The MRI: Noisy, and the Echo Nearly Killed Me.

Update 11/11/11 - wordsmithing, correct some typos and grammar. 

Today I finally had my MRI (Magnetic Reasonance Imaging) so the radiologist could get a picture of what's inside my head.  She needs this so she'll have an idea of how bright to make, and how to aim all those little radioactive flashlights I mentioned earlier in my explanation of IMRT (Intensity Modulated Radiation Therapy).

For those who have not had the pleasure of an MRI, here's what it was like for me: lying inside a large tube while jackhammers of various sizes and speeds are being tested on it from outside.  I have no doubt that the people at Siemens (the company that made this particular machine) know what they're doing, but it sounded like some serious metal on metal action was going on around me -- not what I usually associate with magnets -- that will require lots of maintenance down the road.  I believe this machine was one of the "open" MRI machines you hear advertised on TV, but from what I remember about the "closed" MRI I had about 12 years ago, there's not much difference.

MRI's are essentially a powerful main magnet, a radio frequency broadcaster, a radio antenna, and a few non-super gradient magnets hooked to a computer.  The main magnet has all sorts of super credentials like being super strong because it's made of super conducting wire because it's super cooled with super cold liquid helium with the whole lot being super expensive. Ebay has a used MRI trailer for $145,000, shipped freight, no returns. 

The theory of an MRI goes something like this: All of the atoms of your body are spinning in some quantum way that can be measured; in this case we're only worried about the spin of hydrogen atoms.  Like anything that spins, they wobble, much as the classic spinning top.  The vocabulary word you're looking for is precess.

The main magnet is so strong that it forces all the atoms to stop precessing (wobbling) and line up in a north-south fashion much like the iron filings in your second grade magnet experiment.  Nearly all of the atoms line up in pairs which lock them firmly in place, but like at any high school dance, there's always going to be some that won't (or can't) pair up.  The unpaired ones are still oriented north-south at the dance, they're just freer to move about the paired up atoms.

Once the atoms are lined up, a pulse of radio waves that are "tuned" to hydrogen (the Larmour frequency) is beamed at your body.  This causes the unpaired hydrogen atoms to shift a little bit to the side of the north-south orientation of the rest of the atoms.  When the pulse finishes, the hydrogen atoms snap back to their former north-south position, each giving off a tiny radio wave of its own; this happens hundreds of times each second.  These tiny radio transmissions are then picked up by the MRI antenna and sent to the computer for processing.  While all this tilt and snap business is going on, the non-super gradient magnets "shape" the magnetic field in order to let the machine take glances at different parts of your body.  It's the gradient magnets that are responsible for all the clattering and knocking racket going on around you. 

The computer takes all this, crunches its numbers to make sense of it all and draw a picture for you.

So much for the theory.  Since none of the previous explanation is going to get you through med school, you're probably here looking for practical advice.

If you're having an MRI done for the head and neck area, you'll probably be asked to come in early for a quick x-ray.  The main magnet is so strong that it's been known to pull keys out of pockets, pistols out of holsters (Really!), and demagnetize credit cards from across the room.  The reason for the x-ray is to check for metal filings that may have lodged in your eyes.  Nobody wants to do emergency eye surgery because a bit of metal tore through your cornea from the inside out.

You'll be asked to remove all metal from your person -- I have no idea what they do with folks who have piercings -- in order to avoid distortion of the image and flying metal.  If you've had previous surgeries, find out if any staples were used to repair the incision.  For instance, my 1979 surgery left metal staples in me that were not MRI safe and I had to forgo a renal MRI angiogram because the staples would have interfered with the MRI.  (I found this out about 15 minutes before I was about to climb into the machine, but the trip to the hospital wasn't a total loss thanks to my wife.)  Nowadays, surgical staples are MRI safe.

Don't wear clothes that have any metal on them.  I learned this the hard way at my PET scan and was forced to wear a fashionable hospital gown.

Once you're on the MRI table, you may have a "cage" placed around the part of your body that's being imaged.  The cage is made of metal and is designed to bring the imaged body part further into focus. When I had an ankle MRI a few years back, the tech placed what amounted to a huge bracelet around my ankle.  This time I had a sort of face mask placed over my face and neck for the dual purpose of focusing the MRI's energy on my face and neck and literally bolting my head to the table so that it couldn't move. 

Bring earplugs; the machines are incredibly noisy.  I forgot my earplugs, but the MRI tech kindly supplied me with some.  Not sure about music players and earphones, but if they're allowed, be careful of digital music players.  They may use magnetic storage in which case you'll lose all of your irreplaceable classic 70's and 80's rock that no one ever plays any more.  If it's Smooth Jazz, that's OK.