The simplest geometry, parallel, was used in first generation scanners. As mentioned above, the focal length is not used in this simple geometry. The detector array is set to be the same size as the scan diameter. For optimal scanning in this geometry, the scan diameter should be equal to the phantom diameter. This is accomplished by using the default values of 1 for the view ratio and the scan ratio. If values of less than 1 are used for these two variables, significant distortions will occur.