abdomen or chest), lower contrast is preferable in order to accurately demonstrate all of the soft tissue tones in these areas. Geometric magnification Geometric magnification results from the detector being farther away from the X-ray source than the object. In this regard, the source-detector distance or SDD is a measurement of the distance between the generator and the detector. Alternative names are source/focus to detector/image-receptor/film (latter used when using X-ray film) distance (SID, FID or FRD). The estimated radiographic magnification factor (ERMF) is the ratio of the source-detector distance (SDD) over the source-object distance (SOD). The size of the object is given as: , where Sizeprojection is the size of the projection that the object forms on the detector. On lumbar and chest radiographs, it is anticipated that ERMF is between 1.05 and 1.40. Because of the uncertainty of the true size of objects seen on projectional radiography, their sizes are often compared to other structures within the body, such as dimensions of the vertebrae, or empirically by clinical experience. 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. Geometric unsharpness 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). Geometric distortion 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. Scatter In addition to using an anti-scatter 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. Imaging variations by target tissue Projection radiography uses X-rays in different amounts and strengths