Endovascular Repair of the Aorta: flow quantification of limb hemodynamic parameters
Hadi Mirgolbabaee is a PhD student in the Department Multi-Modality Medical Imaging. (Co)Promotors are prof.dr. M. Versluis, prof.dr. M.M.P.J. Reijnen and dr. E. Groot Jebbink from the Faculty of Science & Technology.
Endografts used in the treatment of abdominal aortic aneurysms (AAA) have been developed and optimized over the past decades. The primary goal of these endografts is to relieve pressure from the weakened vessel wall by preserving natural aortic flow patterns and excluding blood flow from the aneurysm sac, thereby significantly reducing the risk of rupture. While both open surgical repair (OSR) and endovascular aneurysm repair (EVAR) offer distinct advantages and disadvantages in treating AAA, both suffer from post-operative complications, with EVAR being associated with a higher rate of post-operative complications compared to OSR. Therefore, it is crucial to evaluate the performance of endografts and determine whether they function as intended. This thesis evaluates the performance of endografts by investigating hemodynamic factors that may correlate with limb thrombosis (LT) after EVAR, using several in vitro studies. The hypothesis is that by quantifying local disturbed flow patterns known to increase thrombus formation potential, such as local recirculation zones and complex trapped regions where blood flow is confined to areas near or at the flow lumen, potential regions that may lead to LT can be identified.
Throughout this thesis, various components essential for designing an in-vitro ultrasound particle image velocimetry (echoPIV) flow setup were elaborated, ranging from phantom fabrication to hemodynamic parameter quantification. The thesis begins with a systematic review (Chapter 2) to evaluate the existing flow phantoms and benchtop experiments from published in-vitro studies designed to assess challenging EVAR procedures. These studies focused on treating juxtarenal abdominal aortic aneurysms (jAAAs) using fenestrated endovascular aortic repair (FEVAR), chimney endovascular aortic repair (CHEVAR), and chimney endovascular aortic sealing (CHEVAS). Our review highlighted the in-vitro testing characteristics of available studies up to 2021, comprising 7 FEVAR and 11 parallel graft (PG)-based technique studies, i.e., CHEVAR and CHEVAS. Moreover, the anatomical designs of the phantoms used in those studies were categorized as "simplified", "generalized", and "patient-specific" each with their advantages and disadvantages that were discussed in detail. The review recommended that "simplified" designs should be avoided in future studies due to their poor resemblance to real-life conditions. Furthermore, it was proposed that "generalized" flow phantoms should be enhanced to better represent complex anatomies, suggesting the development of advanced averaging techniques, rather than relying on morphometric protocols based on limited anatomical landmarks. Lastly, several recommendations were provided for optimizing future in-vitro studies, including the design of suitable flow setups and the fabrication of realistic phantoms with biomechanical properties that more closely resemble those of the vessel wall.
In Chapter 3, a framework for designing “generalized” AAA phantoms is introduced, demonstrated by creating a cohort-based (n=50) average AAA flow phantom. This framework is not limited to AAA anatomies and can be adapted for other vasculatures, as it only requires the segmented vasculature and its corresponding center lumen lines. A comparison between a patient-specific model generated by this framework and the corresponding reference segmentation revealed differences of 5-10% of the lumen diameter between the two meshes, highlighting its high accuracy compared to traditional averaging protocols that rely on limited diameter measurements at specific anatomical landmarks, e.g., averaging only anterior-posterior and left-right diameters. Additionally, a fabrication method using 3D printing with commercially available Flexible 80A resin (Formlabs, Somerville, MA, USA) was proposed, which proved to be suitable for echoPIV imaging. Overall, the presented flow phantom design and fabrication framework provides an accessible, cost-effective approach for future research to create and optimize phantoms for in-vitro studies, develop vascular models for computational fluid dynamics (CFD) simulations, and to fabricate flow phantoms for clinical training and educational purposes.
Chapter 4 addresses the challenges in fabricating arterial flow phantoms suitable for both echoPIV and optical PIV (laserPIV) imaging. In this study, we present a series of thin-walled, 3D-printed compliant phantoms using commercially available Flexible 80A and Elastic 50A resins designed for dual-modality PIV flow imaging, i.e., suitable for both laserPIV and echoPIV. To overcome the optical opacity and poor surface of flexible and elastic resins, a photo-activated polymeric coating layer was applied. An in-vitro study was then conducted by prescribing a steady-state, fully developed flow field inside the coated 3D-printed pipe phantoms made from both resin types to evaluate the feasibility of dual-modality PIV flow imaging. The results showed that the average normalized root mean square errors, comparing laserPIV and echoPIV velocity profiles against analytical solutions, were 3.2% and 5.1% for the flexible phantoms, and 3.3% and 5.3% for the elastic phantoms, respectively. These findings indicate that dual-modality PIV flow imaging is feasible in the proposed 3D-printed coated phantoms, highlighting its potential for fabricating clinically relevant flow phantoms for future in-vitro studies.
In Chapters 5 to 7, several in-vitro studies were conducted using echoPIV to quantify post-EVAR 2D flow fields in the common iliac arteries. These studies aimed to provide critical insights into flow patterns by quantifying limb hemodynamic parameters, i.e, residence time (RT), vector complexity (VC), and in-plane vortical structures (VS) that represent recirculation zones. These hemodynamic parameters allowed us to identify local disturbed flow regions with potentially unfavorable hemodynamics, which may be associated with LT. The first study (Chapter 5) was conducted using a patient-specific phantom fabricated based on the aortic anatomy of a patient treated with an Anaconda endograft, who developed left-sided LT. The aim was to evaluate the feasibility of echoPIV not only in assessing flow within Anaconda limb grafts but also in determining whether the proposed hemodynamic parameters could pinpoint disturbed flow in the thrombosed limb, i.e., left limb in this case. The results indicated an overall increase in VC after the insertion of the Anaconda endograft, with the anterior wall regions of both left and right iliac arteries showing a two- and four-fold increase in VC, respectively. Additionally, a higher RT was observed after implementing the Anaconda endograft, within the left limb, which developed LT during follow-up, having two trapped zones identified as disturbed flow regions. Thus, in this case study, echoPIV was proven to be a promising tool for predicting the potential occurrence of LT by quantifying unfavorable hemodynamic regions caused by local disturbed flow field.
Chapter 6 presents a parameter scanning study that quantifies the combined influence of iliac anatomy and the Anaconda endograft on limb flow fields using the echoPIV technique. The designed in-vitro setup allowed for the modification of iliac artery anatomy by adjusting the aortic bifurcation angle and the positioning of the iliac arteries in the anterior-posterior plane. The results showed that the implemented Anaconda endograft increased the average VC and RT more in the left common iliac artery (LCIA) than in the right (RCIA), likely due to the specific phantom anatomy used. However, post-operative in-plane VS counts slightly decreased in both limbs, possibly as a result of stiffening in the phantom walls, which reduced arterial pulsation. This study demonstrates that varying anatomical configurations and endograft placement significantly influence limb hemodynamics, both positively and negatively. Thus, no single anatomical characteristic can be directly linked to a higher likelihood of creating disturbed flow patterns, and as such LT. Therefore, the combined influence of patient anatomy and endograft type should be simultaneously evaluated when determining limb hemodynamics, which due to the in-vivo feasibility of echoPIV, can be a promising tool for such investigations.
The in-vitro study conducted in Chapter 7 evaluates the performance of various EVAR endografts (standard Anaconda, custom-made Anaconda with gradual flared limb design, Endurant, and TREO) on limb hemodynamics. Identical AAA phantoms were used to assess the influence of endografts on limb hemodynamics alone, providing insight into disturbed flow regions potentially linked to LT. Endurant and TREO exhibited similar flow patterns during systole, which were also similar to the pre-operative flow patterns, while two version of the Anaconda endografts also showed similar behavior compared to each other. However, significant differences arose during diastole and flow reversal, attributed to the differences in endograft designs. The Anaconda grafts had the highest increases in VC, especially the custom-made version (2.4 times higher) in the posterior wall compared to the control case. RT simulations based on the measure flow field further revealed disturbed flow in some regions of the LCIA and RCIA, but these results are not yet generalizable to clinical settings due to the limitations associated with the use of averaged AAA flow phantoms. To summarize, this set of in-vitro studies (Chapters 5 to 7) represent initial steps in identifying unfavorable hemodynamic factors using echoPIV that identify local regions with disturbed flow, potentially linked to LT after EVAR. However, follow-up studies are recommended, with a focus on designing in-vitro experiments with e.g. whole blood to better understand the relationship between the proposed hemodynamic parameters and thrombus formation dynamics.
