3-D printed flow-model tests of novel retrievable asymmetric stent flow diverters
Yoganand, Aradhana Ashok
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This research introduces a novel device for the treatment of intra-cranial aneurysms (ICAs). Although the traditional endovascular devices to treat cerebral aneurysms such as coils and intracranial stents have served well, they still remain sub-optimal. Therefore the concept of flow diverters is introduced to achieve blocking of the aneurysm orifice while maintaining the parent vessel patent. Currently, the uniform flow diverters are commercially in use. Even though they are minimally invasive, they lack repositioning ability once deployed. This results in intra-procedural complications and perforator occlusions. The new device developed at the Toshiba Stroke and Vascular Research Center, affiliated to University at Buffalo offers an asymmetric design model. This design consists of a solid polymer region responsible for flow diversion/deflection and a high porosity zone that offers mechanical support to the vessel and maintains patent vessel branches or perforators. The most critical challenge that one encounters with an asymmetric flow diverter, is its ability to reposition. This current fabricated new fully retrievable asymmetric flow diverter (RAFD) offers a 100 % retrievability even after complete deployment, thus allows convenient repositioning of the device. A number of trials were conducted to test for its retrievability and the latest record of frictionless recapture or crimping was achieved in a 2F neuro micro-catheter with no polymer delamination. The greatest hurdle was to achieve a thin solid uniform polymer patch that could be safely and effectively reseathed. The most effective polymer patch weighed only 29 milligrams and was between 10-15 micrometers thick covering three-quarters of the stent's circumference at the aneurysm's neck. In order to supplement the device evaluation, flow studies were conducted in a carotid relevant clinical condition. Both simple side-wall phantom and 3D printed patient specific vessels were treated using the device prototypes. The study was conducted using Digital Subtraction Angiography (DSA) guidance. The prototypes were also tested with platinum markers in order to locate the patch facing the aneurysm orifice. DSA maps obtained using parametric imaging were used to study the consequent effects of the prototype-placement on flow dynamics in the aneurysm sac. This was performed by analyzing the contrast behavior at each pixel in an angiographic image sequence while the RAFD was placed at different anatomical positions with reference to the aneurysm orifice ie. Proximal, distal, middle, center-left and center right positions. This revealed information about bolus jet dispersion and inception of coagulation in the aneurysm sac. Comparison between treated and untreated (control) maps quantified the RAFD positioning effect. Average MTT (Mean Transit Time Arrival), related to contrast presence in the aneurysm dome increased, indicating flow decoupling between the aneurysm and parent artery. Maximum effect was observed in the center and proximal position (∼75%) of aneurysm models depending on their geometry. BAT (Bolus Arrival Time) maps, correlated well with inflow jet direction and magnitude. Thus, the reduction and jet dispersion were recorded to be as high as about 50% for various treatments. Thus, the new RAFD prototypes have great potential to replace the conventional flow diverter in the long run and bring forth a much safer treatment technology. The results obtained during these studies have been published and are available in the SPIE Journal Proceedings of Medical Imaging 2015, Volume 9417.