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Computed Tomography

Computed Tomography. CT, CAT tomos = slice, graphein = to write. Magdalena Bazalova. 1. What is a CT scanner?. an X-ray device capable of cross-sectional imaging creates images of slices through the patient. What is a CT scanner?. doughnut shaped gantry with moving patient table. Why CT?.

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Computed Tomography

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  1. Computed Tomography CT, CAT tomos = slice, graphein = to write Magdalena Bazalova

  2. 1. What is a CT scanner? • an X-ray device capable of cross-sectional imaging • creates images of slices through the patient

  3. What is a CT scanner? • doughnut shaped gantry with moving patient table

  4. Why CT? • conventional radiography suffers from collapsing of 3D structures onto a 2D image • although the resolution of CT is lower, it has extremely good low contrast resolution enabling the detection of very small changes in tissue type • CT gives accurate diagnostic information about the distribution of structures inside the body

  5. CT scanning applications • very wide ranging – good for imaging bone and soft tissue • diagnostic imaging • radiotherapy planning • 3D applications

  6. CT imaging applications

  7. CT imaging applications

  8. CT imaging applications

  9. Why CT for radiotherapy? • Radiation therapy planning is done on the basis of patient CT images and is therefore patient specific • the target and organs at risk are delineated in CT images (possibly with help of other imaging modalities – PET) • dose calculation algorithms use CT images for determination of dose delivered to the patient during treatment

  10. Why CT for radiotherapy? • Tissue inhomogeneities can be taken into account in most treatment planning systems • Dose to soft tissue is different than dose to cortical bone - mass density variations between tissue types are the most important factor • Therefore, mass densities of tissues have to be known for an accurate dose calculation • CT images do not represent mass densities of patient body directly but they can be converted into mass densities using a calibration curve

  11. 2. CT scanner components X-ray tube X-ray beam detector ring

  12. X-ray journey

  13. X-ray tube

  14. Beam shaping filter

  15. Detectors

  16. Detector arrangement

  17. Philips CT simulator

  18. Questions on CT apparatus • How do we call the device that produces X-ray beam? • (X-ray tube ) • What have the X-rays pass through on their way to the detector ring? • (beryllium window, Al filters, bow-tie filter, patient, anti-scatter grid)

  19. 3. CT image definition and formation

  20. What are we measuring in CT? • the linear attenuation coefficient, µ, between the X-ray tube and the detector • the linear attenuation coefficient is a measure of how rapidly are X-ray attenuated

  21. p 2D-projection data set - sinogram • projections • I, I0 - intensities -d x-ray source -d d d Projection angle

  22. Reconstruction algorithms • Computer based • simple back-projection • filtered back-projection • iterative techniques

  23. Simple back-projection • reverse the process of measurement of projection data to reconstruct an image • each projection is uniformly distributed across the reconstructed image

  24. Simple back-projection 1/r blurring

  25. Filtered back-projection • simple back-projection produces blurred images • projection data need to be filtered before reconstruction • different filters can be applied for different diagnostic purposes • smoother filters for viewing soft tissue • sharp filters for high resolution images • back-projection is the same as before

  26. Filtered back-projection

  27. Filtered back-projection Image reconstruction Simple back-projection FBP

  28. Patient image reconstruction

  29. Patient filtered back-projection

  30. CT number scale HU represents the linear attenuation of a material.

  31. CT number window

  32. CT number window

  33. CT for radiotherapy – calibration, HU to mass density conversion • HU do not represent mass density, needed for dose calculation, directly. To obtain mass densities of each voxel: • A set of tissue equivalent materials with known mass densities is scanned and a calibration curve is created

  34. Calibration curve for treatment planning

  35. Questions on reconstruction • How do we call picture and volume elements? • (pixels and voxels) • What do CT images represent? • (linear attenuation coefficients of voxels) • How do we call raw detector data? • (a sinogram) • Name two reconstruction techniques? • (simple and filtered back-projection)

  36. 4. CT technology

  37. Third generation CT scanners

  38. Fourth generation CT scanners

  39. Fifth generation CT scanners

  40. Helical CT scanning

  41. Advantages of helical mode

  42. Questions on CT technology • How many CT generations exist? • 5 (maybe more) • Which one is the third one? • rotate/rotate • What are the advantages of helical scanning? • arbitrary image position, faster scanning

  43. 5. CT image quality

  44. Image noise

  45. Image noise

  46. Image contrast

  47. Image contrast

  48. Factors affecting image noise

  49. Reconstruction filters

  50. Factors affecting detector signal

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