite presence of fibrous structures, tissue resistance between ox-LDL-absent fibroatheroma and the lesion-free regions was statistically insignificant (0.86±0.30, n=12, p>0.05). Hence, we demonstrate that the application of EIS strategy was sensitive to detect fibrous cap oxLDL-rich lesions and specific to distinguish oxLDL-absent fibroatheroma.

Radio aneurysm coils for noninvasive detection of cerebral embolization failures: a preliminary study

The rupture of a cerebral aneurysm is the most common cause of subarachnoid hemorrhage. Endovascular embolization of the aneurysms by implantation of Guglielmi detachable coils (GDC) has become a major treatment approach in the prevention of a rupture. Implantation of the coils induces formation of tissues over the coils, embolizing the aneurysm. However, blood entry into the coiled aneurysm often occurs due to failures in the embolization process. Current diagnostic methods used for aneurysms, such as X-ray angiography and computer tomography, are ineffective for continuous monitoring of the disease and require extremely expensive equipment. Here we present a novel technique for wireless monitoring of cerebral aneurysms using implanted embolization coils as radiofrequency resonant sensors that detect the blood entry. The experiments show that commonly used embolization coils could be utilized as electrical inductors or antennas. As the blood flows into a coil-implanted aneurysm, parasitic capacitance of the coil is modified because of the difference in permittivity between the blood and the tissues grown around the coil, resulting in a change in the coil's resonant frequency. The resonances of platinum GDC-like coils embedded in aneurysm models are detected to show average responses of 224-819 MHz/ml to saline injected into the models. This preliminary demonstration indicates a new possibility in the use of implanted GDC as a wireless sensor for embolization failures, the first step toward realizing long-term, noninvasive, and cost-effective remote monitoring of cerebral aneurysms treated with coil embolization.

Electric impedance spectroscopy of human atherosclerotic lesions

OBJECTIVE

The aim of this in vitro study was to investigate the feasibility of a new highly flexible microelectrode on human tissue and its potential of differentiating atherosclerotic lesions by electric impedance spectroscopy (EIS).

METHODS

Electric impedance measurements (EIM) were performed on 148 spots of 7 aortic and 6 femoral human arteries at 1kHz, 10kHz and 100kHz.

RESULTS

According to the AHA classification 33 (25%) grade I lesions (PI), 34 (26%) grade II (PII), 21 (16%) grade III (PIII), 21 (16%) grade IV (PIV), 13 (10%) grade Va (PVa) and 10 (8%) grade Vb (PVb) could be identified by histology. At 1kHz, 10kHz and 100kHz the mean electric impedance (MEI) of PI, PII, PIII and PIV was statistically not different. At 100kHz the MEI of PVa showed significantly higher values compared to the MEI of PI (455+/-66Omega vs. 375+/-47Omega, p=0.05), PII (455+/-66Omega vs. 358+/-63Omega, p=0.007), PIII (455+/-66Omega vs. 342+/-52Omega, p=0.003), PIV (455+/-66Omega vs. 356+/-41Omegap=0.013) and the MEI of PVb was significantly increased compared to the MEI of PI (698+/-239Omega vs. 375+/-47Omega, p<0.001), PII (698+/-239Omega vs. 358+/-63Omegap<0.001), PIII (698+/-239Omega vs. 342+/-52Omegap<0.001), PIV (698+/-239Omega vs. 356+/-41Omegap<0.001), PVa (698+/-239Omega vs. 455+/-66Omega, p<0.001). Performing ROC analyses for the detection of grouped PVa/PVb lesions, the largest AUC was found at 100kHz with a cut-off value of 441Omega presenting a sensitivity of 74% and a specificity of 94%.

CONCLUSIONS

EIM could be performed on human aortic and femoral tissue. The results show that EIS has the potential to distinguish between different plaque types.

Maxi program at IEEE EMBS Student Club of Beijing University of Posts and Telecommunications

This paper presents the recently launched Maxi Program at IEEE EMBS Student Club of Beijing University of Posts and Telecommunications. The program initiates a variety of seminar series covering biomedical expertise and professional communication skills, forms a cooperative partnership between students, university and industry through guest speakers events and industry tours, and sets in motion personal consultative services (PCS) to foster the individualized competence of students. This extended program could be an innovative model of self-development as an affiliated student chapter/club with IEEE EMBS.

Influence of electrode position on the characterization of artery stenotic plaques by using impedance catheter

Use of balloon impedance catheters (BIC) for the characterization of plaques in vessels can support an optimal medical treatment of plaques. The sensitivity of impedance diagnoses with BIC is related with the distribution of electric fields determined by the electrode configuration. Using the three-dimensional finite element method (FEM) simulation, it was estimated how the relative positions of electrode array to the lipid in the vessel affect on the total impedance magnitude. Further, the short-circuiting effect was investigated with respect to the separation distance on the angular axis between the electrode arrays of angular set. By aid of FEM simulations, it is possible to design the sets of multielectrode arrays which have an optimized resolution for individual vessels.

Intravascular electric impedance spectroscopy of atherosclerotic lesions using a new impedance catheter system

Newer techniques are required to identify atherosclerotic lesions that are prone to rupture. Electric impedance spectroscopy (EIS) can characterize biological tissues by measuring the electrical impedance over a frequency range. We tested a newly designed intravascular impedance catheter (IC) by measuring the impedance of different stages of atherosclerosis induced in an animal rabbit model. Six female New Zealand White rabbits were fed for 17 weeks with a 5% cholesterol-enriched diet to induce early forms of atherosclerotic plaques. All aortas were prepared from the aortic arch to the renal arteries and segments of 5-10 mm were marked by ink spots. A balloon catheter system with an integrated polyimide-based microelectrode structure was introduced into the aorta and the impedance was measured at each spot by using an impedance analyzer. The impedance was measured at frequencies of 1 kHz and 10 kHz and compared with the corresponding histomorphometric data of each aortic segment.Forty-four aortic segments without plaques and 48 segments with evolving atherosclerotic lesions could be exactly matched by the histomorphometric analysis. In normal aortic segments (P0) the change of the magnitude of impedance at 1 kHz and at 10 kHz (|Z|(1 kHz) - |Z|(10 kHz), = ICF) was 208.5 +/- 357.6 Omega. In the area of aortic segments with a plaque smaller than that of the aortic wall diameter (PI), the ICF was 137.7 +/- 192.8 Omega. (P 0 vs. P I; p = 0.52), whereas in aortic segments with plaque formations larger than the aortic wall (PII) the ICF was significantly lower -22.2 +/- 259.9 Omega. (P0 vs. PII; p = 0.002). Intravascular EIS could be successfully performed by using a newly designed microelectrode integrated onto a conventional coronary balloon catheter. In this experimental animal model atherosclerotic aortic lesions showed significantly higher ICF in comparison to the normal aortic tissue.

Four-point electrode measurement of impedance in the vicinity of bovine aorta for quasi-static frequencies

Results are presented here of experimental measurements using a four-point electrode technique to measure the complex impedance of bovine aorta submerged in Ringer's solution. Impedance measurements were taken at 250 microm intervals, ranging from 0 (the electrode directly on the surface of the tissue) to 10 mm. Frequencies ranged from 1 kHz to 10 MHz. Throughout this range, the measured impedance changed by an average of 400% when the electrode was 10 mm from the tissue as compared to when the electrode was in direct contact with the tissue. The change in impedance made it possible to determine when the electrode made contact with the arterial wall.