Eur Radiol (2008) 18: 1031–1037 DOI 10.1007/s00330-008-0850-z NEURO G. Fesl B. Braun S. Rau M. Wiesmann M. Ruge P. Bruhns J. Linn T. Stephan J. Ilmberger J.-C. Tonn H. Brückmann Received: 4 May 2007 Revised: 16 December 2007 Accepted: 2 January 2008 Published online: 29 January 2008 # European Society of Radiology 2008 Is the center of mass (COM) a reliable parameter for the localization of brain function in fMRI? Abstract The center of mass (COM) in functional MRI studies is defined as the center of a cerebral activation cluster. Although the COM is a well- accepted parameter for exactly loca- lizing brain function, the reliability of COMs has not received much atten- tion until now. Our goal was to investigate COM reliability as a func- tion of the thresholding technique, the threshold level, and the type of COM calculation. Therefore 15 subjects were examined repeatedly using simple hand and tongue movement paradigms. Postprocessing was per- formed with uncorrected, corrected, and proportional thresholding as well as different threshold levels. Geo- metric and T-weighted COMs of left- hemispheric primary hand and tongue motor clusters were calculated. The COM variation was evaluated within and between repeated sessions depending on the different postpro- cessing setups. Mean COM variations over three repeated sessions varied between 1.6 mm and 9.8 mm for the hand paradigm and between 7.0 mm and 14.4 mm for the tongue task. Stringent thresholding techniques and high threshold levels were required to assess reliable results, whereas the kind of COM calculation was of lesser relevance. Thus, COM reliability cannot be presupposed; it depends strongly on the individual postpro- cessing techniques. This should be considered when using COMs for localizing brain function. Keywords Center of mass . fMRI . Reliability . Hand . Tongue Introduction The center of mass (COM) or center of gravity (COG) is widely used in functional imaging. This parameter indicates the center of a cerebral activation cluster consisting of a defined number of voxels [1–3]. Such a punctual description of function is advantageous primarily in somatotopic and validation studies, since it allows the accurate localization of adjacent and partly overlapping functional areas [4–9] as well as the exact measurement of distances between results assessed with fMRI and other methods such as PET, TMS, and EEG [10–14]. However, the assumption that the COM is a reliable and precise measure for localizing brain function has been investigated in only a few studies. COM distances of less than 3 mm over repeated measurements have been reported G. Fesl (*) . B. Braun . M. Wiesmann . P. Bruhns . J. Linn . H. Brückmann Department of Neuroradiology, University of Munich, Klinikum Grosshadern, Marchioninistr. 15, 81377 Munich, Germany e-mail: Gunther.Fesl@med.uni- muenchen.de Tel.: +49-89-70952501 Fax: +49-89-70952509 S. Rau . J. Ilmberger Department of Physical Medicine and Rehabilitation, University of Munich, Klinikum Grosshadern, Marchioninistr. 15, 81377 Munich, Germany M. Ruge . J.-C. Tonn Department of Neurosurgery, University of Munich, Klinikum Grosshadern, Marchioninistr. 15, 81377 Munich, Germany T. Stephan Department of Neurology, University of Munich, Klinikum Grosshadern, Marchioninistr. 15, 81377 Munich, Germany