Virtual 10–20 measurement on MR images for inter-modal linking of transcranial and tomographic neuroimaging methods Valer Jurcak, Masako Okamoto, Archana Singh, and Ippeita Dan * National Food Research Institute, 2-1-12 Kannondai, Tsukuba 305-8642, Japan Received 21 December 2004; revised 8 March 2005; accepted 14 March 2005 Available online 28 April 2005 It is important to create a link between stereotaxic coordinates and head-surface-based positioning systems in order to share data between tomographic and transcranial brain mapping studies. In our previous studies, we established the probabilistic correspondence of the interna- tional 10 – 20 positions to the standard stereotaxic coordinate systems and made a reference database. However, its expansion required the physical marking of the 10 – 20 positions and the subsequent acquis- ition of MR images. To avoid such tedious procedures, we developed a virtual 10 – 20 measurement algorithm that can be applied to reanalyze any structural MR image that covers the whole head. As in the physical 10– 20 measurements, with the reference points given, the algorithm automatically determines each 10 – 20 position step by step. Using the virtual 10 – 20 measurement method, we reanalyzed the MR images of 17 healthy subjects for whom we had determined 10 – 20 positions by physical marking in our previous study. The acquired 10– 20 positions were normalized to the Montreal Neurological Institute (MNI) stereo- tactic coordinates and compared with the positions previously determined by physical measurements. 10 – 20 positions determined using the virtual and physical methods were roughly consistent. Average standard deviations for virtual and physical methods were 7.7 mm and 9.0 mm, respectively. There was a systematic shift in the virtual method, likely due to the absence of hair interference. We corrected the shift with affine transformation. The virtual 10 – 20 measurement method proved to be an effective alternative to physical marking. This method will serve as an essential tool for expanding the reference database and will further strengthen the link between tomographic and transcranial brain mapping methods. D 2005 Elsevier Inc. All rights reserved. Keywords: Inter-modal data sharing; International 10 – 20 system; Com- mon stereotaxic platform; MNI coordinate system; Human brain mapping; Near-infrared spectroscopy; Optical topography; Transcranial magnetic stimulation Introduction There is no single perfect modality for assessing human brain function. Recent advent and development of various brain mapping techniques theoretically allow researchers to look at human brain function from different perspectives. In reality, however, such inter- modal trends seem to generate a Tower-of-Babel effect, where researchers using different modalities feel that there is some difficulty in communicating with each other. The true value of functional data acquired from different modalities cannot be appreciated until they are assessed in a common arena. Recently, in pursuit of a common arena for inter-modal assessment, a movement described as a probabilistic approach to expressing all functional brain data as entries in a brain atlas that expands into space and time has appeared (Abbott, 2003; Mazziotta et al., 2000,a,b; Toga and Thompson, 2001). The probabilistic atlas per se has not been realized, but its philosophy has become widespread so that by and large, the essential concept for this integrative approach has already been realized in different forms, namely, as stereotaxic standard coordinate systems such as Talairach or MNI coordinates (Talairach and Tournoux, 1988; reviewed in Brett et al., 2002). In these stereotaxic systems, structural brain imaging data are fit to the standard template brain by linear and nonlinear trans- formation processes (normalization). A template for the Talairach space is based on a single patient’s brain with detailed expression of anatomical features including Brodmann estimates (Talairach and Tournoux, 1988). The MNI standard template was generated by fitting brains of multiple subjects to the Talairach template and subsequently averaging them (Collins et al., 1994). Functional data are also registered for the normalized brain, and hence to the MNI or Talairach coordinate systems. Consequently, the stereotaxic sys- tems have virtually culminated in producing a common platform, where researchers can handle functional localization data with a set of three coordinate values within the standard template brains. Accordingly, presenting functional mapping data on the standard coordinate space has turned out to be the golden standard for tomographic functional brain mapping methods, such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET). 1053-8119/$ - see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.neuroimage.2005.03.021 * Corresponding author. Fax: +81 29 838 8122. E-mail address: dan@nfri.affrc.go.jp (I. Dan). Available online on ScienceDirect (www.sciencedirect.com). www.elsevier.com/locate/ynimg NeuroImage 26 (2005) 1184 – 1192