Which statement regarding the 15% rule is true?

• A. A 15% increase in kVp will roughly double radiodensity
• B. A 15% increase in kVp will roughly halve radiodensity
• C. A 15% increase in kVp will leave radiodensity unchanged
• D. A 15% increase in kVp will triple radiodensity

Answer: (A)

The key idea is how a modest change in kilovoltage affects the amount of radiation that reaches the image receptor. In film-screen radiography, increasing kVp by about 15% increases receptor exposure roughly twofold even when the exposure time (mAs) is kept the same. This happens because higher kVp produces more penetrating photons and creates a broader energy spectrum, so more photons make it through the patient and reach the film.

So the statement that a 15% increase in kVp will roughly double radiodensity is the best match. It reflects the practical rule of thumb used in technique adjustments: raise kVp by about 15% and reduce mAs by about half to maintain the same density.

The other options don’t fit the typical behavior: increasing kVp does not halve radiodensity, nor does it leave it unchanged, and tripling radiodensity would require a larger change than 15% or different circumstances. Remember, this is a guideline, and exact changes can vary with film speed, filtration, patient size, and geometry.