Microwave spectrometry for the evaluation of in-stent neoatherosclerosis

Abstract

In this work we assess the feasibility of a method based on microwave spectrometry (MWS) to detect atherosclerotic transformation of the artery wall around an implanted stent. Four commercial coronary bare-metal stents were submitted to a staged process of increasing cholesterol deposition in order to simulate the onset and subsequent development of in-stent neoatherosclerosis (ISNA) in open air conditions. At each stage, we acquired the forward voltage gain of the stent-cholesterol sample over a full 360° rotation around an axis passing through the center of the stent and perpendicular to its longitudinal axis, in a frequency range between 1.5 and 17.5 GHz. The resulting frequency-angle absorbance diagrams show characteristic resonance frequencies that can be understood in terms of the microwave scattering produced by a dipole antenna. We found that such resonance frequencies shift down as the cholesterol mass deposited on the stent increases, reaching maximum relative values at the end of the deposition process that range from −1.10 ± 0.16% to −3.19 ± 0.10%. We ascribed this downshift behavior to an increase of the effective relative permittivity in the stent surroundings due to dielectric material perturbation. This suggests that MWS is able to detect the presence of cholesterol in a stent as a downshift of the characteristic resonance frequencies of such devices. The extension of the technique to more realistic conditions, like biological tissue or a medical phantom, should help to envisage the actual capabilities of MWS to monitor ISNA.