Diagnostic Radiology IV

1. Diagnostic Radiology IV Factors affecting x-ray emission Power ratings and heat loading

2. Factors affecting x-ray emission • Output of an x-ray tube described by the terms quality, quantity and exposure • Quality describes penetrability of an x-ray beam • Quantity refers to the number of photons comprising the beam • Exposure is nearly proportional to the energy fluence of the x-ray beam and therefore has quality and quantity associated characteristics

3. Factors (cont.) • X-ray production efficiency, exposure, quality and quantity are determined by: • X-ray tube target material • Voltage • Current • Exposure time • Beam filtration • Generator waveform

4. Target (anode) material • Affects efficiency of bremsstrahlung radiation production • Output exposure roughly proportional to atomic number • Energies of characteristic x-rays depend on target material • Target material affects quantity of bremsstrahlung radiation and the quality of characteristic radiation

5. Tube voltage (kVp) • Determines the maximum energy in the bremsstrahlung spectrum and affects the quality of the output spectrum • Efficiency of x-ray production is directly related to tube voltage • Exposure approximately proportional to the square of the kVp in the diagnostic range:

7. Tube voltage (cont.) • Changes in kVp must be compensated by corresponding changes in mAs to maintain the same exposure • Additional consideration of technique adjustment concerns the x-ray attenuation characteristics of the patient • To achieve equal transmitted exposure through a typical patient, the mAs varies with the fifth power of the kVp ratio:

8. Tube current (mA) • Tube current is equal to the number of electrons flowing from the cathode to the anode per unit time • Exposure of the beam for a given kVp and filtration is proportional to the tube current

9. Exposure time • Exposure time is the duration of x-ray production • Quantity of x-rays is directly proportional to the product of the tube current and exposure time (mAs)

10. Beam filtration • Beam filtration modifies the quantity and quality of the x-ray beam by selectively removing low-energy photons in the spectrum • This reduces the photon number (quantity) and shifts the average energy to higher values, increasing the quality

11. Generator waveform • Generator waveform affects the quality of the emitted x-ray spectrum • For the same kVp, a single-phase generator provides a lower average potential difference than a three-phase or high-frequency generator • Both the quantity of x-rays produced and the quality of the x-ray spectrum are affected

13. Summary • X-ray quantity is approximately proportional to: • X-ray quality depends on kVp, generator waveform, and tube filtration • Exposure depends on both quality and quantity • Compensation for changes in kVp with radiographic techniques requires adjustments of mAs on the order of the fifth power of the kVp ratio

14. Power rating • Describes the energy per unit time that can be supplied (generator) or received (tube) • Power rating in kilowatts (kW) is the average power delivered by the maximum tube current for 100 kVp and 0.1-second exposure time:

15. Heat loading • The heat unit (HU) is used to express the energy deposition on and dissipation from the anode of an x-ray tube

16. Heat loading (cont.) • HU underestimates energy deposition for three-phase, high-frequency or constant potential generators • Multiplicative factor of 1.35 to 1.40 compensates for this difference • For fluoroscopy:

17. Exposure rating charts • Used to determine operational limits of the x-ray tube for single and multiple exposures and permissible heat load of the anode and the tube housing • Specific to a particular x-ray tube and must not be used for other tubes

18. Single-exposure rating chart • Provides information on allowed combinations of kVp, mA, and exposure time for a particular x-ray tube, focal spot size, anode rotation speed, and generator type (no accumulated heat on the anode) • Family of charts for specific focal spot size and anode rotation speed

19. 0.3 mm focal spot, 10 kW power, 3000 RPM rotor speed

20. 0.3 mm focal spot, 10 kW power, 10,000 RPM rotor speed

21. 1.2 mm focal spot, 120 kW power, 3000 RPM rotor speed

22. 1.2 mm focal spot, 120 kW power, 10,000 RPM rotor speed

23. Use of rating charts • Find the intersection of the requested kVp and exposure time • Determine the corresponding mA (interpolating between adjacent curves, if necessary). This is the maximum mA allowed by the tube focal spot • Compare the desired mA to the maximum mA allowed. If the desired mA is larger, the exposure is not allowed. • For mA versus time plots with various kVp curves, the rules are the same but with an exchange of kVp and mA labels

24. Anode heat input/cooling chart • Shows the remaining heat load of the anode versus time as the anode cools • Maximum anode heat load is the upper value on the y-axis of the chart • After a series of exposures, total heat load accumulated on the anode is calculated as the sum of the HU incident per exposure • If it is necessary to wait before reusing the tube in order to avoid anode damage, the cooling chart specifies how long to wait

26. Anode heat chart (cont.) • Same chart shows heat input curves that correspond to to continuous heat input resulting from fluoroscopy operation • Curves initially rise very fast but reach a plateau • Rate of heat energy input into anode equals rate of heat energy dissipation by radiative emission • Useful for determining amount of accumulated heat on anode after a given amount of fluoroscopy time

27. Housing cooling chart • Heat generated in the anode eventually transfers to the tube housing • Chart is used in much the same way as the anode cooling chart • Housing heat load typically exceeds that of the anode