3. Reading Activity Answers

4. IB Assessment Statements Option I-2, Medical Imaging: X-Rays I.2.1.Define the terms attenuation coefficient and half-value thickness. I.2.2.Derive the relation between attenuation coefficient and half-value thickness. I.2.3.Solve problems using the equation,

5. IB Assessment Statements Option I-2, Medical Imaging: X-Rays I.2.4.Describe X-ray detection, recording and display techniques. I.2.5.Explain standard X-ray imaging techniques used in medicine. I.2.6.Outline the principles of computed tomography (CT).

6. IB Assessment Statements Option I-2, Medical Imaging: Ultrasound I.2.7.Describe the principles of the generation and the detection of ultrasound using piezoelectric crystals. I.2.8.Define acoustic impedance as the product of the density of a substance and the speed of sound in that substance. I.2.9.Solve problems involving acoustic impedance.

7. IB Assessment Statements Option I-2, Medical Imaging: Ultrasound I.2.10.Outline the difference between A-scans and B-scans. I.2.11.Identify factors that affect the choice of diagnostic frequency.

8. IB Assessment Statements Option I-2, Medical Imaging: NMR and Lasers I.2.12.Outline the basic principles of nuclear magnetic resonance (NMR) imaging. I.2.13.Describe examples of the use of lasers in clinical diagnosis and therapy.

9. Objectives  State the properties of ionizing radiation  State the meanings of the terms quality of X- rays, half-value thickness (HVT), and linear attenuation coefficient  Perform calculations with X-ray intensity and HVT,

10. Objectives  Describe the main mechanisms by which X- rays lose energy in a medium  State the meaning of fluoroscopy and moving film techniques  Describe the basics of CT and PET scans  Describe the principle of MRI  State the uses of ultrasound in imaging  State the main uses of radioactive sources in diagnostic medicine

11. Properties of Radiation  Two uses in medicine:  Diagnostic imaging (this lesson)  Radiation therapy (next lesson)

12. Properties of Radiation  Types of Radiation:  Alpha (α)  Beta (β)  Gamma (γ)

13. Properties of Radiation  Intensity – power as if it were radiated through a sphere

14. Attenuation  Intensity drops exponentially when passed through a medium capable of absorbing it  The degree to which radiation can penetrate matter is the quality of the radiation  μ is a constant called the linear attenutation coefficient

15. Attenuation  Attenuation depends not only on the material the radiation passes through, but also on the energy of the photons

16. Attenuation  Half-Value Thickness (HVT) – similar to radioactive decay law, the length that must be travelled through in order to reduce the intensity by a factor of 2

17. Attenuation  Half-Value Thickness as a function of photon energy

18. Attenuation  X-rays absorbed via photoelectric and Compton effects  Photoelectric effect – X-ray photons absorbed by an electron which is then emitted by the atom or molecule  Compton effect – photon gives part of its energy to a free electron and scatters off it with a reduced energy and increased wavelength (elastic collision)

19. X-ray Imaging  First radiation to be used for imaging  Operate at voltage of around  15-30 kV for mammogram  50-150 kV for chest X-ray

20. X-ray Imaging

21.  Most energy lost through photoelectric effect  Photoelectric effect increases with atomic number of elements in tissue  Bone will absorb more X-rays than soft tissue  X-rays show a contrast between bone and soft tissue  Energy will pass through soft tissue and expose the film on the other side  Energy absorbed by bone tissue will cast a shadow

22. X-ray Imaging  When there is no substantial difference between Z-numbers in the material, patients are give a contrast medium, usually barium  Barium absorbs more X-rays to give a sharper image

23. X-ray Imaging  Image is sharper if:  Film is very close to patient  X-ray source is far from patient  Lead strips are moved back and forth between patient and film to absorb scattered X-rays  Low-energy X-rays removed by filtering  Intensifying screens used to enhance energy of photons passed through patient to reduce exposure time

24. X-ray Imaging

25.  X-rays on TV  Capability to project real-time X-ray images on a monitor  Advantages outweighed by increased exposure time/radiation dosage  Does have advantages for examining cadavers and inanimate objects (jet engines)

26. Computed Tomography (CT Scan)  Computed (axial) tomography or  Computer assisted tomography (CAT)  Still uses X-rays, but  Reduced exposure time  Greater sharpness  More accurate diagnoses

27. Computed Tomography (CT Scan)  Thin X-ray beam directed perpendicular to the body axis  Beam creates an image slice that can be viewed from above Source then rotates to take a slice from a different angle

28. Computed Tomography (CT Scan)  Many detectors are used to record the intensity of X-rays reaching them  Information is sent to a computer to reconstruct the image  Similar to digital camera processing Detector grids are also called pixels

29. Magnetic Resonance Imaging (MRI)  Based on a phenomenon called nuclear magnetic resonance  Superior to CT Scan  No radiation involved (don’t let ‘nuclear’ throw you)  But, much more expensive

30. Magnetic Resonance Imaging (MRI)  Electrons, protons and most particles have a property called spin – See Eric  Particles with an electrical charge and spin behave like magnets – magnetic moment  In the presence of a magnetic field, the moment  Will align itself parallel (‘spin up’)  Or anti-parallel (‘spin down’) to the direction of the field

31. Magnetic Resonance Imaging (MRI)  Hydrogen protons have specific energy levels  In the presence of a magnetic field, the energy level will change based on how the magnetic moment aligns with the field  Difference in energy levels is proportional to the external magnetic field strength

32. Magnetic Resonance Imaging (MRI)  A radio frequency (RF) source (electromagnetic radiation) is introduced  If the frequency of the RF source corresponds to the difference in energy levels, the proton will jump to the higher state, then go back down and emit a photon of the same frequency

33. Magnetic Resonance Imaging (MRI)  Detectors register the photon emissions and a computer can reconstruct an image based on the point of emission  Rate of photon emission important to identifying tissue type

34. Magnetic Resonance Imaging (MRI)  Point of emission determined by using a second magnetic field to break up uniformity of original magnets used to align the spins  External magnetic field regulates photon emissions

35. Magnetic Resonance Imaging (MRI)  Process dependent on hydrogen saturation  Newer techniques can measure rate at which protons return to ground state to better identify tissue type

36. Magnetic Resonance Imaging (MRI)  Show and Tell Show and Tell

37. Positron Emission Tomography (PET Scan)  Similar to a CT Scan  Involves annihilation of an electron and a positron (anti-particle of the electron) and detection of two photons that are then produced

38. Positron Emission Tomography (PET Scan)  Patients injected with radioactive substance that emits positrons during decay  Emitted positron collides with an electron in the patient’s tissue  Electron-positron collision annihilates in two photons each of energy 0.511 MeV

39. Positron Emission Tomography (PET Scan)  Total momentum is conserved an the photons move in opposite directions with same velocity  Detectors can then located the point of emission  Can give a resolution of 1mm  Especially good for brain images

40. Ultrasound  Uses sound in the 1 to 10 MHz range – not audible  No radiation  No known adverse side effects  Can produce some images X-rays can’t (lungs)  Not as detailed as X-rays

41. Ultrasound  Sound emitted in short pulses and reflection off various surfaces is measured  Very similar to sonar and radar  Diffraction limits resolution size, d, to λ < d  Wavelength determined by speed of sound in tissue  In practice, with the frequencies used, pulse duration and not diffraction limits resolution

42. Ultrasound  Frequency determined by the type of organ tissue studied  Rule of thumb is f = 200(c/d) where c is speed of sound and d is depth (depth of 200 wavelengths

43. Ultrasound  Transition into a body an into different tissues means some of the waves will be reflected  Amount transmitted into second tissue depends on impedance of the two media

44. Ultrasound  For the most energy to be transmitted, impedances should be as close as possible  Gel is used between transducer and body to improve impedance matching

45. Ultrasound  A-Scan

46. Ultrasound  A-Scan

47. Ultrasound  Combined A-Scans

48. Diagnostic Uses of Radioactive Sources  Used to monitor organs and their functions  Measurement of body fluids  How food is digested  Vitamin absorption  Synthesis of amino acids  How ions penetrate cell walls  Radioactive iodine used to monitor thyroid functions

49. Diagnostic Uses of Radioactive Sources  Most commonly used is technetium-99  Horse example (27 minutes) Horse example (27 minutes)  Abridged version Abridged version

50. Summary of Imaging Methods

51. Σary Review  State the properties of ionizing radiation  State the meanings of the terms quality of X- rays, half-value thickness (HVT), and linear attenuation coefficient  Perform calculations with X-ray intensity and HVT,

52. Σary Review  Describe the main mechanisms by which X- rays lose energy in a medium  State the meaning of fluoroscopy and moving film techniques  Describe the basics of CT and PET scans  Describe the principle of MRI  State the uses of ultrasound in imaging  State the main uses of radioactive sources in diagnostic medicine

53. IB Assessment Statements Option I-2, Medical Imaging: X-Rays I.2.1.Define the terms attenuation coefficient and half-value thickness. I.2.2.Derive the relation between attenuation coefficient and half-value thickness. I.2.3.Solve problems using the equation,

54. IB Assessment Statements Option I-2, Medical Imaging: X-Rays I.2.4.Describe X-ray detection, recording and display techniques. I.2.5.Explain standard X-ray imaging techniques used in medicine. I.2.6.Outline the principles of computed tomography (CT).

55. IB Assessment Statements Option I-2, Medical Imaging: Ultrasound I.2.7.Describe the principles of the generation and the detection of ultrasound using piezoelectric crystals. I.2.8.Define acoustic impedance as the product of the density of a substance and the speed of sound in that substance. I.2.9.Solve problems involving acoustic impedance.

56. IB Assessment Statements Option I-2, Medical Imaging: Ultrasound I.2.10.Outline the difference between A-scans and B-scans. I.2.11.Identify factors that affect the choice of diagnostic frequency.

57. IB Assessment Statements Option I-2, Medical Imaging: NMR and Lasers I.2.12.Outline the basic principles of nuclear magnetic resonance (NMR) imaging. I.2.13.Describe examples of the use of lasers in clinical diagnosis and therapy.

59. #1-8 Homework

60. Stopped Here 4/10/14