<P> The source - detector distance (SDD) is roughly related to the source - object distance (SOD) and the object - detector distance (ODD) by the equation SOD + ODD = SDD . </P> <P> Geometric unsharpness is caused by the X-ray generator not creating X-rays from a single point but rather from an area, as can be measured as the focal spot size . Geometric unsharpness increases proportionally to the focal spot size, as well as the estimated radiographic magnification factor (ERMF). </P> <P> Organs will have different relative distances to the detector depending on which direction the X-rays come from . For example, chest radiographs are preferably taken with X-rays coming from behind (called a "posteroanterior" or "PA" radiograph). However, in case the patient cannot stand, the radiograph often needs to be taken with the patient lying in a supine position (called a "bedside" radiograph) with the X-rays coming from above ("anteroposterior" or "AP"), and geometric magnification will then cause for example the heart to appear larger than it actually is because it is further away from the detector . </P> <P> In addition to using a Bucky - Potter grid, increasing the ODD alone can improve image contrast by decreasing the amount of scattered radiation that reaches the receptor . However, this needs to be weighted against increased geometric unsharpness if the SDD is not also proportionally increased . </P>

When looking at x-rays the description of a pa projection would indicate
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