Pediatric patients who could benefit greatly from an MRI of the abdomen, chest, or pelvis, often undergo CT scans instead. One reason is that in order to acquire a faithful MR image, patients must hold completely still while lying in a scanner, sometimes for over an hour. For young children, such a feat is nearly impossible. In addition, the confined space of the MRI machine combined with the loud noises it generates can upset children, making it even more difficult for them to remain still.
Another challenge is that young kids have trouble holding their breath on command, a task that is required during a scan of the torso to prevent movements of the chest and abdomen from causing image distortion.
In some cases, doctors are able to acquire satisfactory MRI images by giving children enough anesthesia to temporarily suspend movement of their lungs and abdomen. The child’s breathing is controlled throughout the scan by a ventilator, which is halted by the anesthesiologist for short intervals when the MRI technician needs the torso to be still.
Shreyas Vasanawala, M.D., Ph.D., an associate professor of radiology at Stanford University, says this approach can be problematic. “You are taking a procedure that’s otherwise non-invasive and turning it into a much bigger deal. There are some risks associated with being under anesthesia, and there’s also a high cost associated with it. The net impact is that a child’s access to MRI gets markedly reduced, because when you have an anesthesiologist available, you may not have the MRI scanner available and vice versa. It puts a pretty big barrier up to getting an MRI.”
Though a CT scan can be sufficient in many cases, an MRI often provides additional information that can influence treatment. For example, MRI is particularly suited for revealing abnormalities in soft tissues, such as ligaments and cartilage, and organs such as the brain and heart. In addition, it can provide information about blood flow and be used to detect metabolic changes in tissue or reveal molecular changes that occur in the early stages of disease, well before larger structural changes can be observed.
Moreover, unlike X-ray or CT, MRI doesn’t expose patients to ionizing radiation. This makes it a particularly attractive imaging option for children,who are thought to be most susceptible to harmful effects of ionizing radiation.
With research support from NIBIB, Vasanawala is beginning to break down the barriers that keep children from receiving MRIs. Using a multi-pronged, team-science approach that involves adapting MRI equipment for pediatric use, developing better motion correction strategies, and implementing state-of-the art image reconstruction techniques, Vasanawala and his colleagues have significantly reduced the amount of time it takes for a child to undergo an MRI scan at Stanford.
“With our approach, we’re able to increase the speed of imaging quite a bit so children can tolerate the procedure much better,” says Vasanawala. “Some of our MRIs are under 10 minutes now, whereas they used to be an hour.”
Dr. Vasanawala’s collaborators for this research include John Pauly, Ph.D., Greig Scott, Ph.D., and Brian Hargreaves, Ph.D., at Stanford University; and Michael Lustig, Ph.D., and Kurt Kuetzer, Ph.D., at U.C. Berkeley.
Creating an MRI scanner for children
Because MRI machines are built for adults, one of Vasanawala’s first goals was to design and build MRI signal-receiving coils that are tailored to a child’s body. Signal-receiving coils surround the part of the body that is being imaged and are responsible for capturing the radiofrequency signal produced by the body during an MRI scan. The coils are designed to maximize the amount of true signal that is received while minimizing the noise or interference. However, standard coils are often larger than needed for children and pick up extra noise, causing images to become less sharp. In collaboration with GE Healthcare, Vasanawala and colleagues constructed parallel arrays of child-size receiver coils for imaging the abdomen. While the reduced size of the coils enhances image clarity, the parallel array layout speeds the scan time by allowing individual coils to pick up the signal from different parts of the body simultaneously, rather than sequentially.
To further decrease scanning times, Vasanawala collaborated with Lustig, an electrical engineer at U.C. Berkeley, to implement a technique called compressed sensing. Compressed sensing reduces scan times by gathering only a small fraction of the data conventionally needed to enable reconstruction of a complete magnetic resonance image; this is called under-sampling. The key to the technique is a special algorithm used after the scan that can reconstruct the full MR image from these few data points with high fidelity.
“In MRI pictures, there is not as much information in those pictures as one would initially think. You’re able to, in a sense, under-collect information and still reconstruct images,” says Vasanawala.
The process can be likened to filming a movie with a very fast, but low-pixel camera and then using an algorithm to convert the image to high-definition quality.
In addition to compressed sensing, Vasanawala and colleagues also created an imaging strategy that helps to correct for the motion that occurs during a scan so that sharp images can be formulated even when a child is breathing.
While Vasanawala is quick to point out that his research team is not the first to work on some of these advanced MRI approaches, he believes the combination of state-of-the art MRI techniques and technologies for use in a pediatric population is unique.
“There’s a lot of work going on at other places that have looked at all of these issues. For example, many people have developed dedicated receiver coils for specific purposes, such as high-density coils for brain imaging. Others have looked at various accelerated imaging approaches, and many groups are doing work on motion correction. I think what’s unique about what we do is that we’ve married all of these approaches and have always done it with children in mind first.”
Guoying Liu, Ph.D., program director for Magnetic Resonance Imaging and Spectroscopy at NIBIB, agrees that Vasanawala’s approach is unique.
“Despite being most vulnerable to the potential negative effects of ionizing radiation caused by CT, children are often an overlooked population when it comes to technological developments for medical imaging,” she says. “Dr. Vasanawala’s team has achieved an important advancement in pediatric body imaging by developing thoughtful, systematic approaches to tackle well-known issues associated with pediatric MRI.”
Pediatric MRI in the clinic
At Stanford, Vasanawala’s pediatric abdominal MRI program is changing the way children are imaged. “We’ve gone completely to a free breathing approach,” says Vasanawala. “Instead of having to have a tube put down their throat for anesthesia, the children are lightly sedated. And, for some children in borderline age groups where in the past we would have had them undergo anesthesia, they are now getting MRI without anesthesia altogether.”
These shifts in protocol have been significant enough to greatly decrease the number of CT scans given to children at Stanford.
“The patients who end up going to CT are trauma patients or those in the intensive care unit who are very fragile. But, otherwise, it’s pretty much an MRI practice, which is quite a turnaround from five years ago.”
While limiting exposure to ionizing radiation from CT scans is a worthy goal in pediatric medicine, for some children, the stakes are much higher. Recently, 5-year-old Finn Green was able to avoid a liver transplant as a result of Vasanawala’s pediatric MRI program. After an initial standard MRI revealed a large tumor on Finn’s liver, doctors at three different hospitals unanimously recommended a liver transplant, a treatment option that would require him to take daily doses of anti-rejection medications for the rest of his life.
But at the recommendation of a neighbor, the boy’s parents met with chief of clinical transplantation at Stanford University Medical Center, who suggested they get another picture of the tumor using Vasanawala’s pediatric MRI technology. The resulting images persuaded doctors that the tumor was, in fact, operable. During a four-hour surgery, doctor’s removed the tumor along with 60% of Finn’s liver; most of it grew back within six weeks. Finn is now cancer free and living a normal life.
When asked about how Stanford’s pediatric MRI program might be implemented at other hospitals, Vasanawala states that GE Healthcare currently has a pediatric MRI coil in their pipeline that is based on the approaches developed by his research team. In addition, he points out that many of the approaches his team developed, specifically for the image reconstruction algorithm, have been publically posted on the web for anyone to leverage.
In regards to the field of pediatric MRI imaging, Vasanawala is confident things are moving in the right direction:
“From the time we started working on this and began presenting results at meetings, I think more folks have become very interested in pediatric MRI. To the benefit of children, there is more of a focus on pediatric MRI than there was five years ago. In that sense, our work helped motivate some broader pediatric translational efforts in the MRI research community.”