Because the detection area of the photoreceptors is relatively small, they may be positioned beneath an area of fatty tissue in an otherwise glandular breast, resulting in under exposed image of the denser portion of the breast. The size of the focal spot, its position and its distance from the film are also of the utmost importance. This is very important for visualisation of small structures such as microcalcifications or very fine radial cancerous ramifications. The area of the anode that emits x-rays is referred to as the focal spot. However, magnification techniques require extremely small focal spot sizes with low load- ability, requiring longer exposure times.
Some tubes have a second focal spot used for magnification procedures.
Focal spots used for magnification should be small to compensate for the suboptimal geometry. Commercially available mammographic units have distances that range from about 46cm to over 80cm. The longer distances result in greater geometrical sharpness, particularly with large breasts. Also, the longer the FFD the larger the focal spot size can be and still achieve good image detail. As the FFD increases, however, the beam intensity at the image receptor decreases.