1 / 40

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

erica-sullivan
Télécharger la présentation

X-Ray Medical Imaging Physics – IB Objectives

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related