Not the Usual Kid Stuff
By Beth W. Orenstein
Radiology Today
Vol. 21 No. 4 P. 14

PET/MRI at CHOP adds value and helps reduce radiation dose in young patients.

A goal of radiologists everywhere has long been to reduce radiation dose, especially when imaging children. “Because children grow quickly, their cells, which are rapidly dividing, are more sensitive to radiation,” says Lisa J. States, MD, a pediatric radiologist at Children’s Hospital of Philadelphia (CHOP), with additional expertise in nuclear medicine. And because the effects of this radiation can take years to develop, children who undergo ionizing radiation remain vulnerable for an extended period of time, she adds.

Over the past decade, combined PET/MRI machines have gone from a basic technical concept to a clinical research tool to a clinically viable imaging modality. The hybrid modality promises a significant reduction in ionizing radiation when compared with PET/CT; the combined scan exposes patients to approximately 50% to 74% less radiation than a PET/CT, according to several publications focused on pediatric patients.

About four years ago, when the chief of radiology at CHOP and the department business manager learned they could acquire a PET/MRI scanner for their radiology department, they were anxious to do so. “They saw it as innovative technology that could reduce ionizing radiation in children who needed to undergo scans for oncology, neurology, and other diseases,” according to States. Her department acquired the 3 T hybrid machine in the fall of 2015 and performed its first clinical scan in December 2016. States has performed nearly 400 PET/MRI scans on children and adolescents over the last four years. “CHOP is highly innovative in its practice of pediatric radiology,” and it is one of only three sites in the country that has a busy clinical PET/MRI service for children, she says. The other two facilities with PET/MRI for children are Children’s Hospital of Wisconsin in Milwaukee and the Lucile Packard Children’s Hospital Stanford in Palo Alto, California.

A Shift in Perspective
CHOP’s PET/MRI machine is not designed specifically for children; it’s a 3 T MRI scanner containing a PET ring of detectors that can be used for both adults and children, according to States. To date, the youngest patient scanned was 5 weeks old. It’s also used for young adults who are still being treated by the clinicians who saw them as children.

When it is used in place of a PET/CT, it can be extremely valuable because it exposes young patients to much less ionizing radiation, States indicates. Some children may need repeated scans to help determine whether a treatment is working or a cancer has spread, and the reduction in dose over time can be even more significant, she says.

“As a pediatric radiologist, I do try to adhere to the ALARA concept—as low as reasonably achievable,” States says. “My goal is to always make sure that the lowest absorbed radiation dose is used to create high-quality images. PET/MRI helps me achieve this goal.

“Many of us have shifted to PET/MRI vs PET/CT to diagnose, stage, and treat cancer for the main reason of decreasing radiation,” she continues. “When it comes to treatment, our pediatric surgeons are very used to MRI and very comfortable with using MRI instead of CT.”

States notes that performing an MRI and a PET scan concurrently is also an advantage. “You run the MRI sequences at the same time as the PET scan so you know what you’re looking at is in the right spot,” she says. In the past, a child might undergo a CT scan and then have a PET scan on the same table, but when the scans are separate, “there’s always the chance that they might not match up exactly, especially if the child moves a little bit,” States says.

Also, when the scans—whether MRI and PET or CT and PET—are separate, it may require two visits for patients and their families, which means more time and sometimes more travel. “We try to do them back to back,” States indicates. “But it still can be a super long day for the patients and their families, especially when you have to figure doctor’s visits in there as well.” Although the PET/MRI alone can take an hour or so, “it can drastically decrease the time they are in our department,” she says.

Adjusting Protocols
One disadvantage to doing a whole-body PET/MRI vs PET/CT is the time it takes. “It takes 15 to 20 minutes to do the PET portion with CT, where PET/MRI takes about an hour,” States says. Because the child has to stay still for a long time, he or she may require sedation. All children under 4 years of age would likely get sedation for either a PET/CT or a PET/MRI, States says. “Then you have the 4- to 10-year-old age group where you may not sedate for PET/CT but probably in most cases you do for PET/MRI.”

There is some concern about toxicity to the brain with sedation, she says. The FDA has a black box warning for sedation in children younger than 3 years of age. “But you always wonder about the older age groups,” States says. For those caring for children, it’s a dilemma: Do you sedate the child so you can do a scan with less ionizing radiation? “We always try to decrease both of these risks if we can,” she adds.

Children with cancer tend to respond better to treatment than adults, but they continue to get scanned, States says. Some parents want to have their child receive sedation, and some do not; “it really depends,” she says. It can be difficult to tell whether the older child is anxious because they have a disease such as cancer, if they are anxious about undergoing the MRI itself, or they are claustrophobic. In some cases, the staff doesn’t discover whether the child is claustrophobic until he or she is placed in the scanner. But the staff does try to determine, with the parents’ help, whether it will be a problem before attempting the scan, States says. For a variety of reasons, “We end up sedating children who are outside the age we usually sedate,” she says.

Reducing motion is critical to the quality of the PET/MRI scan. That’s always on her mind when deciding the best scanner for that child, States says; at CHOP, they don’t use contrast for any of the scans, she notes. In all the scans she’s done, States has only had a few where the child has asked to get out after the PET/MRI has started. “In that case,” she says, “we’re lucky because we have a new PET/CT scanner, too, and are able to put them there and complete the study.”

Ongoing Challenges
One challenge in interpreting the PET/MRI is that bone marrow is different in children than it is in adults. “Children have red and yellow (fatty) bone marrow and, slowly over time, red marrow becomes fatty marrow,” States says. “When they are teenagers, the marrow composition will approach what adults have. That does change how PET/MRI scans are read. Knowing these changes in children is important, and it’s different to interpret.”

States has found that the most common uses for PET/MRI are oncological and neurological for seizures. “Lymphomas and sarcomas seem to be the most common diseases we’re using it for,” she says. “But when I looked at our first 100 patients, we had 35 different indications for the scan. It’s not cut and dry. Children get a variety of rare tumors. They even have ovarian tumors that are metastatic and so we see those kinds of patients as well.” The whole-body PET/MRI is ideal for sarcoma patients because sarcomas can occur anywhere in the body, she says. Most of the scans are whole body, but they may be used for a specific body part to help diagnose or treat patients with seizures, States notes. The PET/MRI scanner at CHOP cannot be used for children with cochlear implants or vagal nerve stimulators.

States indicates that it takes her about an hour of reading time to issue a report. It is necessary to read and reread the scan “and make sure you’re not missing anything,” she says. A whole-body PET/MRI takes significantly longer to interpret than a PET/CT. “You don’t have the soft tissue resolution on PET/CT, and it’s not expected that you’re going to give them any extra information,” States says.

Another difference, she adds, “is that with PET/MRI, you’re reading a diagnostic study, where with PET/CT, you’re doing a localization study. We try to make the PET/MRI a diagnostic study. That’s our goal, and it takes a long time to look at every sequence and double check everything.” On a good day, in a quiet room, “I could get it done in an hour,” she says. “But sometimes, with interruptions, it takes a lot longer.”

Focus on Patient Care
To make the children who must undergo PET/MRI more comfortable, the department has videos for patients and their families that describe the experience and explain what to expect. Children are also given a list of movies they can watch or music they can listen to while inside the scanner. Of note: The movies aren’t overly stimulating because they don’t want the children laughing hysterically or becoming frightened and moving, States says. The staff is able to talk to the children while they are in the scanner through microphones and headphones and help to comfort them.

“Our nurses and our technologists are very good at working with kids,” States says. “Once the child is in the scanner, it’s the technologist’s job to help make that child comfortable.” One way to make them more comfortable is to tell them how much time is left and how they are doing well, she says. The scanner is a little noisy, but if the children are well prepared, it usually doesn’t bother them.

The department schedules up to 12 PET/MRI scans a week but only one case at a time. “We don’t have enough technologists to run multiple cases at the same time,” States says. But it isn’t necessary, anyway, she says, and the current setup allows everyone to devote their full attention to the patient in the scanner.

— Beth W. Orenstein of Northampton, Pennsylvania, is a freelance medical writer and regular contributor to Radiology Today.