If you keep inflating a balloon, it’s only a matter of time before it pops. The balloon will burst apart at the point where the plastic is weakest. And the same thing can happen with an aneurysm, explains Judith Fonken. For the past four years, she and her colleague Vera van Hal have been researching abdominal aneurysms in the Photoacoustics and Ultrasound Laboratory (PULS/e). They refer to these using the professional term Abdominal Aortic Aneurysms (AAA). Their expertise in the field goes back even further, because for both Fonken and Van Hal, this doctoral project is a continuation of their master’s final project. This week, after defending their dissertations, their years-long collaboration will come to an end.

Ticking time bomb

What causes such a weak spot in an (abdominal) artery is not fully understood. However, it is known that high cholesterol, obesity, smoking and male sex are risk factors for developing an aneurysm. The problem is that an aneurysm often causes no health symptoms, so patients walk around with a ticking time bomb without realizing it, Fonken emphasizes. “A vascular surgeon typically considers surgery advisable once the diameter reaches 5.5 centimeters (5 centimeters in women). However, some aneurysms rupture before that threshold, and we’ve also seen a patient with a stable dilation of as much as 16 centimeters.” For comparison: normally, the abdominal aorta has a diameter of about 2 centimeters.

A vascular surgeon wants to make a well-informed risk assessment: when should I operate? A ruptured abdominal aorta carries a high risk of death, but surgery is not entirely without risk either. That is why a patient-specific model and improved imaging are essential, according to Fonken. “We want a better predictor of how an AAA develops. Currently, specialists take a 2D ultrasound image at the point of maximum diameter, but our work shows that properties of the vessel wall can provide a lot of additional information. My aim is to capture that in a model that is useful to the physician.”

What we really want, Van Hal clarifies, is to create a sort of 3D representation of the risk of rupture. “It would show the specific shape of the aneurysm in the patient and indicate where the risk of rupture is highest. And how high that risk is. To do that, we need a different way of visualizing – one that also factors in the strength of the vessel wall and the stresses acting on it.”

A pad on your belly

Patients with abdominal aneurysms are examined several times a year to monitor the growth of the AAA. Fonken: “A CT scan provides a very accurate picture of the entire affected blood vessel, but because of the amount of radiation and the use of contrast medium, you can’t do that repeatedly. Ultrasound is a very safe and accessible alternative.”

However, current ultrasound images do not show the entire aneurysm. Information about the aneurysm’s sides is often missing, as Van Hal demonstrates using an image in her dissertation. The reason for this is that the sound waves reflect in a different direction there, which prevents them from reaching the ultrasound probe. That is why Van Hal developed a new ultrasound technology that allows her to capture images of the abdominal aneurysm using two probes simultaneously. “We use this to measure from two different angles, which already produces a dramatically improved image. The ultrasound probes are currently fixed in a metal arch, but ultimately, we want to switch to a flexible ultrasound probe that you can place on your abdomen like a pad, allowing signals to be captured from even more directions.”

Fat tissue

What started four years ago with the development of experimental techniques on pieces of pig aortas has now been successfully implemented in clinical practice. “In the clinic, you encounter new practical challenges, such as patients of all shapes and sizes. Sometimes, the large differences in the speed of sound between fat and muscle tissue can cause the individual ultrasound images to shift. I can now correct that with a new method. And sometimes, just pressing down the ultrasound probe a little harder is enough too.”

In addition to achieving better image quality with a higher frame rate – the number of images per second has increased from 30 to 130-180 per second – and visualizing parts of the vessel wall that were previously not visible, Van Hal can now also determine the local elasticity of the vessel wall.

Fonken plans to eventually incorporate this information into her computer model, which she initially set up using current patient data. “The direct stretching of the vessel wall can’t be measured in patients with current techniques, but we can simulate it using a biomechanical model. With the new ultrasound techniques, we can actually measure this local stretching and use it as input for this model. This will ultimately allow us to determine the patient-specific material properties of the aneurysm. We can then translate the combination of wall tension and wall strength into a rupture index which will indicate the risk of rupture and show the differences between patients in terms of growth rate depending on location of the aneurysm.”