Kids with pulmonary hypertension face a real risk of a cardiac event every time they undergo catheterization. And yet cath is currently the reference standard to monitor the progress of their disease. At Children's Hospital Colorado, a cadre of researchers is working to adapt MRI to that task — and they're seeing the heart in a way it's never been seen.
The risks of catheterization for monitoring pulmonary hypertension
Pediatric cardiac interventionist Jenny Zablah, MD, straps on a lead vest. Her patient, 2-month-old Marcus *, lies anesthetized in the Cath Lab. An access sheath is placed in his femoral artery and vein. The catheters are fed through. Soon they appear in the monitor above him, one in the aorta, one in the pulmonary artery.
Six weeks premature, Marcus was born with a large patent ductus arteriosus that failed to close. In a healthy baby, pulmonary blood pressure should be about a third of systemic pressure. Marcus's open PDA lets the pressures in the pulmonary artery and the aorta equalize. Today's mission: Get a read on Marcus's pulmonary hypertension and see if Dr. Zablah can safely close the PDA.
"This is a low-complexity case," Dr. Zablah remarks, "but many of our PH patients are very sick. Anesthesia makes the systemic blood pressure drop, so they're at high risk to go under sedation. Sometimes they go into cardiac arrest."
"For every cath procedure these kids get, the chance of a catastrophic event is 3.3%," says pediatric cardiologist Uyen Truong, MD. "But a lot of kids need it serially, over a lifetime. So you can imagine the risk."
Leveraging cardiac MRI to image the heart
Dr. Truong and bioengineer Michal Schafer, PhD, talk cases. On their agenda: a patient with idiopathic pulmonary arterial hypertension, a rare subset of PH, who had pressures so dire surgeons shunted his pulmonary artery to the descending aorta to give his lungs a way to offload pressure. The need to know his status is urgent and constant. He's had several cath procedures.
A day after his last one, he also got an MRI. For the patient, and for most of the people in the room, it wasn't much different from any other scan.
Chance of catastrophic event for every cath procedure a child goes through
Weight of exercise bike developed by Montview Biomedical for use in MRIs
"MRI excites hydrogen," says Dr. Schafer. "Anywhere there's hydrogen, such as in water, you can excite that atom and get a signal. Then you can map the location of those signals, that's how you get the overall image."
"Typically you're exciting a slice," adds Nivedita Naresh, PhD, a bioengineer in the Advanced Imaging Lab specializing in MRI pulse sequence programming. "The difference here is that we're exciting a volume."
That's trickier than it sounds. It changes the pulse sequence programming, which tells the scanner how to gather data, and the processing, which sorts it into a meaningful image. It's even trickier when the sequences run continuously to produce a moving image of blood-flow in an organ constantly in motion.
The sequences Dr. Naresh is working on compensate for that by gating to ECG, essentially syncing to the heart's natural rhythm. But even then, there's no commercial software that can process the data they generate. For that, Dr. Schafer uses code he helped develop.
The result is 4D MRI: images of the heart in three dimensions, over time.
"Here's where the magic happens," he says.
The heart in four dimensions
On his screen, Dr. Schafer pulls up the rough shape of his patient's heart. Rendered in sinews of color, it comes alive with a click: A rush of green pours into the pulmonary artery and then the aorta, the irregular flows of the shunt breaking up into sprays of blue. Each line represents a direction of flow. Each color represents a velocity. From this dashboard, he and Dr. Truong can get a read on most of the information a cath provides. But they're seeing much more than that.
Dr. Schafer rotates the view and points to where the shunt empties into the aorta. "You can see all these spirals and vortices here. The shunt is helping the heart pump less against pressure, but the system is not designed to convey flow in this fashion. Dr. Truong said, 'Let's check the descending aorta.' Well voilà, we find there is some stiffening there."
"We've never been able to see that before," adds Dr. Truong. "We're putting together the first case report on this. When I presented it, people were blown away."