

Sunday, February 19, 2012

Terminology change: characterizing EPI artifacts

After considering the rest of the topics that I want to cover in the series of posts on EPI artifacts, I've decided to change the terms "static" and "dynamic" to "persistent" and "intermittent," respectively. I think the new terms better reflect the dominant temporal character of each artifact. The idea is to sort them based on whether they are likely to plague every frame of an EPI time series, or come and go.

Take external RF interference, for example. Say you fail to close properly the RF-sealed door to the magnet room and the door opens slightly during your scan, leading to RF contamination from "environmental" sources. In this instance the RF interference itself isn't likely to be static - it will vary with whatever sources of RF happen to be in your scanner environment - but it will persist (at some level) until the door is closed. With a good eye or some appropriate diagnostics it would be possible to show the persistence of the problem throughout an acquisition. Contrast this situation with a static electrical discharge somewhere within the scanner room; tiny sparks that cause a broad range of electromagnetic frequencies, including radiofrequencies. These can arise if the humidity of the magnet room air becomes too low. Depending on the source of the sparks, the humidity, etc. you might find that only one or two TRs of a time series are contaminated, or you could find the entire time series is affected. I will therefore characterize static electrical discharges as an intermittent artifact.

Pedants will spot that the first example can be modulated by the position of the magnet room door, rendering the artifact intermittent, while in the second example the propensity for static electrical discharges will persist as long as the source exists, while the humidity remains low, etc. So, yeah, in some ways the distinctions I'm making are subjective. FMRI, like life, is complicated! Still, I'm hopeful that a more practical characterization of artifacts will assist you in differentiating and diagnosing them when it matters: during your experiment. And once you're an expert you will find it easy to comprehend the nuances of temporal behavior, when my artificial distinctions will be all but irrelevant to you.

So, there you have it. I'll be going back to edit the existing posts in this series over the next couple of days. Apologies for any confusion the switch creates.