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  2. 2. PRINCIPLE • Internal structure of an object can be reconstructed from multiple projections of the object
  3. 3. HOW CT WORKS? • A thin cross section of the body is scanned with a narrow beam of Xrays and the transmitted radiation is measured with a sensitive radiation detector • The radiation detector adds up the energy of all the transmitted photons • A numerical data is obtained which is then computer processed to reconstruct an image • A series of projection data is obtained, these data are used to reconstruct cross-sectional images
  4. 4. FIRST GENERATION CT • For head imaging only • Single narrow pencil shaped beam • Single detector • Translate-rotate motion • Scan time: 5 mins for a pair of tomographic section
  5. 5. • Starting at a particular angle , the x-ray tube and detector system translated linearly across the FOV acquiring 160 parallel rays per view . • After end of translation , tube and detector assembly rotated around the subject by 1 degree. • This procedure was repeated until 180 projection. • A total of 180 x 160 = 28,800 rays were measured. • This combination of linear translation followed by incremental rotation is called translate – rotate motion . FIRST GENERATION CT
  7. 7. • Early detector system couldn’t accommodate large change in signal so patient head was recessed via a rubber membrane into a water filled box / water bath . • It acted to bolus the x-rays so that the intensity outside the head is similar to the intensity inside head . • Though water bath cannot be used for body scanning , it was used because it allowed Hounsfield to maximize accuracy of attenuation coefficient measurement (limitation of dynamic range , beam hardening correction) FIRST GENERATION CT
  8. 8. • ADVANTAGE - Less scatter radiation • DISADVANTAGE -The major drawback was nearly 5 minutes was required to complete a single image. -Contrast resolution of internal structures was unprecedented , images had poor spatial resolution .
  9. 9. SECOND GENERATION CT • Full body scanner • Single Fan shaped beam • Multiple detectors • Translate-rotate motion • Scan time: 10-90 sec •
  10. 10. • Since more detectors are used, only few linear movements are needed • So the gantry rotated through a greater arc, upto 30 degree • The number of repetitions depends on the number of detectors used
  12. 12. • DISADVANTAGE • Even small deviations(because of vibration or other misalignment)of scanner hardware position relative to reconstruction voxels would cause data to be back projected through the wrong voxels, creating severe artifacts. • disadvantage of fan beam is the increased radiation intensity towards the edge . But it is compensated with the use of bow-tie filter (limits the range of intensity reaching detector and hardens beam) SECOND GENERATION CT
  13. 13. THIRD GENERATION CT • Fan shaped beam • Curvilinear detector array • Rotate-rotate motion • Scan time: 1 sec or less • Ring artifacts are produced
  15. 15. DISADVANTAGE • Any error or drift in the calibration of detectors relative to other detectors is back projected along these ray path and reinforced along a ring where they cross . • The result is the ring artifact. THIRD GENERATION CT
  16. 16. FOURTH GENERATION CT • Fan shaped beam • Multiple individual detectors used • The xray source is rotated around a fixed detector array • As the fan beam passes across each detector, image projection is acquired • Scan time: 1 sec or less
  17. 17. • DRAWBACKS  Size and geometric dose inefficiency Because tube rotated inside the detector ring large ring diameter was needed .  Scatter could not be removed FOURTH GENERATION CT
  18. 18. FIFTH GENERATION CT • specifically for cardiac imaging • Scan time: 10-20ms. • large bell shaped x- ray tube.
  19. 19. • A large arc of tungsten encircles the patient and lies directly opposite to the detector ring. • X-rays are produced from a focal track as a high energy electron beam strikes the tungsten. • There are no moving parts in the gantry. • Electron beam is produced in cone like structures behind the gantry and is electronically steered around the patient so that it strikes the annular target. • Wherever it strikes – produces x-rays. • The concept is known as EBCT(Electron Beam CT) FIFTH GENERATION CT
  20. 20. • Helical or Spiral CT • As the table is smoothly moved through the rotating gantry , the tube and detector moves in a helical or spiral path in relation to the patient • Three technological developments were required -slip ring technology -high power x-ray tubes -interpolation algorithms SIXTH GENERATION CT
  21. 21. Slip ring • A slip ring passes electrical power to the rotating components without fixed connections. • It allows the complete elimination of interscan delays except for the time required to move the table to next slice position. SIXTH GENERATION CT
  22. 22. Pitch • It is defined as the table movement per rotation divided by beam width. Interpolation • Helical CT scanning produces a data set in which CT images can be reconstructed at any position along the length of scan. • It allows the production of additional overlapping images with no additional dose to the patient. SIXTH GENERATION CT
  23. 23. Advantages • Fast scan times and large volume of data collected. • Minimizes motion artifacts. • Reduced patient dose. • Improved spatial resolution. • Enhanced multiplaner or 3D renderings. • Improved temporal resolution. SIXTH GENERATION CT
  24. 24. SEVENTH GENERATION CT MULTI DETECTOR CT • A body section can be scanned faster with a multiple row of detectors system with multiple fan beams scanning simultaneously. • Crucial for covering a large body section with thin beams for producing thin , high-detail slice images or 3-D images.
  25. 25. • When multiple detector is used , the collimation spacing is wider therefore more of x-rays that are produced by x-ray tubes are used in producing image data. • With conventional single detector array scanners , opening up the collimator increases slice thickness which is good for utilization of x-ray but reduces spatial resolution in the slice thickness dimension. • With introduction of multiple detector arrays , the slice thickness is determined by the detector size and not by the collimator. SEVENTH GENERATION CT
  26. 26. Pitch •Pitch is equal to the table rotation per gantry rotation divided by width of detector. SEVENTH GENERATION CT
  28. 28. COMPONENTS XRAY TUBE: -Rotating anode with a smaller focal spot -Have large heat loading and heat dissipating capacities COLLIMATORS: -used at 2 places -detector collimator controls scatter -control the thickness of ct slice
  29. 29. DETECTORS SCINTILLATION DETECTOR - combination of scintillation crystal and light detector • With interaction with crystal, energy of Xray photon is converted into light photon. This light output is converted into electric signal using a light detector • Initially thallium activated NaI crystals with photomultiplier tubes were used •
  30. 30. LIMITATIONS: • NaI : -hygroscopic and requires a air tight container -long afterglow NaI is replaced by CsI, BGO, CdWO4 • Photomultiplier tubes are replaced by silicon photodiodes
  31. 31. XENON GAS IONISATION CHAMBERS: • Photon interacts with a gas atom by ionisation of atom into a electron-ion pair • Voltage between anode and cathode drives negative ion(electrons) to anode which in turn produces a small current in the anode
  32. 32. • Current produced is directly proportional to the intensity of incoming radiation LIMITATIONS: -This type of detector cannot be used in rotate- fixed CT -Reduced efficiency
  33. 33. PIXEL AND VOXEL • Each square in the image matrix is called pixel • The 3D volume element of pixel is called voxel
  34. 34. CT NUMBER- HOUNSFIELD UNITS • Numerical values for each pixel • It is directly related to the Xray linear attenuation coefficient for the tissue contained in the voxel
  36. 36. BACK PROJECTION • Depending on the amount of radiation passed through the tissue, a grey scale density is assigned to each projection • When different projections are superimposed or back- projected, they produce a crude reproduction of original object
  37. 37. ITERATIVE RECONSTRUCTION • It starts with an assumption and compares this assumption with measured values, makes corrections to bring the two into agreement, and repeats the process over and over until the assumed and measured values are the same or within acceptable limits • 3 variations -simultaneous reconstruction -ray by ray correction -point by point correction
  38. 38. ANALYTIC METHODS • Currently used in CT • Fournier analysis • Filtered back projection
  39. 39. FOURNIER ANALYSIS • Any function of time or space can be represented by sum of various frequencies and amplitudes of sine and cosine waves • In a wave, height corresponds to amplitude and length corresponds to frequency
  40. 40. FILTERED BACK PROJECTION • Similar to back projection except that the projected image is modified by filtration to eliminate the frequencies responsible for blurring .
  41. 41. THANK YOU