In medical radiology, the source to image-receptor distance (SID) is a measurement of the distance between the radiation source (such as an X-ray generator or gamma ray–generating radiopharmaceutical) and the radiation detector (such as a film, flat panel detector, or gamma camera).
The SID is roughly related to the source to object distance (SOD) and the object to image-receptor distance (OID) by the equation SOD + OID = SID. The relative and absolute values of these measurements will affect various qualities of the radiographic image as well as the amount of radiation to which the patient is exposed. For example, increasing the OID alone can improve image contrast by decreasing the amount of scattered radiation that reaches the receptor; however it will also result in image magnification if the SID is not also proportionally increased.