Focal Length

The focal length, F, is the distance of the X-ray source to the center of the phantom. The focal length is set as a ratio, FR, of the view radius. Focal length is calculated as
F = (Vd / 2) x FR

For parallel geometry scanning, the focal length doesn't matter. However, for divergent geometry scanning (equilinear and equiangular), the focal length ratio should be set at 2 or more to avoid artifacts. Moreover, a value of less than 1 is physically impossible and it analagous to having the x-ray source inside of the view diameter.