In a completely uniform magnetic field, the spatial location of the measured MR signal cannot be determined. However, by temporarily imposing a separate magnetic field that varies linearly across a volume, known as a gradient, it becomes possible to temporarily alter the strength of the external magnetic field in a spatially specific manner. This spatial encoding is achieved by the application of gradient fields in three dimensions at critical times in relation to the RF pulse. Gradients affect which portions of the brain receive the RF energy (slice selection gradient, z-direction), the phase in which the excited nuclei are precessing (phase encoding gradient, y-direction), and the frequency in which the excited nuclei are precessing at the time that the emitted RF energy is reradiated (frequency or "read-out" gradient, x-direction). The application of these gradients allows for the transformation of an acquired free induction decay signal into an image in Cartesian space, where the matrix size of the volume is a function of the number of steps in the three gradients. For example, if an image was created using 20 z, 128 y, and 128 x encoding steps, it would result in a volume of 20 slices containing 128 x 128 pixels or voxels on each slice (a pixel refers to a distinguishable square of information within a two-dimensional image, whereas a voxel corresponds to a box of information in a three-dimensional image).
The type of information that is obtained by an MRI scan depends on how and when magnetic gradients and RF pulses are applied (commonly referred to as the pulse sequence). Fast echo planar imaging (EP1) is by far the most dominant pulse sequence technique used for fMRI (Buxton, 2002; Jezzard, Matthews, & Smith, 2001; Moonen & Ban-dettini, 1999). EPI differs from other standard imaging methods in that it acquires multislice volumes of MR images very rapidly. It does this by applying a rapid cycling phase encoding gradient where all the phase encoding steps are done in a single repetition time (TR) after a single RF pulse. In contrast, more traditional structural MRI techniques apply a single phase encoding gradient step per TR. Depending on the slice thickness and matrix size, fMRI studies using EPI may obtain whole brain coverage every 1 to 3 seconds, (TR = 1-3 s). A number of alternative pulse sequences, such as spin echo or spiral sequences, can be used to detect changes in BOLD signal (Haacke et al., 1999; Noll, Cohen, Meyer, & Schneider, 1995). These tech niques vary in terms of aspects of the RF pulse or the ordering of gradient steps and have both advantages and disadvantages relative to EPI (Kennan, 1999; Noll, Stenger, Vazquez, & Peltier, 1999).
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