Neuroimaging Primer

Keith A. Johnson, M.D., Harvard Medical School

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Tome is Greek for slice. The standard slice orientation in most brain imaging is transaxial or "axial". Left is shown at right. Note that, like the "lower organs", we look up to the brain. Other standard planes of view are coronal and sagittal. Non-tomographic images represent "projections" from a single point of view and include bolus contrast x-ray angiograms and MR angiograms.

Tomographic images are made up of little squares called "pixels" (picture elements), each of which takes a grey-scale value from 1 (black) to 256 (white). Each pixel represents brain tissue which is about 1 mm. on each of two sides. The thickness of the slice is often 3 or 5 mm, thus creating a three-dimensional volume element, or "voxel", which is shaped like a shoe box. Pixel intensity represents an average from tissue within the voxel.

Image types