Calvin Sun was a healthy 37-year-old when a full-body MRI scan showed a cyst in his kidney. Sun saw a urologist who was cautiously optimistic that it wasn’t cancerous and offered him a surgery appointment several weeks away to inspect the kidney and operate if necessary. “I was like, how about tomorrow?” Sun recalls.

As an ER doctor, Sun is used to decisive problem-solving. It’s the “right mindset” for undergoing a whole-body MRI, he says. “You have to be willing to take 100% responsibility for the consequences, good and bad.”

Instead of traditional scans, like CTs or MRIs of a specific part of the body, full-body MRI scans require just an hour to image you from head-to-toe. Celebrities and influencers are holding them up as a pillar of preventive health to catch problems early on, wherever they’re hiding—before they become hard-to-treat diseases. Dwyane Wade, for example, recently credited a whole-body MRI with alerting him to an early-stage kidney cancer.

However, most medical experts are more wary. “The odds that you’re going to be hurt are higher than the odds you’re going to be helped,” says Dr. Matthew Davenport, professor of urology and radiology at the University of Michigan.

Here’s what to know about this relatively new technology—both its promise and shortcomings.