The telltale heart

Twice a week, scores of people gather at Boston Childrenâ€TMs Hospital to plan the most difficult cardiac surgeries. They analyze three-dimensional, digital copies of hearts projected on a screen, every damaged blood vessel or malformed ventricle a threat to the life or health of a child.

These duplicates can be rotated or taken apart piece by piece on the computer screen, allowing the surgeons to precisely plan an upcoming operation. With the help of biomedical engineers, the doctors can see the impact on other cardiac functions such as blood and oxygen flow, the heartâ€TMs electrical system and the pressure on valves. They can even gauge the impact of the patches or repairs they intend to use â€" all before the first incision is made.

Someday, sensors or wearables may be added to the technology, creating a pathway for the human heart to transmit data to any patientâ€TMs virtual heart. This feedback loop would create a digital twin, sometimes referred to as a virtual twin. This is the next step up from more traditional simulations and models, providing doctors with another way to ensure their approach is the best one.

To "understand what the options are, and what the challenges are, itâ€TMs unbelievable," said David Hoganson, a pediatric heart surgeon and director of the hospitalâ€TMs computational 3D visualization program. Digital models have been used in approximately 1,300 surgeries so far.

Digital twinning has come of age in medicine during the last several years, moving into models for lungs, livers, brains, joints, eyes, blood vessels and other body parts. A virtual twin of an entire human being is somewhere in the future. The technology is also being used to test new medical devices and even drugs, with computer models powerful enough to predict a new moleculeâ€TMs impact on organs and cells. It holds the potential to scale back, or even replace, the use of animals in experiments and humans in clinical trials.

The technology "is revolutionary," said Ellen Kuhl, a Stanford University professor of engineering, who is modeling the way a heart translates electrical impulses into physical pumping. "If you do this right, the models you create generalize to a wide population."

The first virtual heart In 1989, Steve Levine, an engineer at Dassault Systà ̈mes, and his wife had a daughter.

The child was born with a rare, serious heart defect. Doctors ultimately decided to manage her condition with pacemakers as she grew. But the amount of guesswork employed in finding a solution left a lasting impression on Levine.

He knew from working at a company that develops software for three-dimensional product design, simulation and manufacturing that no one builds a new aircraft or vehicle and then takes it out for a test drive anymore. Software programs and computer systems enable designers to craft and test a wing, or a rudder, or a wheel in a virtual environment first. Only after designs have passed scrutiny virtually are they assembled into a product.

Why were heart procedures and coronary devices not tested the same way as an aircraft part, he wondered. A heart is, after all, a pump, one subject to the laws of physics.

In 2014, Levine launched the Living Heart Project, eventually pulling together hundreds of doctors, engineers, government officials and industry representatives from around the world with the expertise to build the first three-dimensional, fully functioning heart in a virtual environment.

At first, many doubted the project could be accomplished: A life is at stake in every heart surgery. The risks might be too great to abandon the traditional approach of relying on a surgeonâ€TMs skills and experience, or the whole endeavor of twinning a human heart might be too complicated. Gradually, however, the models were created from common CT scans and MRIs, the software was refined, and the virtual heart was validated, comparing favorably when tested on animals and ultimately humans.

"I knew if the building blocks were correct, the final product would be correct," Levine said. "The secret to it working was the crowdsourcing model."

Within a year the collaborators had a functioning heart and in two years they had a product that could be made available to users such as Boston Childrenâ€TMs Hospital surgeons, he said. The open-source collaboration has continued for a decade without a single intellectual property dispute, Levine said. The cost over 10 years has probably run into hundreds of millions of dollars, he said, primarily for the labor of experts around the world.

"When you have this model, you can personalize with certain features, certain anatomy. Then you can try things," said Kuhl. "In heart surgery, you canâ€TMt try 20 different things. You only have one shot."

In a December 2023 report, the National Academies of Sciences, Engineering and Medicine, the independent panel founded by Congress to advise the federal government and the public, evaluated the rapidly spreading technology. It defined a digital twin as a virtual replica that "mimics the structure, context, and behavior of a natural, engineered, or social system … is dynamically updated with data from its physical twin, has a predictive capability and informs decisions that realize value." The Food and Drug Administration, which reviews medical devices, has been developing standards for the software in this emerging technology and methods of evaluating it as it progresses.

"I think the challenge has been in the very large adoption of these technologies is making sure that everyone is on the same page in how theyâ€TMre being used and how theyâ€TMre being evaluated," said Tina Morrison, director of the office of regulatory science and innovation in the FDAâ€TMs chief scientistâ€TMs office.

In surgeries alone, advocates of digital twins envision cost savings from shorter surgeries, fewer complications and reduced need for follow-up operations. Revenue from digital twins in health care in 2023 was estimated at $1.6 billion but predicted to reach $21.1 billion by 2028.

The removal of a brain tumor Levineâ€TMs work became more personal this year when doctors found a benign golf-ball-size tumor at the base of his brain. It was not immediately life-threatening, but it had eroded part of his skull, invaded his sinuses and was pressing on his optic nerve. The tumor had taken over the function of his pituitary gland, sending out excess growth hormone. Levineâ€TMs feet and hands were actually growing.

Thomas Beaumont, the neurosurgeon at University of California at San Diego who performed Levineâ€TMs surgery, created a digital replica of Levineâ€TMs head, including the tumor, to plan the delicate operation. With the image on a screen in the operating room, he performed the surgery noninvasively by passing a camera up one nostril and his surgical tools up the other, removing the tumor piece by piece and repairing damaged tissue.

Every patientâ€TMs bone structure is somewhat different, Beaumont said. The carotid arteries curve in individual ways. All of it has to be visualized in three dimensions for surgery to be successful.

"Now that I can actually see it in the O.R., does that decrease the mental strain? I think the answer to that is †̃yesâ€TM," he said. "It allows me not to have to project all of that anatomy into what I see. It allows me to be in a more confirmatory state."

All this work costs large sums of money and neither government programs nor private insurance pays much for the engineering work that goes into these surgeries. At Boston Childrenâ€TMs Hospital, Hoganson has more biomedical engineers than surgeons on his staff, but is fortunate that, unlike many smaller hospitals, his institution can afford them.

NASEM also warned of runaway enthusiasm for twinning in its comprehensive report. "The publicity around digital twins and digital twin solutions currently outweighs the evidence base of success," the panel of experts said. It called on the federal government to spend more on cross-agency efforts to "advance mathematical, statistical, and computational foundations for digital twins."

This month the National Science Foundation, in collaboration with the National Institutes of Health and the FDA, has awarded over $6 million in research funding to seven projects to explore the development of digital twins for use in health care and biomedical research .

NASEM also noted the need to continually validate and verify that virtual twins can do what their backers intend. "Verification and validation play key roles in building trustworthiness, while uncertainty quantification gives measures of the quality of prediction," NASEM said. It also highlighted cybersecurity concerns.

Numerous private companies, academic medical centers and governments are exploring digital twinning. Designers and the experts who evaluate it believe the technology holds promise in industrial design such as tweaking the performance of aircraft engines in real time or building atmospheric, climate and sustainability digital twins. In coming years, artificial intelligence may be used to construct the virtual models.