Anatomical atlas-guided diffuse optical tomography of brain activation
Introduction
Diffuse optical tomography (DOT) is the tomographic variant of Near Infrared Spectroscopy (NIRS). These methods complement functional Magnetic Resonance Imaging (fMRI) of human brain activation by providing measures of changes in both oxygenated (HbO) and deoxygenated (HbR) hemoglobin with a superior temporal resolution (see Obrig and Villringer, 2003, Strangman et al., 2002, Shibasaki, 2008, Toronov et al., 2007 for a review). NIRS requires only compact experimental systems, is less restrictive, and is relatively forgiving of body movement, and thus provides one with a wide variety of flexible measurement options of brain function in adults and infants (reviewed in Aslin and Mehler, 2005, Irani et al., 2007).
NIRS for the non-invasive monitoring of brain hemodynamics was introduced more than 30 years ago by Jobsis (1977). NIRS was first applied to measure hemodynamics associated with functional brain activity in the early 1990's (Chance et al., 1993, Hoshi and Tamura, 1993, Kato et al., 1993, Villringer et al., 1993). In these early days, the number of source–detector pairs (aka. channels) was limited to one or at most a few distantly placed to avoid light interference. Multichannel NIRS instruments were then developed with an array of multiple source–detector pairs that allowed simultaneous monitoring across brain regions (Maki et al., 1995). Multichannel NIRS data are often treated in a discrete manner, and subjected to channel-wise statistical analysis within a subject (e.g., Schroeter et al., (2004)) or among a group of subjects (e.g., Okamoto et al., (2006)). To form images, these multichannel NIRS data have been transformed via spatial interpolation to generate 2 dimensional topographic images of brain activation (Maki et al., 1995). Moreover, NIRS has been recently implemented with a relatively large number of densely-placed sources and detectors, allowing tomographic methods, or Diffused Optical Tomography (DOT), to reconstruct 3D images of brain activation (Bluestone et al., 2001, Zeff et al., 2007, Culver et al., 2003). DOT uses short and long distance measurements to provide depth resolution and enables separation of superficial scalp signals from deeper brain signals.
DOT generally requires subject-specific spatial priors of the head anatomy for constructing 3D images of brain activation (Boas and Dale, 2005), but it is not always feasible to obtain subject-specific head anatomy. We thus propose a method to utilize a general head template (atlas) to construct 3D DOT images while maintaining capability of anatomical interpretation of the resultant images. This MRI-free approach to obtaining optical images is based on registering a selected atlas to the subject headsurface and solving the photon migration forward problem on the registered atlas; optical measurements are acquired on the physical subject; and then a map of the cortical absorption coefficient changes is calculated.
We first describe the atlas-based DOT method, including the atlas selection criteria, the dataset, the registration algorithm, the tissue optical properties, the probe design, the photon migration simulation, and the inverse problem solution. The hemodynamic response to median-nerve stimulation measured in three healthy individuals is reconstructed using the subject-specific brain anatomy as well as the atlas, and the spatial localization accuracy is estimated. The results indicate that by using an atlas as a general head model, it is possible to reconstruct the location of the activation focus within the correct gyrus.
Section snippets
Atlas-guided image processing
Diffuse optical tomography comprises two steps: In the forward model (y = Ax), photon migration is simulated in a segmented head obtained from an anatomical MRI. This forward model produces, for source–detector pair, a spatial map of the sensitivity to absorption changes within the head. This spatial sensitivity to absorption changes for each source–detector pair is represented by a row in matrix A. The second step is the solution of the inverse problem (): the Tikhonov
Results
The brain activation images reconstructed in the subject and registered atlas heads are shown in Figs. 4, Fig. 5, Fig. 6. Fig. 4 shows a visual representation of the results for subject c (the same as in Fig. 5c). The subject (left image) and corresponding registered atlas (right image) activation maps are plotted on the cortical surfaces and the probe location is also shown. The activation contrast has been thresholded at half maximum, for better visualization and to focus the comparison
Conclusions
In this paper we introduced an imaging protocol that uses diffuse optical tomography (DOT) to reconstruct brain activation images without the need of the subject's anatomical MRI. The approach consists of using an atlas-based anatomical model instead of the subject's anatomy and solving the photon migration forward model on the atlas model, while acquiring optical measurements from the subject, and then reconstructing the cortical activation in the registered atlas. Although subject's and
Acknowledgments
This work was supported by NIH U54-EB-005149, NIH P41-RR14075, and NIH P41-RR13218.
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