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Tube ratings are the defined input parameters (kVp, mA, exposure) that can be safely used during its operation without causing damage to the x-ray tube itself and unique to each individual x-ray tube model. An x-ray tube rating is the maximum allowable kilowatts (kW) in 0.1 second 2.
When the electron beam strikes the target material in the anode, only 1% of the kinetic energy of the electrons is converted into x-rays whilst the rest is converted into thermal energy. Increasing the kVp, mA, or exposure time increases the thermal energy produced per examination. Thermal energy is dissipated in the anode and surrounding X-ray tube. If too much heat is created (or not enough is dissipated), excess residual thermal energy will damage the anode and tube.
A heat unit (HU) is the total amount of heat produced by X-ray tube using single-phase electric potential with a waveform of 0.71. KVp is the peak kilovoltage of the electric potential applied while MAS is the milliampere-seconds 3.
Heat (HU) = KVp x MAS
For an X-ray tube using three-phase electric potential, the waveform is 0.96, which is 1.35 times higher than single-phase (0.71). Thus the waveform factor (w) is 1.35. Therefore, the HU is calculated as follows. For an X-ray tube with constant potential, the waveform is 1.0 3.
Heat (HU) = 1.35 x KVp x MAS
On the other hand, the power (rate of heat production) of the tube is described with equation below. Note the use of "MA" instead of "MAS". With the use of "MAS" it becomes the second equation above. Power of the tube in 0.1 second is the tube rating 3.
Power (watts) = w x KVp x MA
By creating tube ratings the operator can ensure that the parameters set are appropriate for the examination whilst minimizing the risk of damage to the x-ray tube. Typical x-ray tube ratings are between 5-100kW and are dependent on focal spot size 2. The larger the focal spot size, the higher the ability of the anode to withstand heat (thus higher heat capacity), leading to higher tube ratings. Doubling the focal spot size increases the tube rating by three 3.
Other factors that affect power rating of the x-ray tube are anode angle, anode rotation speed, kV waveform, heating and cooling curves of anode body, and heat capcity of the tube housing 3. Rotating anode allows heat to be distributed over a larger area, thus increasing the power rating of the tube by 60%. Constant electric potential allows uniform heat distribution, thus increasing tube rating. The anode body should not be heated beyond the maximum heat capacity and an idle time is required to cool down the anode body before being used again later. This action flatten the amount of heat added to the anode body overtime, thus improving the ability of X-ray tube to withstand heat. The tubing housing heat capacity is way higher than the anode body because is cooled by air circulation. The heat capacity of tubing housing indicates the ability to withstand heat after a prolonged use 3.
- 1 S. Webb. The Physics of Medical Imaging. ISBN: 9780852743492
- 2. Tube Ratings and Exposure. (1942) Radiology. 50 (1-2): 7-17. doi:10.1159/000484410 - Pubmed
- 3. Perry Sprawls. Physical Principles of Medical Imaging. ISBN: 9780843203097 X-Ray Tube Heating and Cooling
- 4. Walter Huda, Richard M. Slone. Review of Radiologic Physics. (2003) ISBN: 9780781736756