Although modifying any distances affects both magnification and sharpness of detail - One does not cause the other. The greater the O.I.D., the greater the magnification. Remember: for magnification you must have an increase in umbra and Penumbra, not just penumbra. We always have some magnification since all anatomy cannot be directly against the IR. will also increase the umbra of the image. The Larger the O.I.D., the less the sharpness of recorded detail. gap is introduced the penumbra shadow is spread out. Effect on Sharpness of Detail When an O.I.D. to a minimum for reasons related to patient condition (trauma) we must increase it, we would generally want to increase our mAs by 35% O.I.D. is doubled the scatter intensity falls to 1/4th the original For geometrical reasons (recognizability) we generally try to keep O.I.D. equals a reduction in total exposure to image Scatter also follows the inverse square law As the Patient (Source of scatter) is moved farther away from the IR, the intensity of the scatter radiation reaching the IR is inversely proportional to the square of the O.I.D.Įach time the O.I.D. Effect on exposure Total Exposure Total exposure is the sum of both penetrating primary radiation and scatter radiation Reduction in scatter from increased S.I.D. Penetrating primary radiation still reaches the IR the same Does not interact with the Patient The ratio of penetrating primary to scatter radiation has been improved and subject contrast in the remnant beam increased. Scatter is allowed to spread out more before it reaches the IR and therefore it does not all reach the IR Because scatter is caused by an interaction within the patient.
This is the only positive effect on image quality that increasing O.I.D. means less scatter reaching the IR and therefore improved contrast and less noise. Effect on Subject Contrast and Noise Increase O.I.D. Recognizability Functions: Magnifications, Sharpness of Detail. impacts Visibility functions: Noise, Subject Contrast, Exposure. impacts all of the image qualities except shape distortion O.I.D. is doubled (2x) - increase mAs by 2 squared or 4 times Object to Image Distance O.I.D. Square Law is used to compensate mAs in order to maintain the original exposure level (tells you how much to change your mAs) The Square Law Formula mAs1/mAs2 = (D1)2/(D2)2 When S.I.D. Where E is the exposure and D is the Distance Compensating Technique: The Square Law Changes in S.I.D alter only intensity of the beam.Įnergy levels and penetrability are not affected by changing S.I.D. has on the intensity (think quantity or mAs) of the x-ray exposure (the amount of exposure to the IR) Inverse Square Law Formula E1/E2 = (D2)2/(D1)2 greater than 10 percent should be compensated for by an increase in mAs Inverse Square Law Expresses the effect of any change in S.I.D. In film imaging, a loss in density will be apparent Any increase in S.I.D. In digital imaging, a loss in brightness is not apparent due to compensation by the software.
increase by a factor of 2, the area increases by 2 squared or 4 times Intensity has spread out such that it is now 1/4 as much per square inch In digital imaging, a reduction in intensity means a reduction in signal and an increase in image noise from quantum mottle.
effect on exposure X-rays diverge as they are emitted form the focal spot, cover an increasingly larger area with lessened intensity as they travel Inverse Square Law the area in which the radiation spreads out increases as the square of the distance S.I.D. equals increased penumbra or blur Magnification can be used to our advantage to blur and magnify anatomy that is superimposing the anatomy of interest S.I.D. Effect on Magnification Using Shorter S.I.D. Measuring anatomy needs to be as accurate as possible for orthopedic appliances and pathology conditions S.I.D. Reasons to reduce magnification Most of the time magnification is undesirable.Īccurately determine the size of the heart. does not effect shape distortion It may seem like it because the anatomy is already distorted and when we increase S.I.D. reduces magnification Magnification defined an increase in the umbra of an image Note: S.I.D. Effect on Magnification Increasing S.I.D.
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