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How does an MRI work?

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How does an MRI work?  What's all that noise?  What does it all mean?   

Next time you're scheduled for an MRI, read this first! 

An MRI works by finding the hydrogen protons in water.  It moves the protons around with a powerful superconducting magnet, housed in the MRI scanning tube.  When the magnet is on, it's thousands of times more powerful than the magnetic field of earth.  So the magnet is left on, and all the hydrogen protons line up.  Then the scanner stirs them up with radiofrequency waves, generated by copper coils in that tube that you're laying in.  The machine turns those coils on and off.  This happens hundreds of times a minute, so that's what all that noise is about.  The hydrogen protons act differently, depending upon what kind of tissue they're in, so this can generate a pretty accurate picture. 

This is why a 1.5 Tesla machine is better than a .5 Tesla - bigger magnet means more protons jump up and beg to be noticed.  A 3 Tesla is even better, but they're hard to find. 

• So what's all the deal with T1 and T2? 

In a plain ol' MRI, signal intensity is proportional to the amount of hydrogen protons.  More protons, the brighter the image.  They can tell the machine to only watch for longitudinal movement (T1, aka spin-lattice relaxation time) or tranverse relaxation movement (T2, aka spin-spin relaxation time) and they can twiddle this even further with variations in the strength, orientation, and timing of RF pulses.  T1 weighted is best for looking at brain structures (like black holes,) while T2 is better for looking at non-enhancing brain lesions.  T1 is best for looking at the brain and spinal cord after gadolinium is given, as the gadolinium shows up really brightly. 

• And what the heck is FLAIR? 

Fluid-attentuated recovery, if that helps.  To put it simply, it's just a pulse sequence that blacks out the CSF so that the rest of the brain can be seen better.  It's easier to see hyperintense lesions with FLAIR.   

Evidently, with the right combination of numbers, any particular tissue can be blacked out.  Think of T2 and T1 as mathematical vectors that the radiologist plugs into the machine to get different looks at your brain.

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