7 Safety
What are the risks of being in a scanner? How do we control those risks?
There are 4 categories of risk we always think about. The first two have fancy names and require more abstract understanding of the physics to control, but they are managed by engineering controls on the scanner (watchdogs). The last two are easier to understand and fully our responsibility to manage:
- dB/dt: we turn magnetic field gradients on and off rapidly during the imaging process, and those changing fields can induce currents in heart tissue and nerves that cause unwanted muscle contractions and sensory experiences
- SAR: “Specific Absorption Rate”. Not the most intuitive name. This refers to heating in tissue caused by the fact that the radio waves we use during the imaging process can induce currents in any conductive material (any tissue in the body!). With currents comes resistance, and with resistance comes heat.
- projectiles: any ferromagnetic material brought into the scanner room will become a projectile and can do serious harm to people!
- acoustic noise: when imaging gradients turn on and off, the currents they are carrying experience forces. That makes the gradients kick back and forth, which in turn creates noise. The noise can be very loud! Scanners are regulated so they don’t get too loud, but the manufacturers trust that we are providing adequate hearing protection to our participants.
First, some general notes about safety:
It’s a bit complicated to put a person in a scanner, because in addition to making sure they’re safe, it’s also important to make sure they’re comfortable!
A huge part of safety is communication, so you need to make sure they have a squeeze ball that sounds an alarm to get your attention whenever they have a question or concern. And you need to make sure — make sure! — that you have 2 way communication. Testing the squeeze ball and lines of communication is the first thing you do as soon as someone is settled in a scanner.
You also want to make sure you arrange pads, pillows, and blankets for comfort. This is for your benefit as well as theirs — the more comfortable your participant is, the better they’ll hold still and the longer they’ll be happy to stay in the scanner participating in your experiment!
There’s no one-size-fits-all prescription for comfort, but always make sure you participate in your own experimental set-up, so you have a first-person perspective on comfort.
Exercises
- List 3 objects that are definitely projectile risks.
- List 3 metal objects that are not likely to become projectiles (which doesn’t mean you should take them into a scanner room!)
- If your participant is wearing only ear plugs for hearing protection, what’s the loudest pulse sequence you should run?
- How would you find out how loud your pulse sequence is?