X-Ray Production, Tubes, and Generators

X-Ray Production, Tubes, and Generators


This chapter describes the x-ray production process, characteristics of the x-ray beam, x-ray tube design x-ray generator components, and factors that affect exposure and exposure rate.


  • 1. Bremsstrahlung spectrum

    • a. Conversion of kinetic energy of electrons into electromagnetic radiation (x-rays).

    • b. An environment and requirements to produce x-rays are shown in Figure 6-1.

    • c. Voltage applied to the cathode and anode accelerates electrons to a kinetic energy = voltage.

    • d. Electrons interact with other electrons to produce heat; electrons that interact with the nucleus of the tungsten target are decelerated through coulombic interactions as illustrated in Figure 6-2.

    • e. X-ray energies produced have a distribution described by the bremsstrahlung spectrum (Fig. 6-3).

    • f. Efficiency of x-ray production relative to heat production is typically less than 1%.

  • 2. Characteristic X-rays

    • a. Incident electrons can interact with inner orbital electrons of the target atom.

    • b. Requires incident electron kinetic energy greater than binding energy of the electron in the atomic orbital.

    • c. Vacant shell via ejected electron from the target atom is immediately filled with electrons of lower binding energy, generating a “characteristic x-ray” of discrete energy equal to energy difference.

    • d. Electron binding energies of pertinent target materials are listed in Table 6-1.

    • e. Characteristic x-ray formation is illustrated in Figure 6-4 and resultant spectrum in Figure 6-5.

    • f. KA and KB characteristic x-ray energies result from adjacent shell and nonadjacent shell transitions.


X-ray tube cross-sectional diagram (Fig. 6-6) and actual tube insert/housing (Fig. 6-7)

  • 1. Cathode

    • a. The negative electrode comprised of electron emitters (filaments) and focusing cup; filament length provides small and large focal spot selections (Fig. 6-8).

    • b. Filament circuit activates filament to release electrons due to thermionic emission. A flow of electrons occurs when a voltage is applied; note difference of filament and tube current (Fig. 6-9).

    • c. Focusing cup shapes the electron distribution accelerated toward the anode; biased cups (more negative) can produce smaller distributions; grid biased focusing cups can stop flow (Fig. 6-10).

  • 2. Anode

    • a. The target electrode maintained at positive potential difference with respect to the cathode.

    • b. Tungsten (W), Z = 74, has a high melting point—used for most diagnostic systems.

    • c. Molybdenum (Mo), Z = 42, and rhodium (Rh), Z = 45, produce characteristic x-rays beneficial for mammography imaging (see Chapter 8).

  • 3. Anode configurations

    • a. Stationary tungsten target embedded in a copper block (copper efficiently conducts heat) (Fig. 6-11)

      • (i) Used in low-power applications such as dental x-ray and handheld fluoro units

    • b. Rotating anode—higher heat-loading capability achieved by spreading out area of power deposition

      • (i) Requires induction motor (stator-rotor design) to rotate at 3,000 to 10,000 rpm (Fig. 6-12)

      • (ii) Focal track increased by circumference of anode (2πr) for radius r (Fig. 6-13)

  • 4. Anode angle, field coverage, focal spot size

    • a. Anode angle: surface of the focal track to the perpendicular of the anode-cathode axis (Fig. 6-13).

    • b. Actual focal area length is foreshortened when projected down central axis according to the line focus principle: Effective focal length = Actual focal length × sin θ, where θ is the anode angle.

    • c. Focal spot effective size: width (determined by the focusing cup) × length (filament length).

    • d. Field coverage: dependent on anode angle and source to image distance (Fig. 6-14).

    • e. Effective focal spot varies in size along cathode-anode direction of the projection beam (Fig. 6-15).

    • f. Measurement of focal spot size can be achieved by a pinhole camera, slit camera, star pattern, or resolution bar patterns (see Figs. 6-17, 6-18, and 6-19 in the textbook).

    • g. Nominal focal spot sizes are specified by the International Electrotechnical Commission (see Table 6-3 in the textbook).

  • 5. Heel effect

    • a. Reduction in beam fluence on the anode side of the projected field caused by anode self-attenuation.

    • b. Heel effect is more prominent at short source-image distances (Fig. 6-16).

    • c. Orientation of the x-ray tube is important for equalizing transmitted x-ray fluence (Fig. 6-17).

  • 6. Off-focal radiation

    • a. Incident electrons elastically rebound and impact the anode at areas outside of the focal spot.

    • b. Causes extra radiation dose, background signal, and geometric blurring (see textbook Fig. 6-22).

    • c. This effect is reduced with a grounded anode and hooded structure next to the anode focal area.

  • 7. X-ray tube insert

    • a. Evacuated glass or metal enclosure containing the x-ray tube structures (see Figs. 6-6 and 6-7 above).

    • b. “Getter” circuit is used for trapping released outgassing of internal x-ray tube components.

    • c. Tube port made of beryllium (Be) is necessary for x-ray tubes employed for mammography.

  • 8. X-ray tube housing

    • a. Provides mechanical, electrical, thermal, and radiation protection.

    • b. “Leakage x-rays” are reduced by lead shielding in the housing (typically 2 mm or more thick) to radiation levels no greater than 0.88 mGy air kerma (100 mR) per hour at 1 m from focal spot when using the highest kV at the highest continuous tube current (typically 125 to 150 kV and 3 to 5 mA).

    • c. Heat exchangers are used with high-power x-ray tubes by circulating the housing transformer oil.

  • 9. X-ray tube filtration

Apr 18, 2023 | Posted by in GENERAL RADIOLOGY | Comments Off on X-Ray Production, Tubes, and Generators

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