Combining EEG and MRI Techniques - Quantitative EEG Analysis Methods and Clinical Applications - page 318

Three-dimensional structural MRI datasets are stacks of two-dimensional
images (slices), each typically 256 × 256 pixels in size (rarely, 512 × 512). The pixel

dimensions of such a 256 × 256 image showing a cross section of the head are

1 mm². The slice distance in the stack of images is somewhere
between 1 and 2 mm. The field of view should be adjusted so that the whole head is
captured, not only the brain—this allows for the definition of skin landmarks and
realistic head models. T1 protocols deliver good gray matter-white matter contrast
and short acquisition times. When loaded into the computer, the stack of slices
becomes a three-dimensional image (see Figure 12.1) with cuboid (brick-shaped)
voxels.
Although some of the benefits of using MRI can also be achieved using a nor-
malized and averaged dataset such as the ICBM152 brain from the Montreal Neuro-
logical Institute (MNI) [1], exact head models can be derived only from individual
image data, and cortical structures should not be used across subjects at all due to
the high intersubject variability of cortical gyri and sulci.

approximately 1 ×

(a)

(b)

Figure 12.1 (a) Three-dimensional structural MRI datasets are stacks of slices, each typically 256

×
256 pixels in size. (b) When loaded into the computer, the stack of two-dimensional slices becomes a
three-dimensional image.