BrainWeb is an online interface providing access to a 3D simulated brain MRI database (Simulated Brain Database, SBD) for the neuroimaging community. It allows researchers to examine the effects of image acquisition parameters on anatomical brain mapping and quantitative analysis. The database is generated by varying specific imaging parameters and artifacts in an MRI simulator, starting from a fuzzy digital phantom with probabilistic tissue distribution. Signal intensities are computed based on Bloch equations, accounting for partial volume effects. Image noise is specified as a percentage of the mean signal intensity, and multiplicative receive intensity nonuniformity (INU) is simulated using real scan fields. The SBD includes simulated volumes for three imaging sequences (T1, T2, PD) with various parameters, including slice thickness, noise, and INU levels. BrainWeb also allows users to run custom MRI simulations with different pulse sequences and phantoms. All 3D images are in stereotaxic space and can be interactively explored. The database is available online at "http://www.bic.mni.mcgill.ca/brainweb/". SBD can be used to study the performance of anatomical brain mapping techniques, validate quantitative analyses of neuropathology, and test medical pattern recognition techniques. The main advantages of using these simulations include known answers and independent parameter control. BrainWeb is supported by the U.S. Human Brain Map Project and the International Consortium for Brain Mapping. It is currently being extended to include fMRI and PET simulated data.BrainWeb is an online interface providing access to a 3D simulated brain MRI database (Simulated Brain Database, SBD) for the neuroimaging community. It allows researchers to examine the effects of image acquisition parameters on anatomical brain mapping and quantitative analysis. The database is generated by varying specific imaging parameters and artifacts in an MRI simulator, starting from a fuzzy digital phantom with probabilistic tissue distribution. Signal intensities are computed based on Bloch equations, accounting for partial volume effects. Image noise is specified as a percentage of the mean signal intensity, and multiplicative receive intensity nonuniformity (INU) is simulated using real scan fields. The SBD includes simulated volumes for three imaging sequences (T1, T2, PD) with various parameters, including slice thickness, noise, and INU levels. BrainWeb also allows users to run custom MRI simulations with different pulse sequences and phantoms. All 3D images are in stereotaxic space and can be interactively explored. The database is available online at "http://www.bic.mni.mcgill.ca/brainweb/". SBD can be used to study the performance of anatomical brain mapping techniques, validate quantitative analyses of neuropathology, and test medical pattern recognition techniques. The main advantages of using these simulations include known answers and independent parameter control. BrainWeb is supported by the U.S. Human Brain Map Project and the International Consortium for Brain Mapping. It is currently being extended to include fMRI and PET simulated data.