Monday, April 2, 2012
Time to get back to the artifact recognition series of posts, all of which have the Artifacts label in the footer. RF interference (RFI), or more generally electromagnetic interference (EMI), is another one of the insidious artifacts that can be difficult to diagnose online, during an experiment, unless it becomes catastrophically bad. Your scanner is equipped with sensitive, specific tests for RFI that are used by the service engineer (and probably your physicist) to check for problems, but imaging isn't a sensitive test. Consequently, avoidance rather than diagnosis is usually the preferable option during an fMRI experiment, and a little bit of care and standard operating procedures should suffice to ensure minimal hazards to your data.
I'll begin this post with a description of the nature and sources of RF interference in the MR environment, then provide an example of RF interference in EPI time series data. Next I'll describe the sorts of things you should expect to do when you want to interface a new device, such as a button response box or a physiological monitoring unit, to your scanner as a component of your experiment. It's not - at least, it shouldn't be - a case of "plug n' play!" Finally, I'll describe a simple procedure you can follow to ensure minimal to no problems for your experiment, assuming that your facility has been set up properly.
What is RFI and where does it come from?
A nominal 3 tesla scanner is operating somewhere in the range 120-130 MHz. My scanner is parked at 123 MHz, with a magnetic field strength of 2.89 tesla. (Correct, it's only a 3 T scanner to one significant figure!) A quick glance at the FM dial on an analog radio receiver suggests immediately that the operating frequency of your MRI isn't all that different to your local broadcast radio stations. MRIs aren't the only devices operating at tens and hundreds of MHz in normal operation.