The heat unit (HU) is directly proportional to which factors?

• A. Only kVp
• B. Only mA
• C. Only seconds
• D. All of the above

Answer: (D)

Heat units measure the thermal energy deposited in the x-ray tube during an exposure. That energy comes from the electrons accelerated toward the anode by the kilovoltage and the number of electrons flowing per second, times how long they hit the target. So increasing the peak voltage increases energy per electron, increasing the tube current increases the number of electrons, and increasing the exposure time increases how long those electrons deposit heat. Because all three factors contribute to the heat produced, the heat unit is directly proportional to kVp, mA, and exposure time (seconds), with only minor adjustments for the power waveform. That’s why all of the above is the best answer.