4D MRI: A “Massive Leap Forward” in Fetal Cardiac MRI Imaging

October 7, 2020

A new technology overcomes the challenges to cardiac imaging presented by spontaneous fetal movement.

MRI scans with 4D visualization capabilities can help doctors detect congenital heart disease in babies prior to birth, offering an improvement over current in-utero imaging methods.

In a study published in Nature Communications, investigators from King’s College London discussed their method that can examine the fetal heart in four dimensions. The images, which can capture views of major vessels and blood flow circulation, are themselves 3D, but they morph through time as the heart beats. In doing so, they compensate for the fetal motion that has compromised previous images captured during pregnancy.

This is the first time researchers have been able to capture images that allow cardiologists to see the contracting and beating of the fetal heart in motion, said Dr. Tom Roberts, research associate in perinatal imaging and health at King’s college London.

“The results in the paper are exciting because no one has been able to look at the fetal heart using MRI in four dimensions like this,” Roberts said. “When we give these videos of the beating heart to the doctors, they are able to interact and examine the direction of blood flow instantly.”

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This type of imaging has been available – and is standard – with adult patients, but the spontaneous, unpredictable movements of a fetus, as well as the rapid heartbeat, have prevented these types of images. To capture the images now, he said, 4D volumes are achieved by using a mathematical motion-correction technique and model to stitch them together. 

“The heart is a pump. Our method is very beneficial because doctors can start to measure how much blood is pumped out with each heartbeat, which can be used to tell how effectively the heart is performing,” he said. “We can measure the amount of blood going through the aorta and other major vessels, simultaneously. We can get new types of measurements that you weren’t able to do before in fetal MRI.”

Although many presentations of congenital heart disease can be relatively straightforward and easily diagnosed with ultrasound, there are some pathologies that are more complicated. Overall, Roberts said, the hope is that this new technology and method will change how providers assess this disease in an unborn fetus. Currently, they are only able to judge whether a baby’s heart has formed incorrectly. But, the intent is for this new technique to measure the heart’s performance, as well.

Until now the challenge has been that measurements of fetal blood flow has been unreliable with ultrasound. And, MRI measures blood flow well to determine if a heart is beating efficiently, but only in adults.

“When we scan a fetus, it moves and wriggles around however it wants which can prevent you from measuring the blood flow in the tiny vessels,” Roberts said. “In our work, we’ve developed methods to correct for this motion and build those 3D movies where we can measure blood flow in any region or vessel of the fetal heart.”

By offering detailed, high-resolution images, MRI reveals the tiny vessels of the fetal heart that are less than 5-mm wide. According to study details, the whole heart 4D cine reconstruction consists of two streams. First, a motion-corrected 4D magnitude cine reconstruction was performed with data that consisted of multiple stacks of highly accelerated dynamic bSSFP imaging planes that contained multiple time frames in each slice. The 72 ms per frame temporal resolution of the acquisition was enough to capture fetal cardiac motion in real-time without the requirement for periodic movement and electrocardiography-gating.

The team estimated the fetal heart rate from the frames of each captured slice, and they used that rate to assign cardiac phases to each successive frame. The cardiac phases from the different slice locations were, then, synchronized and rigid body transformations were calculated for each frame. This provided a completely motion-corrected dataset for reconstruction, the team said.

Ultimately, Roberts said, he and his colleagues hope this research will open the door for improved clinical care for infants born with congenital heart disease.

“If [congenital heart disease] is detected prior to birth, then doctors can prepare appropriate care immediately at birth, which can sometimes be life-saving. It also gives parents advance time to prepare when, otherwise, the [congenital heart disease] might have been discovered at birth, which can be very stressful,” he said. We are trying to advance fetal cardiac MRI as a way of potentially improving outcomes in congenital heart disease, either by being able to offer a better diagnosis or being able to look for congenital heart disease at an earlier time during pregnancy.”

Dr. Kuberan Pushparajah, second study author and clinical senior lecturer in pediatric cardiology, seconded Roberts’ thoughts about how this research advanced care for this condition, calling it a “massive leap forward” in the field of fetal cardiac MRI

“We will now be able to simultaneously study the heart strugtures and track blood flow through it as it beats using MRI for the very first time,” Pushparajah said. “This is key in the assessment of congenital heart disease where the heart structures and connections are abnormal and can be very complex.”