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X-Ray Medical Imaging Physics – IB Objectives

X-Ray Medical Imaging Physics – IB Objectives. X-Ray Production. Anode (Tungsten). **Spinning** (Why?). Vacuum chamber. High voltage. X-rays. Hot filament cathode. Electrons. Filament voltage. X-Ray Interaction with Matter and Attenuation. X-rays interact with matter in four ways

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X-Ray Medical Imaging Physics – IB Objectives

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  1. X-Ray Medical Imaging Physics –IB Objectives IB Physics HL 2

  2. X-Ray Production Anode(Tungsten) **Spinning** (Why?) Vacuumchamber . . . High voltage X-rays Hot filamentcathode Electrons Filament voltage IB Physics HL 2

  3. X-Ray Interaction with Matterand Attenuation • X-rays interact with matter in four ways • Photoelectric effect (photon in – electron out) • Coherent scattering off atom as a whole (photon in – photon out) • Compton scattering off electron (photon in – electron + photon out) • Pair production (photon in – electron + positron out) (E > 1 MeV) IB Physics HL 2

  4. X-Ray Interaction with Matterand Attenuation • Photoelectric effect Orbital electronknocked out ofatomic orbitcreating ion Incoming photonscatters offorbital electron IB Physics HL 2

  5. Outgoing photonscatters offatom as a whole Incoming photonscatters offatom as a whole X-Ray Interaction with Matterand Attenuation • Coherent scattering / Rayleigh scattering • Atom not ionized nor excited IB Physics HL 2

  6. X-Ray Interaction with Matterand Attenuation • Incoherent scattering / Compton scattering Electron scatteredout of atom Incoming photonscatters offsingle electron(as if electron werefree) Outgoing photonafter scattering offelectron IB Physics HL 2

  7. X-Ray Interaction with Matterand Attenuation • Pair production • Enough energy in initial beam to create e+e- pair Nucleus interactswith incomingphoton e- Electron-positronpair created fromincoming photonand nuclear interaction Incoming photonscatters off nucleus e+ IB Physics HL 2

  8. X-Ray Interaction with Matterand Attenuation • For carbon (~people) below 12 keV, increasing energy decreases interaction • Interaction mainly from photoelectric effect • Bones (heavier nuclei) attenuate X-rays more than soft tissue (carbon) IB Physics HL 2

  9. X-Ray Attenuation Coefficient • Similar to radiation half-lives and decay coefficients • Decrease in intensity (W/m2) is proportional to initial intensity: • With solution: I = I0e-x •  is the linear attenuation coefficient (m-1) does depend on energy • This gives the intensity at depth x meters IB Physics HL 2

  10. X-Ray Half-Value Thickness • Similar to the radioactive decay half-life, we can define a half-value thickness at which the beam drops to one-half its initial intensity • I0/2 = I0e-x1/2 • or 0.5 = e-x1/2or ln(0.5) = -x1/2or  = ln(2) / x1/2 (just like radioactive decay) IB Physics HL 2

  11. X-Ray Choice of Wavelength • Choice of wavelength depends on what is being imaged • Bone • Soft tissue • Also want to minimize absorbed energy IB Physics HL 2

  12. X-Ray Attenuation Sample Problem • The attenuation coefficient for an X-ray of a specific wavelength through muscle is 0.045 cm-1 • What is the half-value thickness? • The half-value thickness of bone, for the same X-ray, is 150 times smaller • What is its attenuation coefficient? • In which of these materials does the X-ray intensity drop off more quickly? IB Physics HL 2

  13. X-Ray Attenuation Sample Problem (Cont’d) • If the initial X-ray intensity is 2.00 W/m2, what is its intensity after traveling through 13.0 cm of muscle? • How much is absorbed by the muscle? • What is the intensity of the X-ray after traveling through 3.47 cm of bone? IB Physics HL 2

  14. X-Ray Beam Techniques • Improve penetrating quality of beam by absorbing out low-energy X-rays • With large attenuation coefficients, X-rays get absorbed easily by soft tissue • Use ~1 mm to 1 cmof Al IB Physics HL 2

  15. X-Ray Beam Techniques • Tube voltage • Increasing tube voltage increases penetrating power of X-rays • Bremsstrahlung • K, Lspectra IB Physics HL 2

  16. X-Ray Beam Techniques • Beam current • Increasing beam current increases intensity of X-rays • Does not changepenetrating power IB Physics HL 2

  17. X-Ray Beam Techniques • Target material • Changing target material changes characteristic K, L lines • Bremsstrahlungspectrum staysthe same (more orless) IB Physics HL 2

  18. X-Ray Imaging Techniques • Putting a lead grid in front of imaging material will improve the sharpness of the image • Scattered X-rays areabsorbed by gridbefore getting tofilm IB Physics HL 2

  19. X-Ray Imaging Techniques • Direct image • Bone (white) • Higher energy X-ray • Soft tissue (gray) • Lower energy X-ray • Gaps – air (black) • Contrast medium • Opaque material outlines soft tissue • Barium, bismuth (intestines) • Iodine (blood) IB Physics HL 2

  20. X-Ray – Coronary Arteries IB Physics HL 2 From: http://www.ajronline.org/cgi/content-nw/full/179/4/911/FIG8

  21. X-Ray Detection, Recording, and Display • Detection • Film, image-enhanced film, digital computer-read screens and detectors • Recording • Film, digital film, computer memory • Display • Film, computer display, television (real-time) display (~fluoroscopy) IB Physics HL 2

  22. X-Ray Detection, Recording, and Display • Film • Person placed between X-ray tube and film • Film is detection, recording, and display mechanism all in one X-raytube X-raysensitivefilm IB Physics HL 2

  23. X-Ray Detection, Recording, and Display • Enhanced film (basically all modern X-rays) • Person placed between X-ray tube and film • Film is placed in cassette with X-ray sensitive phosphors • Provides better image • Film as recording and display device X-raytube X-rayfilm cassette IB Physics HL 2

  24. X-Ray Detection, Recording, and Display • Enhanced film cassette • Intensifying screens contain X-ray sensitive phosphors that create light when struck with X-rays • Film displays X-rays detected by film and screen IB Physics HL 2

  25. X-Ray Detection, Recording, and Display • Digital Radiology • Instead of normal film, X-rays detected by a plate sensitive to X-rays • Plate is “read” by laser • Stored in computer memory • Computer display Digitalscanningprocess X-raytube X-raysensitive plate IB Physics HL 2

  26. X-Ray Detection, Recording, and Display • Computer Radiology • Instead of film, X-rays detected by a computer-readable screen • Computer reads screen, and stores image in memory • Computer display X-raytube Computer-readableX-ray phosphor screen IB Physics HL 2

  27. X-Ray Detection, Recording, and Display • Real-Time Displays • Observe operation of heart, intestines, throat, etc. • Instead of film, X-rays detected by phosphors on screen • Television camera observes phosphor screen • Display real-time image on television screen X-raytube X-ray sensitivephosphor screen IB Physics HL 2

  28. X-Ray Medical Imaging –Fundamental Ideas • What are they? IB Physics HL 2

  29. Drawbacks of Normal X-Ray Scans • X-rays show only one view of body • Shadow of everything between X-ray tube and film • Difficult to interpret soft-tissue images -> Idea: take X-ray scans in multiple directions IB Physics HL 2

  30. 8 4 4 10 5 5 Idea of Multiple Scan Directions • Imagine taking X-ray image of 2 x 2 square • Take image in horizontal direction A B X-rayintensities C D X-rays Film IB Physics HL 2

  31. 8 4 4 10 5 5 11 7 Idea of Multiple Scan Directions • Imagine taking X-ray image of 2 x 2 square • Take second image in vertical direction X-rays A B C D Film X-ray intensities IB Physics HL 2

  32. 8 3 5 10 4 6 11 7 Idea of Multiple Scan Directions • Imagine taking X-ray image of 2 x 2 square • Use both intensities to determine relative X-ray absorption • Show relative absorption with different shading • This is the principle of Computed Tomography (CT) A B C D X-ray intensities IB Physics HL 2

  33. Computed Tomography (CT) Scan Schematic • Use more then just 2 x 2 resolution • Typical: 256 x 256 IB Physics HL 2

  34. Computed Tomography (CT) Scanners IB Physics HL 2

  35. Computed Tomography Scanner IB Physics HL 2 From http://en.wikipedia.org/wiki/Computed_Axial_Tomography

  36. Computed Tomography Scanner - Internals IB Physics HL 2 From http://en.wikipedia.org/wiki/Computed_Axial_Tomography

  37. Computed Tomography – 2D to 3D • X-ray imaging system can move along the body • CT scans in cross-section • Can build up 3D model of body • Instead of pixels (picture elements): voxels (volume elements) IB Physics HL 2

  38. Computed Tomography – Usage • Brain scans • Bleeding • Stroke • Tumor • Other organs (soft tissue) • Heart • Kidneys • Etc • Applications • Tumors • Trauma • Structure IB Physics HL 2 From http://en.wikipedia.org/wiki/Computed_Axial_Tomography

  39. Computed Tomography – Risk Balancing • CAT scans and X-rays use ionizing radiation • Ionizing radiation is damaging to tissue • Normal X-rays give some multiples of background radiation dosage • CAT scans give significantly more than normal X-rays • Balance help to patient from scan vs risk of damage (cancer) from X-rays IB Physics HL 2

  40. Computed Tomography –Fundamental Ideas • What are they? IB Physics HL 2

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