Trouble Thinking

September 15, 2010

What Exactly Is An MRI?

Filed under: Science — Tags: , , , , — Durandal @ 12:23 pm

Throughout Trouble Thinking’s storied several-month history, one question has been relentlessly asked by readers and yet has remained cruelly unanswered: what is the function and design of an MRI?

The time has come to put this questions to rest.

First of all, it looks like this:

Like a cross between Vader's chair-bubble and what Dr. Doom uses to place you into a nightmare from which you will NEVER AWAKE.

A Magnetic Resonance Imaging (MRI) machine is designed to peer through your skin and see the bits inside using immense powers you barely understand. That must be frightening for you.

So why do we need an MRI to look inside ourselves when we already have other methods of looking into the human body, like X-Rays?

Several reasons. First of all, there’s only so often you can X-Ray your genitals before you stop producing children and start producing X-Men. But not the good X-Men, you get this guy:

The BEST X-Man.

Basically, many other imaging techniques use radiation. And while said radiation probably won’t do anything in particular to you, it’s considered ethical to limit the exposure the average research participant has to the potentially harmful effects of radiation no matter how small the likelihood of harm. This is the same reason most men cover their balls during a dental X-Ray. You can never be too careful. An MRI is also non-invasive, requiring only that you sit still. Many other techniques for imaging require the use of “contrast material” that is minutely radioactive, and has to be injected.

Alright so presumably you don’t want anything to do with Beak up there, so you’re intrigued. But how does an MRI work, if not through the wonders of our friend Radiation?

An MRI works by using superconducting magnets of a awesome size and power to align the water molecules in your body to a standing magnetic field. Normally, hydrogen in water molecules kind of spins around a magnetic field at random. The MRI field is so insanely strong (a 3T magnet is 60,000 times as strong as the Earth’s magnetic field) that it grabs hold of all those randomly spinning hydrogen atoms and shoves them into line, either “up” or “down” depending on how they were originally spinning.

Once that great injustice to Hydrogen has been perpetrated, a “coil” is then used to shoot a radiofrequency (RF) pulse at your tissues. The pulse gives a bit of extra energy to some of the Hydrogen atoms, and forces them to spin in a specific direction at a specific frequency. Sort of like when one of your friends holds someone’s arms and you hit them until they start talking. When the pulse is shut off, the Hydrogen returns to the frequency it was forced into by the giant standing magnetic field, releasing the energy the pulse shot into it. Based on how long it takes to do this, you can make some conclusions about the type and composition of the tissue.

In order to make the contrast even clearer for particular tissues or tissue types, usually the magnetic field is altered slightly by less impressive “gradient” magnets, that flick on and off near the bit of a person you want examined. This slight jostling of the magnetic field affects different things in different ways, and makes it easier to discern what’s what.

Once all this is done, you end up with a startlingly clear picture of what’s inside a person.

Horrifyingly clear, actually.

Obviously it’s useful as a sort of “muscle X-Ray” for the diagnosis of things like cysts, tumors, torn ligaments, and such. But probably the most interesting advantage to an MRI is the ability to frequently scan with little to no risk. That allows a much broader range of possible research and research subjects than most other imaging techniques, and you can look at a single subject with much greater detail over time.

Perhaps even more important for brain imaging, though, an MRI can be used to perform a functional MRI (fMRI) scan. I’ll talk about that later, though. I don’t want to overwhelm you with my vast stores of knowledge about subjects of which you’re lucky to grasp the barest sliver.

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