Distance
Field size has a direct relationship to distance. As source to image-receptor distance (SID) increases, the field size gets larger. As the field size gets larger, the photons become more spread out (less concentrated) over a larger area. As SID changes so does the intensity of the beam, assuming that we are using the same technical factors (kVp and mAs). It has an inverse relationship to distance; the greater the distance, the lesser the intensity. As the x-ray tube gets further away (increased SID) from the patient, intensity becomes less.
Inverse Square Law
The inverse square law measures exposure rate in milliroentgens (mR) using a dosimeter. The rules of thumb are that when SID is doubled, intensity decreases to ¼ of the original intensity and when SID is cut in half, intensity increases to 4 times the original intensity.
Formula
or 
Example
An x-ray exposure of 240 mR is recorded at a distance of 20 inches. If the same technical factors are used, what will the exposure be if the distance is increased to 40 inches?
I2 = 240 x 202 I2 = 240 x 400 I2 = 60mR
402 1,600
Examples of changes in intensity with changes in SID
Distance Intensity
20" 40mR
40" 10mR
80" 2.5mR
Contrast
Distance, specifically object to image-receptor distance (OID), affects image contrast as a result of an air-gap between the object and the image receptor. An air-gap technique's increased OID has the same effect as using a grid. The air-gap controls scatter in that scattered photons diverge beyond the borders of the image receptor. This air-gap is equivalent to a 5:1 grid. This technique is commonly used for a portable x-table lateral projection of the cervical spine.
Density
Density has an inverse relationship to distance. The greater the distance, the less density there will be on the image receptor. Although we routinely perform our examinations at the he standard distances of 40" and 72", it is sometimes necessary to change our distance. When that is necessary, we turn to the density maintenance formula.
Density Maintenance Formula
The density maintenance formula is used to calculate the necessary change in mAs in response to a change in SID. The rules of thumb are when SID is doubled, density decreases to ¼ of the original density and when SID is cut in half, density increases to four times the original density.
Formula
= 
or 
= 
Example
An acceptable radiograph of the abdomen is taken using 80 kVp at 25 mAs at a distance of 40 inches. A second radiograph is requested to be taken at 56 inches. What mAs should be used to produce an acceptable radiograph if the distance is increased to 56 inches?
mAs2 = 25 x 562 = 25 x 3,136 = 49 mAs
402 1,600
Examples of changes in mAs with changes in SID
Distance mAs
20" 25
40" 100
80" 400
Detail
Sharpness of detail has a direct relationship to SID and an inverse relationship to OID. The general rules of thumb are when SID increases, detail increases and when OID decreases, detail increases.
Distortion
A certain amount of image distortion is an inherent part of every radiographic image because of the position, thickness, and shape of anatomic structures within the body. Shape distortion is unequal magnification of different portions of the same object and is influenced by the relationship between (1) the x-ray tube and the part to be imaged; (2) the part to be imaged and the image receptor; and (3) the image receptor and the x-ray tube.
Three conditions contribute to image distortion: object thickness, object position, and object shape. In regards to object thickness, thicker objects have more edge distortion than thinner objects. Round objects at the periphery of an x-ray field can appear oval shaped while flat objects can appear round. Regarding object position, objects that are parallel to the imaging plane will not be distorted while objects on the outer edges of the film are slightly distorted due to beam divergence.
In regards to object shape, a misalignment between the tube-part-film of the object being radiographed results in a misrepresentation of that object. Foreshortening of an image occurs when an inclined object appears smaller than it actually is. Foreshortening increases as angle of inclination increases. Elongation of an image occurs when an inclined object appears longer than it actually is. Elongation increases as tube angle increases
Focal Spot Blur
Focal spot blur is also known as penumbra. Focal spot blur is the blurry edges surrounding a radiographed object. Umbra is the sharp edge of an object. Focal spot blur occurs because x-rays are emitted from a focal spot which is roughly a rectangular area, not a single point.
Focal spot blur is influenced by OID, source to object distance (SOD), SID, and focal spot size. It is greater on the cathode side of the image because the effective focal spot is not constant across the radiograph. The general rules of thumb are as OID increases, focal spot blur increases and as SID increases, focal spot blur decreases. To minimize focal spot blur, position the patient as close to the image receptor as possible.
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