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

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-Z metal stents under X-ray imaging, by combining high-Z metal 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-µm; 10 m 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 (50-µm) with W coatings (2-3 µm-thick) were fabricated by magnetosputtering. These samples were X-ray visualised, for quantification of contrast between W-coated and uncoated Fe samples. Fe struts (50-µm) were also coated with W (3.8 ± 0.2 µm), and stent-like objects were X-ray visualised.

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 (580  5 % increase). The dose was also strongly reduced (70 and 75%, for 80 and 120 kVp beams). Finally, for 3D Fe stents visualised at 80 kVp, the highest CNR and CNRD values were achieved with 0.1 mm Sn (18.5 x and 20.1 mGy⁻¹; compared to 15.0 x and 12.0 mGy⁻¹ in 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.