The distance between the probes to be used in a pilot study (dots) can be determined by taking the sum of the areas of the sectional profiles of the region of interest (in this example, nine sections from the striatum of a rat brain) and dividing this by 150. Note that the number of probes in a section is proportionate to the area of the structure of interest in that section. (Redrawn from West et al. 1996.)