Imaging protocols

Three types of coronary stents of different metal composition: Cobalt-Chromium (Stent1), Platinum (Pt)-Chromium (Stent2), and Stainless steel with gold (Au) markers (Stent3) (see Table 4) were imaged using the SPCCT prototype, the IQon and the B64 systems in vitro, and the SPCCT and the B64 in vivo, all with 120 kVp and 100 mAs tube settings. IQon and SPCCT have same tube filtration while B64 has additional Ti filter resulting with 25–30% lower dose.

Description of the three types of coronary stents used in the study.

Tsunami Gold

(Terumo Europe NV)

On the B64 and IQon, the acquisitions were performed in spiral mode using 64 × 0.625 mm collimation, gantry rotation time of 1 second, pitch of 0.56, and a large focal spot on the X-ray tube (standard resolution mode). On the SPCCT, the acquisitions were performed in axial step and shoot mode with 9 × 0.274 mm collimation, rotation time of 1 second and with small focal spot on the X-ray tube (high Resolution mode) to benefit from the potential spatial resolution improvement allowed by the small detector size.

The reconstructed FOV and voxel size were: 160 mm and 0.2 × 0.2 in plane pixel size and slice thickness of 0.25 mm on SPCCT using a “detailed” filter; and 154 mm and 0.2 × 0.2 in plane pixel size, effective slice thickness of 0.67 mm and increment of 0.33 mm on IQon and B64, using the YA sharp recon kernel. For the SPCCT the photon counting thresholds were adjusted to allow for simultaneous detection of iodinated contrast material and platinum K-edge.

Reconstructions of SPCCT acquired data were performed using maximum-likelihood processing method of counting data to compute several basis material sinograms (water, Iodine, and Platinum)²⁷. The individual sinograms are then reconstructed to obtain conventional HU images, water and Iodine material density maps and Platinum specific K-edge images.

The stents were deployed inside individual plastic tubes (internal lumen diameter of 3.5 mm). Presence of calcification was simulated by using a mixture of Calcium Phosphate powder (25%), flour (45%), and water (30%). Small spheres (1–2 mm in diameter) were obtained from the mixture and allowed to dry for 24 hours and were then placed inside the plastic tubes before stent deployment. To assess the impact of the presence of vascular contrast agents on stent imaging, tubes were subsequently filled with water and with a standard iodinated contrast agent (a concentration of ~11 mg/ml of elemental iodine (I)). The entire length of the stents was imaged for each type of filling on the three systems.

This study was approved by the local Institutional Animal Care and Use Committee (Council Directive No. 2010/63/UE on the protection of animals used for a scientific purpose) under the authorization n° APAFIS#1732-2015091411181645v3 and performed under relevant guidelines and regulations.

Similar stents were sequentially deployed in the abdominal aorta of one New Zealand White rabbit (NZW, male, 3.2 kg) while under full anesthesia achieved with an intramuscular injection of medetomidine (0.4 mg/kg) and ketamine (20 mg/kg). An incision was performed at the level of the left hind paw to reach the iliac artery. The guidewire supporting the stent was then inserted in the artery up to the abdominal aorta at the level of the diaphragm and the first stent was deployed, followed by the other two stents. Stent placement was verified by DSA. During the procedure, the animal’s heart rate and oxygen saturation was monitored. At the end of the procedure the incision was sutured and anti-inflammatory cream was applied locally and the animal was allowed to recover for 2 weeks before the first imaging session.

The animal underwent two imaging sessions, one on SPCCT and one on B64. On SPCCT, cross-sectional views of each stent were acquired before and after manual intravenous injection of a standard iodinated contrast agent (400 mg/ml of elemental I, IOMERON, Bracco, Italy). The three stents were imaged individually, each one approximately 5 seconds after the administration of 5 ml of IOMERON, equivalent to a high clinical dose of 1.6 ml/kg, with at least a 5 minute interval between the injections, to allow the blood concentration of iodine to decrease. On B64, one injection of 5 ml was performed and the entire length of the abdominal aorta was imaged approximately 5 seconds after the manual injection.