Low dose contrast enhancement of biodegradable low-density stents by an approach balancing radiopaque coatings and beam filtration

Abstract

Objective.Biodegradable cardiovascular stents made of thin, low atomic number metals (e.g. Zn, Mg, Fe) are now approved for clinical use. However, poor contrast under x-ray imaging leads to longer surgical times, high patient exposure, and sometimes stent misplacement. This study aimed at enhancing the visibility of low-Zmetal stents under x-ray imaging, by combining high-Zmetal coatings and beam filtration.Approach.Photon energy spectra from W-anode x-ray beams operated at 80 and 120 kVp, were generated by the SpekCalc and BEAMnrc softwares. The contrast produced by Fe stent struts (50-10μm W coatings), as well as dose and air kerma values (by BEAMnrc), were simulated. Several types of beam hardening filters (Sn: 0.1, 0.2 mm; Cu: 0.2, 0.7 mm) were also applied. Then, Fe foils (50µm) with W coatings (2-3µm-thick) were fabricated by magnetosputtering. These samples were x-ray visualized, for quantification of contrast between W-coated and uncoated Fe samples. Fe struts (50µm) were also coated with W (3.8 ± 0.2µm), and stent-like objects were x-ray visualized.Main results.Fe samples attenuate 6.4% (120 kVp) and 10.1% (80 kVp) spectra photons, and 25% and 34.5% for W-coated Fe samples (SpekCalc). BEAMnrc calculations revealed the highest contrast improvement in a 120 kVp beam (36.4%, and 38.5%) for W-coated and uncoated Fe samples with Sn (0.2 mm), and Cu + Sn (0.2 + 0.2 mm) filters. Experimentally, the highest contrasts between Fe and W-Fe foils, were obtained with 0.2 mm Sn (77 ± 7% contrast increase at 80 kV). The dose was also strongly reduced (70% and 75%, for 80 and 120 kVp beams). Finally, for 3D Fe stents visualized at 80 kVp, the highest CNR and CNRD values were achieved with 0.1 mm Sn (18.5 × and 20.1 mGy^-1; compared to 15.0 × and 12.0 mGy^-1in no-filter condition).Significance.The contrast of Fe-based stents in x-ray imaging is improved by addition of a thin layer of W and beam filtration with Sn. The precision and rapidity of biodegradable stents implantation would be improved thereby, as well as the dose to patients.

Keywords: Monte-Carlo simulations; X-ray dose; beam hardening; contrast in x-ray imaging; fluoroscopy; radiopaque endovascular devices; stents.