CHAPTER 3: X-ray Production

Transcription

1 PREPARED BY: MR KAMARUL AMIN BIN ABDULLAH SCHOOL OF MEDICAL IMAGING FACULTY OF HEALTH SCIENCES PHYSICS FOR RADIOGRAPHERS 2 (HDR202) CHAPTER 3: X-ray Production

2 LEARNING OUTCOMES At the end of the lesson, the student should be able to:- Explain the principle of x-ray production. Explain the Bremsstrahlung and Characteristic radiation. Explain the X-ray Quality and Intensity including the definition, measurement, and significance in medical imaging and the factors affecting them. Describe the Basic x-ray circuit including each of the components their functions. with Slide 2 of 38

3 TOPIC OUTLINES INTRODUCTION 3.1 History of X-ray 3.7 Factors Affecting the X-ray Emission Spectrum 3.2 Properties of X-ray 3.8 X-ray Quantity 3.3 Principle of X-ray Production 3.9 X-ray Quality 3.4 Interactions with Target 3.5 Conditions for X-ray Production 3.6 X-ray Emission Spectrum Characteristic X-ray Spectrum Bremsstrahlung X-ray Spectrum Slide 3 of 38

4 3.1 History of X-Ray In 1895, Wilhelm Roentgen, German Physicist, was studying high voltage discharges in vacuum tubes in Crookes tube, then he noticed fluorescence of barium platinocyanide screen lying several feet from tube end. These rays where named X-rays which means invisible penetrating radiation. X represent unknown in mathematics. Crookes Tube Slide 4 of 38

5 3.1 History of X-Ray Wilhelm Conrad Roentgen In 8th November 1895, he has produced and detected electromagnetic radiation in a wavelength range today known as X- rays or Röntgen rays. The German physicist won a Nobel Prize for his discovery of the X ray in Wilhelm Roentgen, 1895 Slide 5 of 38

6 3.2 Properties of X-ray X-rays are high energy waves, with very short wavelengths, and travel at the speed of light. X-rays have no mass (weight) and no charge (neutral). You cannot see x-rays; they are invisible. X-rays travel in straight lines; they can not curve around a corner. An x-ray beam cannot be focused to a point; the x-ray beam diverges (spreads out) as it travels toward and through the patient. This is similar to a flashlight beam. Slide 6 of 38

7 3.2 Properties of X-ray X-rays are differentially absorbed by the materials they pass through. More dense materials will absorb more x-rays than less dense material (like skin tissue). This characteristic allows us to see images on an x-ray film. X-rays will cause certain materials to fluoresce (give off light). We use this property with intensifying screens used in radiography. X-rays can be harmful to living tissue. Because of this, you must keep the number of films taken to the minimum number needed to make a proper diagnosis. Slide 7 of 38

8 3.3 Principle of X-Ray Production The x-ray production is caused by the interaction of electrons from the cathode (filament) with the target anode. The electrons are emitted by filament will be targeted to the anode. Then, they will bombard the anode target to produce an x-ray. Slide 8 of 38

9 3.4 Conditions for X-Ray Production A heated filament, which releases negative electrons by thermionic emission. A positive anode, which attracts them. A high tension supply, which accelerates the electrons to very high speeds. A target (part of the anode), whose job is to force the fast moving electrons to deviate very rapidly, thus causing x-rays to be emitted. An x-ray tube must be in vacuum condition to allow the electrons travel in high speed. Slide 9 of 38

10 3.5 Interactions with Target There are TWO major interactions in anode target for x-ray production:- a) Bremsstrahlung Radiation b) Characteristic Radaition Slide 10 of 38

11 3.5 Interactions with Target Bremsstrahlung Radiation Also known as braking radiation or general radiation. Bremsstrahlung x-rays are produced when high-speed electrons from the filament are slowed down as they pass close to, or strike, the nuclei of the target atoms. The closer the electrons are to the nucleus, the more they will be slowed down. The higher the speed of the electrons crossing the target, the higher the average energy of the x-rays produced. The electrons may interact with several target atoms before losing all of their energy. Slide 11 of 38

12 3.5 Interactions with Target High-speed electron from filament enters tungsten atom. + Electron slowed down by positive charge of nucelus; energy released in form of x-ray. Electron continues on in different direction to interact with other atoms until all of its energy is lost. Slide 12 of 38

13 3.5 Interactions with Target Characteristic Interactions Characteristic x-rays are produced when a high-speed electron from the filament collides with an electron in one of the orbits of a target atom; the electron is knocked out of its orbit, creating a void (open space). This space is immediately filled by an electron from an outer orbit. When the electron drops into the open space, energy is released in the form of a characteristic x-ray. The energy of the high-speed electron must be higher than the binding energy of the target electron with which it interacts in order to eject the target electron. Both electrons leave the atom. Slide 13 of 38

14 3.5 Interactions with Target Characteristic x-rays have energies characteristic of the target material. The energy will equal the difference between the binding energies of the target electrons involved. For example, if a K-shell electron is ejected and an L-shell electron drops into the space, the energy of the x-ray will be equal to the difference in binding energies between the K- and L-shells. The binding energies are different for each type of material; it is dependent on the number of protons in the nucleus (the atomic number). K-shell L-shell M-shell Slide 14 of 38

15 3.5 Interactions with Target Electron in L-shell drops down to fill vacancy in K-shell. High-speed electron with at least 70 kev of energy (must be more than the binding energy of k-shell Tungsten atom) strikes electron in the K shell, knocking it out of its orbit. M L K vacancy Ejected electron leaves atom. X-ray with 59 kev of energy produced. 70 (binding energy of K-shell electron) minus 11 (binding energy of L- shell electron) = 59. Recoil electron (with very little energy) exits atom. Slide 15 of 38

16 3.6 X-ray Energy Characteristic x-rays have very specific energies. K-characteristic x-rays require a tube potential of a least 70 kvp Bremsstrahlung x-rays that are produced can have any energy level up to the set kvp value. Brems can be produced at any projectile e- value Slide 16 of 38

17 3.7 X-ray Spectrum A display or graph of the intensity of x-rays, produced when electrons strike a solid object, as a function of wavelengths or some related parameter; It consists of a continuous bremsstrahlung spectrum on which are superimposed groups of sharp lines characteristic of the elements in the target. Slide 17 of 38

18 3.7 X-ray Spectrum Discrete Spectrum Contains only specific values. Slide 18 of 38

19 3.7 X-ray Spectrum Continuous Spectrum Contains all possible values. Slide 19 of 38

20 3.7 X-ray Spectrum Characteristic X-ray Spectrum The discrete energies of characteristic x-rays are characteristic of the differences between electron binding energies in a particular element. A characteristic x-ray from tungsten, for example, can have 1 of 15 different energies and no others. Slide 20 of 38

21 3.7 X-ray Spectrum A plot of frequency with which characteristic x-rays are emitted as a function of their energy. This is called characteristic x-ray emission spectrum. Characteristic x-rays have precisely fixed (discrete) energies and form a discrete emission spectrum. Slide 21 of 38

22 3.7 X-ray Spectrum Bremsstrahlung X-ray Spectrum Bremsstrahlung x-rays have a range of energies and form a continuous emission spectrum. The general shape of the bremsstrahlung x-ray spectrum is the same for all x- ray imaging systems. The maximum energy (in kev) of a bremsstrahlung x-ray is numerically equal to the kvp operation. The greatest number of x-rays is emitted with energy approximately one third of the maximum energy. The number of x-rays emitted decreases rapidly at very low energies. Slide 22 of 38

23 3.7 X-ray Spectrum Bremsstrahlung x-ray emission spectrum extends from zero to maximum projectile electron energy, with the highest number of x-rays having approximately one-third the maximum energy. The characteristic x-ray emission spectrum is represented by a line at 69 kvp. Slide 23 of 38

24 3.7 X-ray Spectrum Factors Affecting The X-ray Emission Spectrum There are FOUR factors affecting the shape of x-ray spectrum:- a) The projectile electrons accelerated from cathode to anode do not all have peak kinetic energy. b) The target of a diagnostic x-ray tube that relatively thick causes multiple interactions of the projectile electrons that it less energy. c) The low energy x-rays that are more likely to be absorbed in the target. d) The external filtration that removes low photons energy from the beam. Slide 24 of 38

25 3.8 X-ray Quantity It is also known as intensity. It is measured in roentgens (R) or miliroentgens (mr) (mgy a ). Another term used is radiation exposure. X-ray quantity is the number of x-rays in the useful beam. Slide 25 of 38

26 3.8 X-ray Quantity Factors Affecting X-ray Quantity There are 4 main factors affecting it:- 1. mas 2. kvp 3. Distance 4. Filtration Slide 26 of 38

27 3.8 X-ray Quantity mas Miliampere-Seconds X-ray quantity is directly proportional to the mas. When mas is doubled, the number of electrons striking the tube target is doubled, and therefore the number of x-rays emitted is doubled. Slide 27 of 38

28 3.8 X-ray Quantity kvp Kilovoltage Peak X-ray quantity varies rapidly with changes in kvp. The change in x-ray quantity is proportional to the square of the ratio of the kvp If kvp is doubled, the x-ray intensity would increase by a factor of four. Slide 28 of 38

29 3.8 X-ray Quantity Distance X-ray intensity varies inversely with the square of the distance from the x-ray tube target. This relationship is known as inverse square law. Slide 29 of 38

30 3.8 X-ray Quantity Filtration X-ray imaging systems have metal filters, usually of 1 to 5 mm of aluminum (Al), positioned in the useful beam. The purpose of these filters is to reduce the number of low energy x-rays. Adding filtration to the useful x-ray beam reduces patient dose because fewer low-energy x-rays are found in the useful beam. It reduces the x-ray quantity. Slide 30 of 38

31 3.9 X-ray Quality It is always related to the penetrability. As the energy of an x-ray beam is increased, penetrability is also increased. Penetrability refers to the ability of x-rays to penetrate deeper in tissue. High energy x-rays are able to penetrate tissue more deeply than low energy x-rays. X-rays with high penetrability are termed high quality x-rays. Those with low penetrability are low quality x-rays. Slide 31 of 38

32 3.9 X-ray Quality Factors Affecting X-ray Quality There are two main factors affecting it:- 1. kvp 2. Filtration Slide 32 of 38

33 3.9 X-ray Quality kvp Kilovolt Peak As the kvp is increased, so is x-ray beam quality. An increase in kvp results in a shift of the x-ray emission spectrum toward the high energy side, indicating an increase in the effective energy of the beam. The result is a more penetrating x-ray beam. Slide 33 of 38

34 3.9 X-ray Quality Filtration The primary purpose of adding filtration to an x-ray beam is to remove selectively low energy x-rays that have little chance of getting to the image receptor. It improves the quality of x-ray beam but reduces quantity. Slide 34 of 38

35 3.10 Basic X-ray Circuit There are two main parts of the circuit, one is the main circuit and the second is the filament circuit. A. Main part of X-Ray Circuit: supplies power to the x-ray tube so that x- rays are produced. B. Filament Circuit: supplies power to the filament of the x-ray tube so that the filament supplies enough electrons by thermionic emission. Slide 35 of 38

36 3.10 Basic X-ray Circuit Slide 36 of 38

37 3.10 Basic X-ray Circuit In the diagram below are the important parts of the circuit. The blue part is the main x-ray circuit and the tan part is the filament circuit. Slide 37 of 38

38 3.10 Basic X-ray Circuit 1. main breaker - this is where the alternating current comes from to power the circuit. 2. exposure switch - when you push the button to start an exposure this switch closes to start the exposure. 3. autotransformer - this is where you adjust the kvp for the exposure. 4. timer circuit - this part of the circuit stops the exposure. 5. high-voltage step-up transformer - this transformer bumps the voltage up so that the x- ray tube has very high voltage to make the electrons have enough energy to form x-rays. 6. four-diode rectification circuit - this makes the current only go in one direction through the x-ray tube. 7. filament circuit variable resistor - this variable resistor adjusts the current going to the filament. 8. filament step-down transformer - this transformer steps the voltage down and therefore the current up. 9. x-ray tube - this is where the x-rays are created. 10. rotor stator - this rotates the anode. Slide 38 of 38

39 2.6 References No. REFERENCES 1 Ball, J., Moore, A. D., & Turner, S. (2008). Essential physics for radiographers. Blackwell. 2 Bushong, S. C. (2008). Radiologic science for technologists. Canada: Elsevier. Slide 39 of 38

40 SUMMARY The x-ray production is caused by the interaction of electrons from the cathode (filament) with the target anode. There are TWO major interactions in anode target for x-ray production: Characteristic and Bremsstrahlung radiation. X-ray spectrum: A display or graph of the intensity of x-rays. X-ray quantity (intensity) is the number of x-rays in the useful beam. X-ray quality (penetrability) refers to the ability of x-rays to penetrate deeper in tissue. There are two main parts of the circuit, one is the main circuit and the second is the filament circuit. Slide 40 of 38

41 NEXT SESSION PREVIEW CHAPTER 3: X-RAY PRODUCTION Slide 41 of 38

42 APPENDIX FIGURE Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 SOURCE 2QLfw4/s1600/John+Dalton.jpg /Thomson's_Model.gif Slide 42 of 38

43 APPENDIX FIGURE Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 Figure 18 SOURCE Bwc/s1600/proton.jpg tomic/basicstructure/atmparts.gif onelectronlight.jpg c_k3_0.jpg Isotopes.jpg Slide 43 of 38

44 Activity Define or otherwise identify the following: a) X-ray Quality Answer b) X-ray Quantity Answer c) X-ray Spectrum Answer Describe how kvp affects x-ray quality. Answer Explain the properties of x-ray. Answer Slide 44 of 38