I found the answer to question #2 in the archives. Regularisation level and degree of bias correction are inversely related, so the observed ranges make sense.
From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] On Behalf Of Dana Perantie
Sent: Tuesday, January 20, 2009 2:08 PM
To: [log in to unmask]
Subject: [SPM] bias correction - what level?
Dear SPM list,
I am using DARTEL in SPM8b to do longitudinal analyses of T1 images. For the initial Segment, I am looking at the different options for bias correction ranging from "very light" (default) to "extremely heavy". I tested all 6 levels of bias correction on one image. I applied the bias correction parameters from *seg_sn.mat to an image of all ones, using cspm_bias_writecorrected.m. This way the bias field that is applied can be visualized.
I have attached the results as viewed in Check Reg, with "very light" entered first and all of them in order progressing up to "extremely heavy". I have a few questions...
1) Is it a concern that the shape of the brain is discernable? Might this mean that intensity data related to brain is being mistaken for intensity variation due to field inhomogeneity?
2) I was surprised that the range of values was greatest using the "very light" setting (.94 to 1.07) and most limited in the "extremely heavy" setting (.98 - 1.01). This was the opposite of what I expected based on the names -- the "very light" setting is making the greatest changes. Is this supposed to happen?
3) In some locations the intensity is adjusted down using the "very light" setting and up using the "extremely heavy" setting. The results appear to be qualitatively different; how can one decide which is best? Is there a recommendation what level of bias correction to use for longitudinal data, where we are especially concerned with potential influence of field change over time?
Thank you for your advice,