1 / 38

Computed Tomography

Computed Tomography. Computed Tomography. Introduced in 70’s Principle: Internal structures of an object can be reconstructed from multiple projections of the object. Philips CTVision Secura. Mechanism of CT. X-ray tube is rotated around the patient

glain
Download Presentation

Computed Tomography

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. Computed Tomography

  2. Computed Tomography • Introduced in 70’s • Principle: Internal structures of an object can be reconstructed from multiple projections of the object

  3. Philips CTVision Secura

  4. Mechanism of CT • X-ray tube is rotated around the patient • Radiation transmitted through the patient is absorbed by a ring of detectors • Absorbed radiation is converted to an image Detectors

  5. Detectors • Scintillation crystals • Xenon-gas ionization chamber

  6. Scintillation Crystals • Materials that produce light (scintillate) when x-rays interact • Similar to intensifying screen • Number of light photons produced a energy ofincident x-ray beam • Light photons need to be converted to electrical signal

  7. Ionization Chamber • X-ray ionizes xenon gas • Electrons move towards anode • Generates small current • Converted to electrical signal

  8. Attenuation • Reduction in the intensity of an x-ray beam as it traverses matter, by either the absorption or deflection of photons from the beam

  9. Pixel - Voxel • Pixel - picture element • Voxel - volume element

  10. CT Number

  11. Image Display: Windowing • Usual CRT can display ~256 gray levels • 2000 CT numbers • Select the CT number of the tissue of interest, then range of ±128 shades

  12. Cone Beam CT • Uses cone shaped x-ray beam. • Beam scans the head in 360 degrees. • Raw data are reformatted to make images

  13. Benefits of Cone Beam Imaging • Less radiation than multi-detector CT due to focused X-rays (less scatter) • Fast and comfortable for the patient (9 to 60s) • Procedure specific to head and neck applications • One scan yields multiple 2D and 3D images

  14. Anatomic Landmarks on CT

  15. Axial CT Sections

  16. Coronal Sections • Zygomatic Arch • Lat. Pterygoid plate • Optic canal • Sphenoid sinus • Soft tissues of nasopharynx

  17. Frontal bone (orbital plate) • Ethmoid air cells • Middle concha • Maxillary sinus • Inferior concha

  18. Vomer • Ramus • Follicle of molar • Gr. wing of Sphenoid • Tongue • Mylohyoid m

  19. Magnetic Resonance Imaging

  20. Magnetic Resonance Imaging • Three steps of MRI • MRR • Magnetic Field • Radio-frequency Pulse • Relaxation

  21. Magnetic Moment Direction

  22. Application of RF Pulse Relaxation

  23. Spin or Angular Moment • 1H, 14N, 31P, 13C, and 23Na has nuclear spin • They spin around their axes similar to earth spinning around its axis • Elements with nuclear spin has odd number of protons, neutrons

  24. Magnetic Moment • When a nucleus spins, it has angular momentum • When the spinning nucleus has a charge, it has magnetic dipole moment • Moving charges produce magnetic fields

  25. Hydrogen Nucleus • Most abundant • Yields strongest MR signal

  26. Radiofrequency Pulse • RF pulse is an electromagnetic wave • Caused by a brief application of an alternating electric current

  27. Receiver Coils • Send or “broadcast” the RF pulse • Receive or “pick up” the MR signals • Types: Body coils, head coils, and a variety of surface coils

  28. Philips Gyroscan Intera

  29. Relaxation • This is the process that occurs after terminating the RF pulse • The physical changes caused by the RF pulse revert back to original state

  30. T1- Spin Lattice Relaxation • At the end of RF pulse, transversely aligned nuclei tend to return back to equilibrium • This return to equilibrium results in the transfer of energy

  31. T2- Spin-spin Relaxation • While the nuclei are in transverse phase, their magnetization interfere with each other. • This interference leads to the loss of transverse magnetization.

  32. Magnetic Field Strengths • Measured in Tesla or Gauss • Usual MRI field strength ranges from 0.5 to 2.0 Tesla • Earth’s magnetic field is about 0.00005 Tesla (0.5 Gauss)

More Related