American Registry of Radiologic Technologists (ARRT) Practice Exam

What can cause a decrease in the quantity of X-rays produced?

1. Increased SID

2. Decreased patient thickness

3. Increased milliamperage (mA)

4. Shorter exposure time

The correct answer is: Increased SID

Increasing the source-to-image distance (SID) leads to a reduction in the quantity of X-rays that reach the image receptor. This is due to the inverse square law, which states that the intensity of radiation decreases with the square of the distance from the source. As SID increases, the X-ray beam spreads out over a larger area, resulting in fewer X-rays per unit area hitting the detector. Consequently, less radiation is detected, contributing to a decrease in the overall quantity of X-ray exposure. When considering other factors: a decrease in patient thickness would typically allow more X-rays to pass through the patient unimpeded, increasing the quantity of X-rays detected. Increased milliamperage (mA) directly increases the quantity of X-rays produced, as it refers to the amount of current running through the X-ray tube, increasing the number of X-rays generated. Shorter exposure times would also normally reduce the amount of radiation produced, but it operates differently; this option also corresponds to a lower quantity of X-rays being produced, but the more direct focus in this question is the effect of distance, which is why increased SID is the definitive factor in reducing X-ray quantity.