#153- Sleeping in an MRI Machine

Matt Walker (0:02)

Hello, and welcome back to the podcast, it's Matt here.

Today's episode is about something hidden in the drawer in your house somewhere, foam earplugs. They're tapered, slightly gritty, the color of a highlighter pen. Roll them between finger and thumb until they're thin enough to slip into your ear, and then hold them there until they expand back out again and seal themselves in place. We've all used them before, right? On an aeroplane, next to a snoring partner, or at a concert we may or may not later regret going to.

But what happens when you take that earplug with you somewhere completely different, into a room with a superconducting magnet in it, cooled with liquid helium doing a brain scan? There's a protocol we know about. 51 minutes at a sound level of between 103 and 111 decibels. Your head inside the thing you're trying to image. At this volume, the earplug is no longer a luxury. It's a piece of safety equipment. And in this room, supine position, head immobilized in a cradle, with all of that going on around you, people fall asleep. They do it systematically, intentionally, as the basis for an entire scientific discipline.

The first systematic review of the literature found 205 studies where people had their brain scanned while they were confirmed to be asleep, spanning 30 years and three different imaging methods.

So the question I want to talk about today is this. Can you sleep at 110 decibels?

Yes, you can. And everything interesting about the process is in the how.

To get a person to sleep in the world's worst bedroom, you first need to understand which aspects of sleep are fragile and which are shockingly resilient.

Once a person is under in there, the machine reveals something we can't see any other way, what falling asleep looks like from the inside. Before the sleep comes, the machine, because it's a lot younger than a lot of people think.

The science rests on two foundational papers, both published in the early 1970s. The first documented that the magnetic signal from tissue within a tumor was different from the signal of normal tissue. It wasn't an image, it was just a difference in the signal. The first image of a living subject followed, it was a clam. A few years later, scientists were able to run the first whole body MRI of a living human. Barely a decade took the technology from clam to clinical practice. The first commercial scanner shipped in 1980, and the US FDA approved MRI for clinical use in 1984

MRI or magnetic resonance imaging is commonly used to scan bodily functions and structures like blood flow, tissue and organs. Think of an MRI almost like a camera. It can take photos or videos, depending on what you want to look at. An MRI does two types of scans, functional and structural. Functional or fMRI scans look for blood flow and oxygenation and act more like a video recording. Structural scans are the photos and look at a specific organ, like the brain or part of the body, to see the anatomy. Beyond the walls of the clinic, that scanner is our main tool for investigating the brain. And the average study is really quite tiny. Imagine 12 people sitting around your kitchen table. That's the median number of subjects in the thousand or so most cited brain imaging papers published between 1990 and 2012

Even in the latest years examined, even in the top tier journals, the median sample size had risen only to 23 or 24

Now, we move on to describe the worst bedroom ever constructed. First, let's talk about the noise. It's the aspect that most people struggle to accept. 26 healthy young adults were scanned with that 51-minute protocol. They wore earplugs. They wore padding. Their hearing was assessed before, within 20 minutes after, and then again, 25 days later. After the scan, their hearing threshold had moved 5 decibels. By the 25th day, it had returned to baseline. There are two interesting follow-up points here. First, it's temporary. It falls well below the 40-50 decibel shift that's been linked to changes in the auditory nerve. Second, the effect is still significant, and it occurs despite the hearing protection. But be cautious, those numbers, 103 to 111, refer to just one protocol and just one machine. An acoustic survey of newborn brain protocols on five different machines found an average of 93 decibels with a range of 81 to 105 over protocols lasting roughly half an hour to nearly 40 minutes. The reality is a broad spectrum. Sound levels between 80 and 110 decibels, for anything from half an hour to nearly an hour, are routine in brain imaging.