HEAT UNITS
What is the formula for Heat Units (HU)?
Heat Units (HU) = kVp x mAs x Modification Factor (MF)
Calculate the heat produced by a single phase x-ray machine using a 70 kVp and 7 mAs. A. 490 HU B. 662 HU C. 691 HU D. 710 H
The answer is A, this is because we are solving for Heat Units (HU). Although we need to remember the modification factor (MF) which for this problem is an 1-phase x-ray so the MF is 1 in this case. HU = kVp x mAs x MF HU = 70 x 7 x 1 HU = 490 *Note: the modification factor will not be provided so you will need to memorize that information)
Calculate the heat produced by a 3-phase 6-pulse x-ray machine using a 120 kVp and 3 mAs. A. 360 HU B. 486 HU C. 508 HU D. 522 H
The answer is B, this is because we are solving for Heat Units (HU). Although we need to remember the modification factor (MF) which for this problem is an 3-phase 6 pulse x-ray so the MF is 1.35 in this case. HU = kVp x mAs x MF HU = 120 x 3 x 1.35 HU = 486 *Note: the modification factor will not be provided so you will need to memorize that information)
A 3-phase 12-pulse x-ray tube is installed in a fluoroscopic imaging system. A procedure is performed using an average of 68 kVp, 4 mA, and the total fluoroscopy time is 1 minute 17 seconds. Calculate the heat produced in the anode. A. 461 HU B. 28,274 HU C. 29,531 HU D. 30,369
The answer is C, this is because we are solving for Heat Units (HU). Although we need to remember the modification factor (MF) which for this problem is an 3-phase 6 pulse x-ray so the MF is 1.41 in this case. Also the mAs is not provided directly so we will need to solve for that first: mAs = 4mA x (60s + 17s) mAs = 4mA x 77s mAs = 308 HU = kVp x mAs x MF HU = 68 x 308 x 1.41 HU = 29,531 *Note: the modification factor will not be provided so you will need to memorize that information)
Your high-frequency machine has a maximum heat load of 2,000 HU. How many consecutive exposures could be made without over-heating the tube? The technical factors being used are 1.5 mAs and 125 kVp. A. 10 exposures B. 11 exposures C. 9 exposures D. 7 exposures
The answer is D, this is we are solving to find out how many repeated exposures can be made before the high frequency machine over heats. So in order to do this we need to first figure out how many heat units are created for a single exposure. We will use the same formula to do this the machine is high frequency so the MF = 1.45. HU = kVp x mAs x MF HU = 125 x 1.5 x 1.45 HU = 271.875 Now we will take the 2000 heat units capacity and divide that by the answer we just got: 2000 ÷ 271.875 = 7.3 So a total of 7 exposures can be made back to back without the x-ray tube overheating.
