A method for the development of an effective flow diverting device for the treatment of cerebral aneurysms
2013-09
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A method for the development of an effective flow diverting device for the treatment of cerebral aneurysms
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2013-09
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Intracranial aneurysms are malformations that occur in the complex network of blood vessels supplying oxygen and nutrients to the brain. Weakening of the blood vessel wall leads to a bulge that ruptures in more than 30 000 Americans every year. Prognosis is very poor. Patients often die or suffer a greatly reduced quality of life. Two predominant methods for treating aneurysms are (1) surgical clipping, where part of the skull is temporarily removed and a metallic clip is placed to circumvent the aneurysm neck, and (2) coiling, where metallic coils are snaked through the blood vessels and packed into the aneurysm.For large aneurysms or those with poorly defined necks, a new class of medical device has recently emerged as a more effective treatment than coiling. A flow diverter is placed inside the parent vessel, spanning the aneurysm neck. The diverter's braided structure keeps most of the blood from entering the aneurysm. The risk of rupture is eliminated when stagnant pools of blood thrombose inside the aneurysm, cutting the aneurysm off from the rest of the circulatory system. However, complications related to the presence of flow diverters are observed clinically. Aneurysms with incomplete clot formation after placement of the flow diverter are still at risk of rupture. The high metallic content of the device presents a risk of in-stent thrombosis and require a lifetime of anti-coagulants for its management. Subarachnoid hemorrhage after placement of the flow diverter is observed, but the underlying mechanism is not well understood. Therefore, a greater understanding of the fluid mechanics underlying flow diversion is needed to facilitate the design of the next generation of flow diverters. Research was pursued in three parallel synergistic paths. (1) Benchtop experiments using a technique called particle imaging velocimetry (PIV) were used to characterize the flow diversion accorded by the Pipeline Embolization Device (PED, designed by Covidien) in a variety of geometries. (2) The computational fluid dynamic (CFD) simulation methods were verified with PIV results, and then applied towards a wider range of vessel geometries to predict how the PED will perform at various locations of the human neurovasculature. (3) Animal studies were pursued to develop surgical techniques for device evaluation in the future. The implementation of PIV was found to be a labor and computationally intensive process. Previous researchers who have used PIV to experimentally investigate the flow diverting effect of the device occasionally interrogated the fluid domain at several planes, but typically only at the center plane bisecting the aneurysm. This limited information was found to be insufficient for verification of CFD simulations or to calculate bulk properties such as flow rate of fluid entering the aneurysm. Evaluation of intraaneurysmal flow was also found to be problematic after placement of the flow diverter. The significantly reduced flow highlighted the difference in densities between the seeded reflective particles and the flowing fluid. Particles also accumulated on the glass model wall in regions of low flow. These complications introduced challenges to the PIV measurement technique.Detailed sets of PIV results were collected in three flow domains by interrogating the flow at parallel planes 400 microns apart. The flow rates of fluid entering the aneurysm before (QUT) and after (QT) placement of the flow diverters were calculated. CFD simulations were conducted with the openings, or pores, of the PED modeled as an array of diamond shaped pores connecting the aneurysm to the parent artery. Since the deployed shape of the PED was variable and depended on the deployment technique, simulations with different diamond pore dimensions were conducted. The QT and QUT values predicted from CFD were in reasonable agreement with the PIV results.CFD simulations were then conducted in an array of idealized blood vessel geometries that typified a portion of the vessel curvatures found in the human neurovasculature. It was discovered that the performance of the PED varied depending on the curvature of the parent vessel, the location of the aneurysm along the curve, and the geometry of the aneurysm neck. The claim of "85% reduction in circulation" made by Covidien (who designed the PED) is a somewhat ambiguous statement. An ~85% reduction in vorticity was observed on the center planes of the aneurysms evaluated in this research effort, but the reduction in flow rate entering the aneurysm was on average only around 65%, and dipped as low as 50% in the most tortuous bends. However, the shapes of the deployed PED, the vessel geometries, and inlet conditions examined in this thesis may have been different than those used to substantiate Covidien's marketing claim. The term "circulation" was also not defined in Covidien's literature. Further research is needed to identify the source of this discrepancy.The present research also provides insight into the fluid mechanics of blood entering aneurysms created in a rabbit model. Residence time was defined as the volume of the aneurysm divided by the flow rate of blood entering the aneurysm. Blood velocities acquired using an ultrasound probe were used as input to CFD simulations. The varying volumes of the aneurysms and the varying angles of the aneurysms relative to their parent arteries led to residence times that varied from rabbit to rabbit. Knowledge of the initial flow conditions is important for an apples to apples comparison of new flow diverter designs. More animal studies combined with clinical data of the PED are needed to determine the minimum threshold in flow reduction, the minimum residence time, or some other metric that will predict healing of the aneurysm. In summary, a comprehensive platform of evaluation techniques was developed and implemented for use in optimizing the design of the next generation of flow diverters. The reduction in flow entering the aneurysm after placement of the PED was found to be less than the claimed "reduction in circulation" and presents an opportunity for a flow diverter that restricts flow more severely. Moving from a metallic braid to a polymeric stent graft platform would allow for easier manipulation of flow diversion characteristics while taking into account other design requirements such as device stiffness, force required to advance it through the catheter, radiopacity, thrombogenicity, stent migration, and others. A better understanding of the underlying mechanism by which flow diverters heal aneurysms will lead to wider adoption and on-label use (officially approved by the European Commission and the Food and Drug Administration) of this class of device as a first-line treatment for all aneurysms.
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University of Minnesota Ph.D. dissertaion. September 2013. Major: Mechanical Engineering. Advisor: Eph Sparrow, PhD. 1 computer file (PDF); xx, 264 pages, appendices A-C.
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Chow, Ricky. (2013). A method for the development of an effective flow diverting device for the treatment of cerebral aneurysms. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/162930.
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