X-Ray Production Processes

1. X-Ray Production Processes Recommended Book: Walter Huda, REVIEW OF RADIOLOGIC PHYSICS By: Maisa Alhassoun maisa@inaya.edu.sa

2. A. Introduction -Diagnostic x-rays are produced when electrons with energies of 20 to 150 kilo electron volts (keV) are stopped in matter, producing electromagnetic radiation in the form of x-rays. -Electrons accelerated to the positive anode gain a kinetic energy of V eV, determined solely by the value of the applied voltage (V). -The kinetic energy of electrons is transformed into heat and x-rays when the electrons strike the anode.

3. -Electrons only penetrate tens of micrometers (μm) into the anode before losing their energy by ionization and excitation of electrons in the anode material. -Energetic electrons loose their energy in matter by excitation, in which electrons are energized to higher energy states; ionization, in which an outer-shell electron is removed; and radiation, in which the energy loss is converted directly to a photon.

4. -X-rays are generated by two different processes known as bremsstrahlung(radiation) and characteristic x-ray production (ionization). -Most incident electrons interact with outer-shell electrons (excitation and ionization). -Energy lost in the form of excitation and ionization appears as heat in the anode.

5. -The efficiency of x-ray production is approximately kV x Z x 10−6 and is approximately 1% for materials with high atomic numbers (Z) at100kVp. -A graph of x-ray tube output showing the number of photons at each x-ray energy is called a spectrum.

6. B. Bremsstrahlung radiation -Bremsstrahlung (braking) x-rays are produced when incident electrons interact with nuclear electric fields, which slow them down (brake) and change their direction. -Fig. 2.2 shows a bremsstrahlung process in which a fraction of the initial electron kinetic energy is emitted as an x-ray photon.

8. -Bremsstrahlung x-rays produce a continuous spectrum of radiation, up to a maximum energy determined by the maximum kinetic energy of the incident electron. -The closer the electron passes to the nucleus, the greater the interaction of the incident electron with the nucleus, and the higher the energy of the resulting x-ray.

9. -Maximum photon energies correspond to minimum x-ray wavelengths. -The majority of x-rays produced in x-ray tubes are via the bremsstrahlung process. -Bremsstrahlung x-ray production increases with the accelerating voltage (kV) and the atomic number (Z) of the anode.

10. C. Characteristic radiation -Characteristic radiation is the result of ionization and is produced when inner-shell electrons of the anode target are ejected by the incident electrons. -To eject a bound atomic electron, the incident electron must have energy greater than the binding energy. -The resultant vacancy is filled by an outer-shell electron, and the energy difference is emitted as characteristic radiation (e.g., K-shell x-rays, L-shell x-rays), as shown in Fig. 2.3.

12. -Characteristic x-rays occur only at discrete energy levels, unlike the continuousenergy spectrum of bremsstrahlung. -After the ejection of a K-shell electron, the excess energy may also be emitted as an Auger electron. -Each anode material emits characteristic x-rays of a given energy, as listed in Table 2.1.

14. -K-shell characteristic x-ray energies are always slightly lower than the K-shell binding energy. (Table 1.4 lists K-shell binding energies). -K-shell electrons are ejected only if incident electrons have energies greater than the K-shell binding energy. -For tungsten, K-shell characteristic x-rays are only produced when the applied voltage exceeds 69.5 kV (K-shell binding energy is 69.5 keV).

15. -For molybdenum, K-shell characteristic x-rays are only produced when the applied voltage exceeds 20 kV. -L-shell radiation also accompanies K-shell radiation, but because L-shell characteristic x-rays have very low energies, they are absorbed by the glass of the x-ray tube. -Only K-shell characteristic x-rays are important in diagnostic radiology.

16. D. Quantity -Intensity is affected by the generator type, beam filtration, and distance from the beam (inverse square law). -For conventional radiography with a tungsten target, the characteristic radiation produced accounts for up to 10% of the x-ray beam intensity. -X-ray output is directly proportional to the current (mA), and to exposure time (sec-onds).

17. -The product of the tube current (mA) and exposure time (seconds) is known as the mAs, and the x-ray tube output is proportional to the mAs. -Doubling the current at constant exposure time has the same effect as doubling the exposure time at constant tube current. -Doubling the mAs doubles the number of x-rays emitted but does not change the energy spectrum.

18. -Fig. 2.4A shows how the number of photons at each energy level increases when the tube current is increased, but the spectrum shape does not change. -The quantity of x-rays produced can also be increased by increasing the kVp, but this also changes the quality or shape of the x-ray spectrum, as shown in Fig. 2.4B. -The quantity (intensity) of x-ray production is approximately proportional to the square of the tube potential.

20. E. Quality -X-ray beams in diagnostic radiology are polychromatic and consist of a range of photon energies. -Quality refers to effective photon energy of the x-rays produced, and relates to their ability to penetrate the patient. -The quality of an x-ray beam is obtained from the effective x-ray energy of the x-ray spectrum.

21. -The effective photon energy is taken to be between one third and one half of the maximum photon energy. -Increasing the peak voltage (kVp) increases the x-ray tube output, peak energy, and mean energy of the beam. -This increases the beam quality as shown in Fig. 2.4B.

23. -Increasing beam quality increases x-ray beam penetrating power because the av-erage photon energy is higher. -A rule of thumb is that increasing the peak voltage by 15% has the same effect on film density as that of doubling the mAs. -For example, changing tube voltage by 10 kVp (from 65 to 75 kVp) normally has the same effect on film density as doubling the mAs.

24. -Reducing the voltage waveform ripple increases average photon energy and thus x-ray beam quality. -Increasing x-ray tube filtration also increases beam quality, as low-energy photons are preferentially removed from the x-ray beam (beam hardening). -Table 2.2 lists typical x-ray outputs as a function of x-ray tube voltage and filtration.