Discovery of X-rays

1. Discovery of X-rays • X-rays were discovered on November 8, 1895, by Dr. Wilhelm Conrad Roentgen. • Accidental discovery • First radiograph of Mrs. Roentgen's hand • Roentgen received the first Nobel Prize presented for physics in 1901. • Public viewed discovery as a novelty • Radiographic imaging and therapy important to the medical sciences

2. X-rays as Energy • A form of electromagnetic radiation • Behave both like waves and like particles • Move in waves that have wavelength and frequency • Wavelength and frequency are inversely related • X-rays also behave like particles and move as photons

3. Properties of X-rays • X-rays are invisible. • X-rays are electrically neutral. • X-rays have no mass. • X-rays travel at the speed of light in a vacuum. • X-rays cannot be optically focused. • X-rays form a polyenergetic or heterogeneous beam. • X-rays can be produced in a range of energies. • X-rays travel in straight lines.

4. Properties of X-rays (cont.) • X-rays can cause some substances to fluoresce. • X-rays cause chemical changes to occur in radiographic and photographic film. • X-rays can penetrate the human body. • X-rays can be absorbed or scattered by tissues in the human body. • X-rays can produce secondary radiation. • X-rays can cause chemical and biologic damage to living tissue.

5. Birth of Radiology • Dry plate- used to record x-ray images • exposure required were extremely long. • Glass plate easily broken. Thomas Edison developed the first intensifying screen WWI x-ray film was produced the cellulose nitrate film base and was highly flammable and a fire hazard

6. X-ray Production • The production of x-rays requires a rapidly moving stream of electrons that are suddenly decelerated or stopped. • The negative electrode (cathode) is heated, and electrons are emitted (thermionic emission). • The electrons are attracted to the anode, move rapidly towards the positive electrode, and are stopped or decelerated.

7. X-ray Tube Housing • Metal or glass envelope • Negatively charged electrode • Positively charged electrode

8. Cathode • Filament • Source of electrons • Filament current • Thermionic emission • Coiled tungsten wire • Large and small • Focusing cup • Space charge effect

9. Anode • Rotating anode • Requires a stator and rotor to rotate • Tungsten metal • High melting point • Efficient x-ray production • Target • Decelerates and stops electrons • Energy converted to heat and x-rays • Bremsstrahlung and characteristic interactions

10. Target Interactions • Bremsstrahlung interactions • Braking or slowing down radiation • 85% of x-ray beam • Characteristic interactions • Projectile electron energy at least 69.5 keV • Inner shell electron ionized • 15% of x-ray beam • X-ray properties the same

11. Review of Interactions in the x-ray tube • Bremsstrahlung- electrons interacts with the atomic nucleus, more energy is lost and a stronger x-ray is produced; account for the majority of the x-ray beam. • Characteristic Radiation - electrons interact with an orbital electron from the atom, the pulling down of another electron from an outer shell causes an x-ray to be produced; account for a small majority of the x-ray beam

12. Target Interactions (cont.)

13. Heterogeneous Beam • Contains low energy rays which will be absorbed by the x-ray tube • Average energy of the beam is 1/3 of the maximum energy • X-rays are an inefficient process • 99% heat, 1% converted to x-rays

14. X-ray Beam • Primary Radiation (PR)- portion of beam from tube to the patient; radiation before it enters the patient • Remnant Radiation (RR)- radiation emerging from patient’s body to expose the film; image forming radiation

15. Primary Beam Distribution • 5% of primary beam passes through the patient without any interactions • 15% of the primary beam interacts with atoms and produce secondary radiation, they make it out of the patient and expose the film. • 80% will be totally absorbed by patient

16. Distribution of Remnant Radiation • 20% or 1/5 of the intensity of the original beam exposes the film • With remnant radiation about 75% to 80% of the beam is made up of secondary radiation

17. Prime Factors of Radiography • mA- milliampere • S- seconds time • kVp- kilovoltage peak • SID- source to image distance These are all controlled by the technologist

18. X-ray Emission Spectrum The range and intensity of x-rays emitted changes with different exposure technique settings on the control panel.

19. Kilovoltage • Creates potential difference • Determines the speed of the electrons in tube current • Greater speed results in greater quantity and quality of primary beam • Increasing electron speed will increase x-ray beam penetrability

20. Milliamperage • Unit to measure tube current or number of electrons flowing per unit time • mA directly proportional to quantity of x-rays produced • Double the mA will double the number of x-ray photons produced

21. Milliamperage and Time • Exposure time determines the length of time x-rays are produced. • Increasing time will increase the total number of x-rays produced. • Exposure time and x-ray quantity are directly proportional.

22. Beam Filtration • Aluminum filtration added to x-ray beam to absorb low-energy photons • Total filtration • Inherent • Added • Reduces patient exposure

23. Compensating Filtration • Added to primary beam to alter its intensity • Wedge filter • Trough filter • Used to image non-uniform anatomic areas • Thicker part of filter lined up with thinner part allowing fewer x-ray photons to reach anatomic area

24. Image Formation • Differential absorption • Anatomic tissues absorb and transmit x-rays differently based on their composition (atomic number and tissue density). • Bone absorbs more x-rays than muscle. • Attenuation: the primary x-ray beam loses some of its energy (number of photons) as it interacts with anatomic tissue. • Absorption • Scattering

25. Differential Absorption

26. X-ray Beam Absorption • During absorption, the energy of the primary beam is deposited within the atoms comprising the tissue. • Photoelectric effect: complete absorption of the incoming photon • X-ray ionizes atom • Low energy secondary x-ray photon created • Probability of photoelectric effect dependent on the energy of the incoming x-ray photon and tissue atomic number

27. Determining Attenuation of the Beam • Three essential aspects of tissues will determine their attenuation properties and the resulting subject contrast: • Tissue Thickness • Tissue density • Tissue atomic number

28. X-ray interaction with matter • When the primary x-ray beam interacts with anatomic tissues. Three processes occur during attenuation of the x-ray beam: • Absorption • Scattering • Transmission

29. Transmission • If the incoming x-ray photon passes through the anatomic part without any interaction with the atomic structures, it is called transmission. • The combination of absorption and transmission of the x-ray beam will provide an image that represents the anatomic part.

30. Scattering • The Compton effect occurs when an incoming photon loses some but not all of its energy, then changes its direction. • It can occur within all diagnostic x-ray energies and is dependent only on the energy of the incoming photon, not the atomic number of the tissue. • Higher kVp reduces the number of interactions overall, but the number of Compton interactions increases in comparison to the number of photoelectric interactions.

31. Absorption versus Scattering

32. Photoelectric Effect • The secondary x-ray photon does not reach the film. • The photoelectric effect is crucial to the formation of the radiographic image. • The photoelectric effect is responsible for the production of contrast on the radiographic image.

33. Photoelectric Effect • During attenuation of the x-ray beam, the photoelectric effect is responsible for total absorption of the incoming x-ray photon.

34. Scattering/ Compton Effect • The Compton photon may be scattered in any direction. • Scatter refers to any x-ray photon which has changed direction from the direction of the primary beam. • The Compton Effect may be considered as scatter, since 99% of all scattered x-ray photons originate from Compton interactions in the patient.

35. Where do interactions occur • Compton interactions occur only in the outer shells of an atom. • Photoelectric interactions occur only in the inner most shell of an atom.

36. Factors Affecting Beam Attenuation • Tissue thickness • X-rays are attenuated exponentially and generally reduced by ~ 50% for each 4 to 5 cm (1.6" to 2") of tissue thickness. • Type of tissue • Tissues composed of a higher atomic number will increase beam attenuation. • Tissue density • Increasing the compactness of the atomic particles will increase beam attenuation. • X-ray beam quality • Higher kVp increases the energy of the x-ray beam and will decrease beam attenuation.

37. Exit Radiation • Remnant or exit radiation is composed of transmitted and scattered radiation. • The varying amounts of transmitted and absorbed radiation create an image that structurally represents the anatomic area of interest. • Scatter radiation reaching the image receptor creates unwanted exposure called fog.

38. Secondary Radiation vs. Scatter Radiation • Secondary Radiation refers to any radiation resulting from interactions within the patient. • Scatter radiation refers only to that secondary radiation which has been emitted in a direction different than the original x-ray beam. • Most secondary radiation is scattered.

39. Radiographic Quality • A quality radiographic image accurately represents the anatomic area of interest, and its information is well visualized for diagnosis. • Visibility of anatomic structures • Density • Contrast • Accuracy of structural lines (sharpness) • Resolution or recorded detail • Distortion

40. Density • A film image is evaluated by the amount of density or overall blackness after processing. A radiographic image must have sufficient brightness or density to visualize the anatomic structures of interest.

41. Image Contrast • The radiograph must exhibit differences in the brightness levels or densities (image contrast) in order to differentiate among the anatomic tissues.

42. Subject Contrast • Subject contrast is a result of the absorption characteristics of the anatomic tissue radiographed and the quality of the x-ray beam. • The ability to distinguish among types of tissues is determined by the differences in brightness levels or densities in the image or contrast. • Contrast resolution describes an imaging receptor's ability to distinguish between objects similar in subject contrast. • Gray scale: number of different shades of gray that can be stored and displayed in a digital image • Scale of contrast: the range of densities visible on film

43. Scale of Contrast Short scale or high contrast Long scale or low contrast

44. Recorded Detail • Anatomic details must be recorded accurately and with the greatest amount of sharpness. • Recorded detail refers to the distinctness or sharpness of the structural lines that make up the recorded film image. • All radiographic images have some degree of unsharpness.

45. Distortion • Radiographic misrepresentation of either the size or shape of the anatomic part • Size distortion or magnification is an increase in the object's image size compared to its true or actual size. • SID and OID affect magnification. • Shape distortion is a misrepresentation of an object's image shape. • Elongation and foreshortening • Central ray (CR) alignment of the x-ray tube, part, and image receptor affect distortion.

46. Scatter • Unwanted exposure to the image receptor resulting in fog • A result of Compton interactions • Provides no useful information • Scatter or fog decreases image contrast.

47. Modern x-ray film • Radiographic film is composed of two main parts • Base • Emulsion

48. Film Construction • Consists of emulsion of finely precipitated silver bromide crystals • Crystals are suspended in a gelatin and is coated on both sides with a transparent blue tinted polyester support called the base

49. Manifest Image • Visible image you see when the film is processed • What you see as your final radiograph

50. Film-screen Image Characteristics • Film used as medium for acquiring, processing, and displaying the radiographic image • Film emulsion: active layer of film that contains the crystals that serve as latent imaging centers • Intensifying screens: used to convert exit radiation intensities to visible light and expose the emulsion crystals • Film is chemically processed to display the range of densities created as a result of the x-ray attenuation characteristics of anatomic structures.