Scanning electron microscopy for flow-diverting stent research: technical tips and tricks

Role of terminal and anastomotic circulation in the patency of arteries jailed by flow-diverting stents: animal flow model evaluation and preliminary results

OBJECTIVE

The authors describe herein the creation of an animal model capable of producing quantifiable data regarding blood flow rate and velocity modifications in terminal and anastomotic types of cerebrofacial circulation. They also present the preliminary results of a translational study aimed at investigating the role of terminal and anastomotic types of circulation in arterial branches jailed by flow-diverting stents as factors contributing to arterial patency or occlusion.

METHODS

Two Large White swine were used to validate a terminal-type arterial model at the level of the right ascending pharyngeal artery (APhA), created exclusively by endovascular means. Subsequently 4 Large White swine, allocated to 2 groups corresponding to the presence (Group B) or absence (Group A) of terminal-type flow modification, underwent placement of flow-diverting stents. Blood flow rates and velocities were quantified using a dedicated time-resolved 3D phase-contrast MRA sequence before and after stenting. Three months after stent placement, the stented arteries were evaluated with digital subtraction angiography (DSA) and scanning electron microscopy (SEM). Patent (circulating) ostia quantification was performed on the SEM images.

RESULTS

Terminal-type flow modification was feasible; an increase of 75.8% in mean blood velocities was observed in the right APhAs. The mean blood flow rate for Group A was 0.31 ± 0.19 ml/sec (95% CI −1.39 to 2.01) before stenting and 0.21 ± 0.07 ml/sec (95% CI −0.45 to 0.87) after stenting. The mean blood flow rate for Group B was 0.87 ± 0.32 ml/sec (95% CI −1.98 to 3.73) before stenting and 0.76 ± 0.13 ml/sec (95% CI −0.41 to 1.93) after stenting. Mean flow rates after stenting showed a statistically significant difference between Groups A and B (Welch test). Mean and maximal blood velocities were reduced in Group A cases and did not decrease in Group B cases. Control DSA and SEM findings showed near occlusion of the jailed APhAs in both cases of anastomotic circulation (mean patent ostium surface 32,776 μm2) and patency in both cases of terminal-type circulation (mean patent ostium surface 422,334 μm2).

CONCLUSIONS

Terminal-type arterial modification in swine APhAs is feasible. Sufficient data were acquired to perform an a priori analysis for further research. Flow diversion at the level of the APhA ostium resulted in significant stenosis in cases of anastomotic circulation, while sufficient patency was observed in terminal-type circulation.

Role of terminal and anastomotic circulation in the patency of arteries jailed by flow-diverting stents: from hemodynamic changes to ostia surface modifications

OBJECTIVE The outcome for jailing arterial branches that emerge near intracranial aneurysms during flow-diverting stent (FDS) deployment remains controversial. In this animal study, the authors aimed to elucidate the role of collateral supply with regard to the hemodynamic changes and neointimal modifications that occur from jailing arteries with FDSs. To serve this purpose, the authors sought to quantify 1) the hemodynamic changes that occur at the jailed arterial branches immediately after stent placement and 2) the ostia surface values at 3 months after stenting; both parameters were investigated in the presence or absence of collateral arterial flow. METHODS After an a priori power analysis, 2 groups (Group A and Group B) were created according to an animal flow model for terminal and anastomotic arterial circulation; each group contained 7 Large White swine. Group A animals possessed an anastomotic-type arterial configuration to supply the territory of the right ascending pharyngeal artery (APhA), while Group B animals possessed a terminal-type arterial configuration to supply the right APhA territory. Subsequently, all animals underwent FDS placement, thereby jailing the right APhAs. Mean flow rates and velocities inside the jailed branches were quantified using time-resolved 3D phase-contrast MR angiography before and after stenting. Three months after stent placement, the jailed ostia surface values were quantified on scanning electron micrographs. The data were analyzed using descriptive statistics and group comparisons with parametric and nonparametric tests. RESULTS The endovascular procedures were feasible, and there were no findings of in situ thrombus formation on postprocedural optical coherence tomography or ischemia on postprocedural diffusion-weighted imaging. In Group A, the mean flow rate values at the jailed right APhAs were reduced immediately following stent placement as compared with values obtained before stent placement (p = 0.02, power: 0.8). In contrast, the mean poststenting flow rates for Group B remained similar to those obtained before stent placement. Three months after stent placement, the mean ostia surface values were significantly higher for Group B (527,911 ± 306,229 μm²) than for Group A (89,329 ± 59,762 μm²; p < 0.01, power: 1.00), even though the initial dimensions of the jailed ostia were similar between groups. A statistically significant correlation was found between groups (A or B), mean flow rates after stent placement, and ostia surface values at 3 months. CONCLUSIONS When an important collateral supply was present, the jailing of side arteries with flow diverters resulted in an immediate and significant reduction in the flow rate inside these arteries as compared with the prestenting values. In contrast, when competitive flow was absent, jailing did not result in significant flow rate reductions inside the jailed arteries. Ostium surface values at 3 months after stent placement were significantly higher in the terminal group of jailed arteries (Group B) than in the anastomotic group (Group A) and strongly correlated with poststenting reductions in the velocity value.

Intravascular optical coherence tomography for the evaluation of arterial bifurcations covered by flow diverters

Background and objective

Due to its high spatial resolution, intravascular optical coherence tomography (OCT) has been used as a valid method for in vivo evaluation of several types of coronary stents at straight lumen and bifurcation sites. We sought to evaluate its effectiveness for flow diverting stents deployed in arterial bifurcation sites involving jailing of a side branch.

Methods

Four large white swine were stented with flow diverting stents covering the right common carotid artery–ascending pharyngeal artery bifurcation. After 12 weeks of follow-up the animals were evaluated by digital subtraction angiography and intravascular OCT and subsequently sacrificed. Neointimal thickness on the parent arteries and the free segments of the stent were measured. The stented arteries were harvested and underwent scanning electron microscopy (SEM) imaging. Ostia surface values were measured with OCT three-dimensional (3D) reconstructions and SEM images.

Results

All endovascular procedures and OCT pullback runs were feasible. Stent apposition was satisfactory on the immediate post-stent OCT reconstructions. At 12-week controls, all stents and jailed branches were patent. Mean neointimal thickness was 0.11±0.04 mm on the free segments of the stent. The mean ostia surface at 12 weeks was 319 750±345 533 μm2 with 3D-OCT reconstructions and 351 198±396 355 μm2 with SEM image-derived calculations. Good correlation was found for ostia surface values between the two techniques; the values did not differ significantly in this preliminary study.

Conclusions

Intravascular OCT appears to be a promising technique for immediate and follow-up assessment of the orifice of arterial branches covered by flow diverting stents.