Magnetic resonance images are simulated by MRISIM [23], a program that predicts image contrast by computing NMR signal intensities from a discrete-event simulation based on the Bloch equations of a given pulse sequence. A realistic high-resolution brain phantom was used to map tissue intensities into MR images. The simulator accounts for the effects of various image acquisition parameters by incorporating partial volume averaging, measurement noise and intensity non-uniformity. The images in Fig. 1 were simulated using the same MR parameters as those described below (c.f., 2.1) for the real MRI acquisitions.