American Registry of Radiologic Technologists (ARRT) Practice Exam

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If the source-to-image distance (SID) is doubled, how does the intensity of the beam change?

Increases two times
Decreases four times
Remains the same
Increases four times

The correct answer is: Decreases four times

When the source-to-image distance (SID) is doubled, the intensity of the beam decreases due to the inverse square law. This law states that the intensity of radiation from a point source is inversely proportional to the square of the distance from the source. Therefore, if the distance is doubled, the intensity is reduced by a factor of the square of that distance change. In mathematical terms, if the original distance is represented as "d" and the initial intensity as "I," when the distance is increased to "2d," the new intensity (I') can be calculated as follows: I' = I / (2^2) = I / 4. This shows that the intensity becomes one-fourth of its original value, explaining why the intensity of the beam decreases four times when the SID is increased. Hence, this relationship is crucial in radiographic techniques as it affects image quality and exposure considerations.