What is mA?

• A. The exposure time
• B. The distance to the focal spot
• C. The quantity of electrons crossing the tube during exposure
• D. The voltage across the tube

Answer: (C)

mA is the tube current—the amount of electric current flowing through the X-ray tube, which corresponds to the number of electrons emitted by the heated cathode and crossing to the anode each second. This directly sets how many x-ray photons are produced, so increasing mA increases the radiation quantity. The energy of the x-rays (how hard they are) is determined by the kilovolt peak, not by mA. The exposure time then multiplies this current to give the total exposure (mA × s, or mAs). The distance affects beam intensity through the inverse square law and beam geometry, not the number of electrons generated. So mA best describes the quantity of electrons crossing the tube per unit time.