MRI: A Guided Tour

Magnets with Muscle

Let’s start with a little tour of the metallic cylinder surrounding you (it’s called a bore, technically).

You’re in the center of a tremendous magnet, weighing tens of thousands of pounds and differing from the little magnets on your fridge in two fundamental ways. First, those fridge decorations are permanent magnets made of alloys. The MRI magnet surrounding you, on the other hand, is a superconducting magnet; it conducts electricity, thereby creating a magnetic field. Secondly, your fridge magnet has a fraction of the power of the one you’re in. Scientists measure magnetic strength in units called tesla and gauss – 1 tesla equals 10,000 gauss. The Earth’s magnetic pull is about .5 gauss. Your fridge magnet is about 10 gauss. The electromagnet you’re inside could be up to 3 tesla – 60,000 times the force of the Earth’s magnetic field.

Let’s take a moment to appreciate the “superconducting” part of that magnet, without which your MRI scanner would not be here. You could (and people do) make a permanent magnet with the strength to run an MRI. For the most part, however, these magnets are prohibitively huge and heavy. That leaves you the option of creating a magnet by running electrical current through wire coils – an electromagnet. The problem is the electrons making up that current are forever bumping into the fidgety atomic particles of the material through which they are traveling, slowing them down considerably. (Brush up with a quick review of electricity at this atomic level, if you need to). Given the resistance the current encounters, providing the vast amount of power required to overcome it and generate a magnetic field sufficient to operate an MRI would be prohibitively expensive.

This is where our hero, superconductivity, saves the day! Take special coils and surround them with something really, really cold – liquid helium, at 452.4 degrees below zero on the Fahrenheit scale, does quite nicely. The result? Those over-caffeinated atoms in the conducting wire are frozen into submission. Slowed to a virtual halt, they allow the current to sail right through the miles of wires snaking through an MRI scanner. This technology allows for the construction of hugely powerful magnets like the one surrounding you right now. Most clinical MRI scanners use superconducting magnets. If you’re interested in learning more about this, you can read a more in-depth overview of superconductivity.

By the way, don’t let that little business about liquid helium worry you. It’s insulated in a vacuum, so you won’t need your parka. It wouldn’t be allowed, anyway; zippers, snaps, jewelry and other metals can become life-threatening projectiles in the vicinity of a magnet as powerful as this one. That’s why technicians are very careful to keep metals outside the exam room, and why people with pacemakers and aneurism clips can’t have MRI scans.

By now the technologist in the control room, who is talking you through the exam via an intercom, has started to place you into the main magnetic field in your scanner. The field is running horizontally through the bore from your head to your toes (or vice versa, depending on your position). Because your spine is what she’s interested in, you’re being positioned so that part of your back is in the middle (or isocenter) of the field.

You’d think that being in the middle of such a powerful force would make you feel different – tingly or something. It doesn’t. However, on an atomic level, it’s quite a different story, which takes us from the “M” of MRI to the “R” – Resonance. We’ll understand this better after first taking a close look at the molecules in your body.

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