Recent developments in minimally invasive approach to cerebrovascular diseases include the placement of stents in arteries for treatment of aneurysms. Preliminary clinical observations and experimental studies have shown that intravascular stents traversing the orifice may lead to thrombosis and subsequent occlusion of the aneurysm. The alterations in vessel local hemodynamics due to the introduction of a stent are not yet well understood. We investigated changes in local hemodynamics resulting from stent implantation. Pulsatile flow patterns in an experimental flow apparatus were visualized using laser-induced fluorescence of rhodamine dye. The test cells were constructed in a rectangular shape to facilitate an undisturbed longitudinal view of flow patterns in parent vessel and aneurysm models with and without porous stents. Woven nitinol stents of various porosities (76%, 80%, 82%, and 85%) were investigated. The selected fluid dynamic similarity parameters (Reynolds and Womersley numbers) represented conditions usually found in high-flow, larger arteries in humans (such as the carotid artery) and low-flow, smaller arteries (such as the vertebral artery). The mean Reynolds number for the larger arteries was 180, with maximum/minimum values of 490/-30 and the Womersley number was 5.3. The mean Reynolds number for the smaller arteries was 90, with maximum/minimum values of 230/2, and the Womersley number was 2.7. For the larger arteries modeled, placement of a stent of the lowest porosity across the aneurysm orifice resulted in reduction of aneurysmal vortex speed and decreased interaction with parent vessel flow. For smaller arteries, a stent of the same porosity led to a substantial reduction of parent vessel/aneurysmal flow interaction and the appearance of a nonrecirculating crescent of fluid rich in rhodamine dye in the aneurysm dome. Our results can help explain in vivo thrombus formation within an aneurysm after placement of a stent that is compatible with local hemodynamics.

Background and Purpose-The potential for successful treatment of intracranial aneurysms by flow diversion is gradually being recognized in the clinical setting; however, the devices currently available (stents) are not designed for flow diversion. We evaluate the long-term response of an appropriately designed flow diversion device in producing thrombotic occlusion of experimental aneurysms. Methods-Three different configurations of an original flow diversion device were implanted across thirty elastaseinduced aneurysm models in rabbits. Ten animals per device configuration were followed-up for 3 weeks (nϭ3), 3 months (nϭ3), or 6 months (nϭ4), and tissue explanted postsacrifice was sent for histology. The temporal variation in angiographic contrast intensity within each aneurysm was fitted with a mathematical model to quantify the alteration in local hemodynamics caused by the implanted device. A predictive index, called the washout coefficient, was constructed to estimate long-term aneurysm occlusion probabilities immediately after treatment with any flow diversion device. Results-The device with a porosity of 70% and pore density of 18 pores/mm 2 performed better at occluding aneurysms than devices with 70% porosity, 12 pores/mm 2 and 65% porosity, 14 pores/mm 2 . A value of the washout coefficient less than 30 predicted greater than 97% angiographic aneurysm occlusion over a period of 6 months with a sensitivity of 73% and specificity of 82%. Conclusions-The flow diversion devices effected successful and stable aneurysm occlusion. Pore density, rather than porosity, may be the critical factor modulating efficacy of such devices.

It has been known for more than a decade that intracranial aneurysms can be successfully treated by deploying a porous meshed tube in the parent vessel of the aneurysm. Such devices are currently called flow diverters because they promote intraneurysmal flow stasis and thrombosis by diverting blood flow away from the aneurysm sac. The objective of this study was to use angiographic data to quantify and compare the performance of flow diverters of original design in successfully occluding an experimental aneurysm model. Three different configurations of a novel flow diverter with varying porosities and pore densities were implanted in thirty rabbit elastase-induced aneurysms. Temporal variations in angiographic contrast intensity within the aneurysms were fit to a mathematical model. Optimized model parameters were supplemented by the angiographic percentage aneurysm occlusion and an angiographic measure of device flexibility to derive composite scores of performance. Angiographic quantification further suggested a parameter, which could be employed to estimate long-term aneurysm occlusion probabilities immediately after treatment. Performance scores showed that the device with a porosity of 70% and pore density of 18 pores/mm2 performed better than devices with 65% porosity, 14 pores/mm2 and 70% porosity, 12 pores/mm2 with relative efficacies of 100%, 84%, and 76%, respectively. The pore density of flow diverters, rather than porosity, may thus be a critical factor modulating device efficacy. A value of the prognostic parameter of less than 30 predicted greater than 97% angiographic aneurysm occlusion over six months with a sensitivity of 73% and specificity of 82%.