Susceptibility-induced loss of signal: Comparing PET and fMRI on a semantic task

Functional magnetic resonance imaging (fMRI) has become a popular tool for investigations into the neural correlates of cognitive activity. One limita tion of fMRI, however, is that it has difficulty imaging regions near tissu e interfaces due to distortions from macroscopic susceptibility effects whi ch become more severe at higher magnetic field strengths. This difficulty c an be particularly problematic for language tasks that engage regions of th e temporal lobes near the air-filled sinuses. This paper investigates susce ptibility-induced signal loss in the temporal lobes and proposes that by de fining a priori regions of interest and using the small-volume statistical correction of K. J. Worsley, S. Marrett, P. Neelin, A. C. Vandal, K. J. Fri ston, and A. C. Evans (1996, Hum. Brain Mapp. 4: 58-83), activations in the se areas can sometimes be detected by increasing the statistical power of t he analysis. We conducted two experiments, one with PET and the other with fMRI, using almost identical semantic categorization paradigms and comparab le methods of analysis. There were areas of overlap as well as differences between the PET and fMRI results. One anticipated difference was a lack of activation in two regions in the temporal lobe on initial analyses in the f MRI data set. With a specific region of interest, however, activation in on e of the regions was detected. These experiments demonstrate three points: first, even for almost identical cognitive tasks such as those in this stud y, PET and fMRI may not produce identical results; second, differences betw een the two methods due to macroscopic susceptibility artifacts in fMRI can be overcome with appropriate statistical corrections, but only partially; and third, new data acquisition paradigms are necessary to fully deal with susceptibility-induced signal loss if the sensitivity of the fMRI experimen t to temporal lobe activations is to be enhanced. (C) 2000 Academic Press.