Virtual surgery: doctors can simulate heart operations with the click of a mouse

Science News, July 28, 2007 by Erica Klarreich

Board an airplane and you can rest assured that it underwent rigorous safety testing before its first flight and, in fact, before it was even built. You can feel confident that engineering software searched across a vast range of design parameters to find the most aerodynamic shape, the sleekest wings, the sturdiest fuselage. Competing designs were "test-flown" in computer simulations to predict how they would perform in both friendly and turbulent skies.

Go into the hospital for open-heart surgery, however, and your doctor will have designed your treatment using a very different process. Most likely, the surgeon will have considered what treatments have worked best on patients with symptoms similar to yours and then used a combination of medical protocols and intuition to decide on your case. Doctors can estimate the risks and benefits of a procedure by looking at how well it has gone in the past, but they can't guarantee the outcome in your particular case. Advanced diagnostic tools and new drugs notwithstanding, medicine remains what it has always been: an empirical science.

Charles Taylor, a bioengineering professor at Stanford University, wants to make the run-up to a heart-surgery procedure more like the design of an airplane. His team has created a simulator, called SimVascular, that converts magnetic resonance imaging (MRI) data into a computerized geometric model of the patient's blood vessels. The model offers surgeons the chance to test-fly various surgical procedures and other treatments to see which approach is most likely to best fit an individual patient's physiology.

"A patient gets only one surgery. You can't do two and see which is better," says Taylor. Using the model, however, surgeons could perform multiple virtual surgeries and test out ideas without ever touching the patient.

Sim Vascular, which has been more than a decade in the works, is due to be released on Aug. 31 as open-source software freely available to researchers all over the world. Taylor's team of cardiologists, surgeons, and others is already using the software to conduct basic research on the value of various heart treatments. For one type of serious congenital heart defect, the team has even used the software to design and test an entirely new surgical procedure. In the coming months, Stanford surgeons hope to perform that procedure for the first time.

"It's a very different way of thinking about medicine, infusing engineering and mathematics into the surgical-planning process," Taylor says.

CAPTURING CIRCULATION For several decades, researchers have been modeling the physics of the human circulatory system. In recent years, they've moved away from traditional glass-and-water models toward computer models that use software tools handed down from the aerospace industry to solve fluid mechanics equations. These models have provided compelling evidence that simple fluid-mechanical forces, such as blood pressure and shear stress, underlie most heart disease, from the buildup of fatty plaques in arteries to the vessel-wall weakenings called aneurysms.

While such computer models have proved illuminating, they remain idealized descriptions of a generic circulatory system. Individual circulatory systems vary widely, not only in the locations of plaques and the widths of vessels but also in how various vessels connect with one another.

As a graduate student in the early 1990's, Taylor realized that it should be possible to reconstruct a particular patient's physiology from the three-dimensional images of blood vessels that MRI creates. At first, Taylor planned to use such patient-specific models simply to continue studying the link between fluid mechanics and vascular health.

"But at some point," he says, "I realized that there was a much more important application: to help predict the outcome of a surgery."

Off-the-shelf fluid mechanics software tools were not designed to study blood flow, so Taylor knew that they would be inadequate for his purposes. The human circulatory system is different from a network of pipes, he explains. For instance, blood pulses in bursts rather than flowing smoothly, and blood vessels, unlike pipes, are flexible tubes that expand and contract with each pulse.

To make matters more complicated, the pictures offered by an MRI, unlike the blueprints for a plumbing system, are far from complete. MRI images can show all the vessels larger than 1 millimeter in diameter, but that amounts to just a few hundred of the millions of blood vessels in a human body. To deal with these complexities, Taylor realized that he would have to build his model from scratch.

The Sim Vascular software starts with MRI images. Using image-recognition techniques, it extracts the geometry of the patient's major blood vessels--all those big enough for the MRI to detect. Next, the software divides the geometric model into millions of tiny chunks. Then it chooses a representative point inside each chunk. At each such point, differential equations govern the blood's velocity and pressure as well as the dynamics of the vessel walls.