This is from a Gina Kolata piece in The New York Times:

Magnetic resonance machines….vary enormously, and not just in the strength of their magnets. Even more important, radiologists say, is the quality of the imaging coils they put around the body part being scanned and the computer programs they use to control the imaging and to analyze the images. And there is a huge variability in skill among the technicians doing the scans…. Then there is the question of how skilled is the radiologist who reads your scans.