Pre-operative Risk Assessment of Abdominal Aortic Aneurysms using 4-D Ultrasound


Previous (VENI-Lopata STW-11885) and ongoing funding (Stichting Lijf & Leven, IMPULS-II) has resulted in a large study on pre- and post-operative risk assessment of abdominal aortic aneurysms using 2-D, 3-D and 4-D functional ultrasound data. This study was originally initiated at the MUMC+ but is currently ongoing in the Catherina Hospital Eindhoven (dr. Marc van Sambeek).

Aneurysm formation is a progressive arterial disease, where the arterial diameter has exceeded its normal size (2 cm in case of an aorta) by 150% or more. Aneurysms are mostly asymptomatic, but cause a life threatening hemorrhage when they rupture, making them silent killers. Surgical intervention is possible, being either endovascular stent grafting or open surgery. However, these procedures are not without risk and complications can lead to a 30-day mortality rate of 2 – 8%, depending on the type of surgery.

Current clinical decision making is based on population data and uses only a generic measure, i.e., the maximum anterior-posterior diameter. If the diameter exceeds the threshold of 5.5 cm or has increased with 0.6 – 0.8 cm in the last year, surgery is decided upon. However, these criteria have proven to be inadequate and are not tailored for the individual patient. To improve clinical decision making, and providing the surgical with clinical decision support, a better criterion is sought for in the combination of functional ultrasound imaging and biomechanics.

Many groups have worked on image-based biomechanical modeling of (mostly) abdominal aortic aneurysms. Computed tomography (CT) is used to build a patient-specific finite element (FE) mesh to simulate local wall stresses. Although this technique is finding its way into the clinic, its use is limited due to the use of ionizing radiation and contrast agents. MRI is a good alternatively but sparsely used due to the low temporal resolution and high costs involved. Ultrasound (US) is already used for diameter assessment and patient monitoring. Now, with the rise of 3-D+t ultrasound techniques, our research is devoted to develop US-based techniques for pre-operative and post-operative risk assessment using the combination of functional ultrasound imaging and biomechanical FE modeling. 

Patient study

In the Catherina Hospital Eindhoven (CZE), all AAA patients are asked to participate in our ongoing study. All AAA patients that visit the hospital at regular intervals are subjected to additional 3-D and 4-D US imaging. In the current framework, almost all patients have been included. The use of US allows multiple acquisitions at regular intervals, enabling studies on growth and aneurysm development

4-D functional ultrasound imaging & elastography

2D and 3D speckle tracking and strain imaging techniques are developed using state-of-the-art radiofrequency (RF) and image processing techniques. Technical challenges are the small displacements, lateral resolution and low depths with limited contrast between blood, thrombus and wall. Material properties are estimated directly from these data and via an FE modeling approach. The latter is also used for wall stress analysis (see next section).

Experimental validation of the US techniques is performed using mock circulation loops. Porcine aortas are tested under physiological yet controlled hemodynamic conditions. Other types of testing (i.e. bi-axial tensile testing) are used to validate the US outcome.

US-based FE modeling

CT-based wall stress analysis uses constitutive models with parameters obtained from population data. In this study, FE models are fit to the US data to provide a) the patient-specific material properties of the AAA and b) the true patient-specific wall stresses. The US techniques are validated using other imaging modalities (CT for geometry, MRI for motion estimation). This study is in collaboration with Philips Research & Philips Ultrasound.

Additionally to pre-operative monitoring in Lijf & Leven, 3-D motion estimation will be performed on post-EVAR AAA patients (IMPULS-II). Measured US data will be compared with wall-stent models to get insight in wall-stent interaction, assess sealing and detect endoleaks.

Projects

Projects for bachelor-end projects, internships and MSc projects are available.

  • 4-D speckle tracking algorithms for functional imaging of AAAs
  • Personalized FE modeling of AAAs and inverse problems
  • Experimental validation (inflation testing on phantoms and tissue, tensile testing)
  • Segmentation of 3-D US images of AAAs

Other projects can be designed in consultation with the supervisors (Emiel van Disseldorp, Niels Petterson & Richard Lopata)

Students working on this project:Niels Petterson (PhD project), Emiel van Disseldorp (PhD project), Ellen van Wesel (MSc project), Marcel van den Hoven (MSc project) & Jesper Schouren (BEP) 
Completed MSc projects:Annette Kok, Tomas Schlepers, Tim Vonk, Germaine Jongen, Patrick Verhulst, Julia Dronkelaar
Completed internships:Paul Kemper, Tomas Schlepers
Completed BSc projects:

Koen Hobelman, Tommy Hetterscheid, Kujtim Gashi, Maarten Heusinkveld, Emiel van Disseldorp, Laura Jacobs, Arjet Nievergeld, Sjoerd Nooijens, Anneloes Munneke