CAMRT Radiography Practice Exam

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What effect does increasing the Source to Image Distance (SID) have on patient dose, assuming all other factors remain constant?

Increased SID = Increased dose
Increased SID = Decreased dose
Increased SID = No change
Increased SID = Variable dose

The correct answer is: Increased SID = Decreased dose

Increasing the Source to Image Distance (SID) has a direct impact on the amount of radiation dose the patient receives, assuming that all other factors remain constant, such as the exposure settings (mA, time, kVp) and the radiographic technique used. When the SID is increased, the intensity of the radiation reaching the patient decreases. This is due to the inverse square law, which states that as the distance from the radiation source increases, the intensity of the radiation diminishes proportionally to the square of the distance. For instance, if the SID is doubled, the intensity of the radiation is reduced to one-fourth of its original value. As a result, the amount of radiation absorbed by the patient also decreases, leading to a reduced patient dose. This reduction is crucial in radiography since minimizing patient exposure to radiation is a key principle in protecting patients while still obtaining the necessary diagnostic images. Hence, increasing the SID results in a decreased dose to the patient, which is why this answer is correct.