Devil physics The baddest class on campus IB Physics

1. Devil physicsThe baddest class on campusIB Physics

2. Tsokos Option I-2Medical Imaging

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. X-ray Imaging • 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-ray Imaging • 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) • Source then rotates to take a slice from a different angle • Thin X-ray beam directed perpendicular to the body axis • Beam creates an image slice that can be viewed from above

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

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

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) • Abridged version

50. Summary of Imaging Methods