Woowon Jeong

Hemodynamics of cerebral aneurysms: computational analyses of aneurysm progress and treatment.

The progression of a cerebral aneurysm involves degenerative arterial wall remodeling. Various hemodynamic parameters are suspected to be major mechanical factors related to the genesis and progression of vascular diseases. Flow alterations caused by the insertion of coils and stents for interventional aneurysm treatment may affect the aneurysm embolization process. Therefore, knowledge of hemodynamic parameters may provide physicians with an advanced understanding of aneurysm progression and rupture, as well as the effectiveness of endovascular treatments. Progress in medical imaging and information technology has enabled the prediction of flow fields in the patient-specific blood vessels using computational analysis. In this paper, recent computational hemodynamic studies on cerebral aneurysm initiation, progress, and rupture are reviewed. State-of-the-art computational aneurysmal flow analyses after coiling and stenting are also summarized. We expect the computational analysis of hemodynamics in cerebral aneurysms to provide valuable information for planning and follow-up decisions for treatment.

Drug perfusion enhancement in tissue model by steady streaming induced by oscillating microbubbles

Drug delivery into neurological tissue is challenging because of the low tissue permeability. Ultrasound incorporating microbubbles has been applied to enhance drug delivery into these tissues, but the effects of a streaming flow by microbubble oscillation on drug perfusion have not been elucidated. In order to clarify the physical effects of steady streaming on drug delivery, an experimental study on dye perfusion into a tissue model was performed using microbubbles excited by acoustic waves. The surface concentration and penetration length of the drug were increased by 12% and 13%, respectively, with streaming flow. The mass of dye perfused into a tissue phantom for 30s was increased by about 20% in the phantom with oscillating bubbles. A computational model that considers fluid structure interaction for streaming flow fields induced by oscillating bubbles was developed, and mass transfer of the drug into the porous tissue model was analyzed. The computed flow fields agreed with the theoretical solutions, and the dye concentration distribution in the tissue agreed well with the experimental data. The computational results showed that steady streaming with a streaming velocity of a few millimeters per second promotes mass transfer into a tissue.

The hemodynamic alterations induced by the vascular angular deformation in stent-assisted coiling of bifurcation aneurysms

The hemodynamic changes induced by stent deployment and vascular remodeling in bifurcation aneurysms were investigated using computational fluid dynamics. The stent deployment reduced the intra-aneurysmal flow activity by decreasing the mean velocity, mean kinetic energy, mean wall shear stress, and mean vorticity. These hemodynamic parameters increased with an increase in the branching angle because of the vessel deformation caused by stent straightening. The maximum wall shear stress and its spatial gradient occurred near the neck of the aneurysm in the stented left daughter vessel, whereas a maximum oscillatory shear index was detected near the neck of the right aneurysm of the right daughter vessel. Theses parameters, which might be related to the recurrence of aneurysms, were also increased by stent-induced vessel deformation.

Effects of framing coil shape, orientation, and thickness on intra-aneurysmal flow

To study the effects of the geometrical characteristics of a framing coil on aneurysm thromboembolization efficacy, the hemodynamics in lateral aneurysms filled with coils having a different shape, orientation, and thickness were analyzed using computational fluid dynamics. The aneurysms packed with vortex and cage-shaped coils were modeled using three different coil orientations: transverse, parallel, and orthogonal. The orthogonal orientation of a vortex coil and parallel orientation of a cage-shaped coil showed higher inflow, vorticity, and wall shear stress in the dome region, which provide an unfavorable hemodynamic environment for thromboembolization. Thicker coils also produced unfavorable hemodynamic conditions compared to normal coils having the same shape, orientation, and total coil volume. Though the effects of coil shape and orientation on the hemodynamic parameters of interest were not consistent, the open area at the distal half of the mid-transverse plane of an aneurysm showed significant positive correlation with flow into the dome region and mean vorticity in the dome region. Therefore, blocking the distal mid-transverse plane of an aneurysm using coils would effectively reduce the intra-aneurysmal flow activity and provide a more efficient hemodynamic environment for thromboembolization.

Computational study of particle size effects on selective binding of nanoparticles in arterial stenosis

In order to elucidate particle size and wall shear effects on the selective binding of nanoparticles to vessel wall, particle binding to the wall of arterial stenosis was computationally analyzed using a transport and reaction model. The attachment rate constant was modeled as a function of shear rate and particle size. The results showed that it had a positive correlation with the shear rate for particles smaller than 600 nm and a negative correlation with the shear rate for particles larger than 800 nm. Small size particles showed high binding selectivity in the stenosis region for the normal and shear-activated wall, whereas large particles showed high binding selectivity in the low and oscillatory zone for the shear-activated wall.

Polymeric Embolization Coil of Bilayered Polyvinyl Alcohol Strand for Therapeutic Vascular Occlusion: A Feasibility Study in Canine Experimental Vascular Models

PURPOSE To investigate the feasibility of polyvinyl alcohol (PVA) polymer coil as a new endovascular embolic agent and to gauge the related histologic response in a canine vascular model. MATERIALS AND METHODS PVA polymer coil was fabricated by cross-linking PVA and tantalum particles. Basic properties were then studied in vitro via swelling ratio and bending diameter. Normal renal segmental arteries and wide-necked aneurysms of carotid sidewalls served as canine vascular models. Endovascular PVA coil embolization of normal renal segmental arteries (N = 20) and carotid aneurysms (N = 8) was performed under fluoroscopic guidance in 10 dogs. Degree of occlusion was assessed immediately and at 4 weeks after embolization by conventional and computed tomographic angiography. Histologic features were also graded at acute (day 1, six segmental arteries and four aneurysms) and chronic phases (week 4, 14 segmental arteries and four aneurysms) after embolization to assess inflammation, organization of thrombus, and neointimal proliferation. RESULTS Swelling ratio declined as concentrations of cross-linking agent increased. Mean bending diameters were 2.05 mm (range, 0.86-6.25 mm) in water at 37 °C and 2.29 mm (range, 0.94-6.38 mm) in canine blood samples at 37 °C. Occlusion of normal renal segmental arteries was sustained (complete occlusion at day 1, n = 20; at week 4, n = 14), whereas immediate outcomes in carotid aneurysms (day 1, complete occlusion, n = 5; residual neck only, n = 3) were not sustained (week 4, complete occlusion, n = 1; minor recanalization, n = 1; major recanalization, n = 2). At week 4, chronic inflammatory cells predominated, with progressive organization of thrombus and fibrocellular ingrowth. All aneurysms bore full neointimal linings on the coil mass in the chronic phase. CONCLUSIONS Vascular occlusion by PVA polymer coil proved superior in normal renal segmental arteries and feasible in surgically constructed carotid aneurysms (with packing densities ≥ 30%), constituting acceptable radiologic feasibility and histologic response.

Computational Study on Aneurysm Hemodynamics Affected by a Deformed Parent Vessel after Stenting

To investigate the hemodynamic alterations of a deformed parent vessel after stenting, flow field change after parent vessel stenting was analyzed using computational fluid dynamics. Effects of branch angle change in the vessel bifurcation after stenting on hemodynamic parameters were considered. The results showed that inflow rate, mean velocity, and mean kinetic energy in an aneurysm decreased in the stented vessel comparing to those in the vessel without a stent, which showed flow diversion effects of a stent. Inflow rate, mean velocity, mean kinetic energy in an aneurysm, and maximum wall shear stresses in the parent vessel and in the aneurysm dome increased in the deformed vessel model due to branching angle increase. Parent vessel deformation after stenting should be considered because it could provide unfavorable hemodynamic environment for aneurysm embolization.

In vitro phase-contrast magnetic resonance investigation on development of human carotid sinus in young age

The morphology of adult carotid sinus and atherosclerosis are major risk factors for ischemic stroke. It has been reported that the function of carotid sinus is pressure sensing and the regulation of heart rate. Seong and colleagues reported the lifelong development of carotid sinus morphology from infancy to maturity using post-image processed digital subtraction angiograms from 36 patients, ages from newborn to 36 years old [1]. The study unveiled the substantial growth of the internal carotid artery (ICA) with aging and the development of a carotid sinus at the root of the ICA during late adolescence. However, the development of the carotid sinus with associated hemodynamics is not clearly understood yet.Copyright

Effects of continuous and intermittent injection of thrombolytic agents on clot dissolution

In order to dissolve a blood clot and restore the patency of a blood vessel, various treatments have been used. Direct or intravenous injection of thrombolytic agents, such as tissue plasminogen activator (tPA), urokinase(uPA), streptokinase (SK), has been used for the treatment of thrombosis. Direct injection of thrombolytic agents to the clot may increase the effectiveness of thrombolysis by enhancing the permeation of a thrombolytic agent into the blood clot. Injection velocity and methodology, such as continuous infuson and pulsed injection, would affect the thrombolytic efficiecy. In order to explore the effectivenesss of injection methods, we modeled clot dissolution numerically. Species transport equation was solved along with three dimensional momentum equations. The blood clots were modeled as prorous media. Pressure, velocity and species concentration fields were calculated by computational fluid mechanics methods. Two different thrombus models - arterial and venous thrombus - were simulated. The results showed that thrombolytic speed increased as the injection velocity increased, and intermittent injection was more efficient in dissolving clots comparing to comtinuous perfusion.