This presentation gives an information about Basic principles of CT scanning covering the syllabus of Non Destructive testing

1. 6/3/2014 1Hareesha N G, Dept of Aero Engg, DSCE

2. The basics of CT
• CT imaging chain
• System components
• Acquisition methods
• Image quality
• Applications
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3. X-ray: The beginning
• X-Rays founded in 1895 by Wilhelm
Conrad Roentgen
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4. CT: The beginning
• CT founded in 1970 by Sir Godfrey Hounsfield
– Engineer with EMI, LTD.
– first applications were in neuroradiology
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5. CT Scanner
• X-Ray modality used to the body in
cross section
• Used to determine
– extent of trauma
– location and type of tumors
– status of blood vessels
– pre surgical planning
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6. CT System
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7. CT System
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8. Basic CT scanner components
• Gantry
• X-Ray Tube
• Detector
• Control Console
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9. Gantry
• CT X-ray tube
• High voltage generator
• Detector array
• Data acquistion system
• Slip ring
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10. The CT X-ray Tube
• Anode heat capacity
– 3.5 MHU up to 6.5 MHU
• Determines maximum mAs
• Determines volume length
• Dictates generator size
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11. Detector Elements
• Capture energy that has not been attenuated
by the patient
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12. Control console
• Set scan parameters
– kVp, mA, scan time, reconstruction filter,
etc.
• Set scan mode
– Digital radiograph, axial or volume
• Houses reconstructor
• Review and archive images
• Post-processing
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13. CT
• CT - Computed Tomography
• CAT Scan - Computerized Axial Tomography
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14. Scanning methods
• Digital projection
– AP, PA, Lat or Oblique projection
– Surview, Scanogram
• Conventional CT
– Axial
• Start/stop
• Volumetric CT
– Helical or spiral CT
• Continuous acquisition
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15. Digital Projection
• X-ray tube and detector remain stationary
• Patient table moves continuously
– With X-rays “on”
• Produces an image covering a range of anatomy
– Similar to a conventional X-ray image, e.g. flat plate of
the abdomen
• Image used to determine scan location
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16. Axial CT
• X-ray tube and detector rotate 360°
• Patient table is stationary
– With X-ray’s “on”
• Produces one cross-sectional image
• Once this is complete patient is
moved to next position
– Process starts again at the
beginning
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17. Volume CT
• X-ray tube and detector rotate 360°
• Patient table moves continuously
– With X-ray’s “on”
• Produces a helix of image information
– This is reconstructed into 30 to 1000 images
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18. Volume Scanning: Pitch
1:1 2:1
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19. Advantages of Volume CT
• More coverage in a breath-hold
– Chest, Vascular studies, trauma
• Reduced misregistration of slices
– Improved MPR, 3D and MIP images
• Potentially less IV contrast required
• Gapless coverage
• Arbitrary slice positioning
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20. Fundamentals of Multislice CT
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21. Multislice Fundamentals
• Everything is better
• (R)esolution
– Z-axis, spatial, low contrast
• (S)peed
– Temporal -
bolus capture, stopped motion
• (V)olume
– Thin slice -
– organ-specific coverage
• (P)ower
– Enough photons -
uncompromising image quality
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22. Single Slice = One 10mm slice per rotation
Dual Slice = Two 5mm slices per rotation
Quad Slice = Four 2.5mm slices per rotation
Multislice Effectiveness
• Everything is better
– Resolution 2x 4x-8x
– Speed same same
– Volume same same
– Power same same
Dual Quad
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23. . .
single detector arc dual detector arc
pre-patient collimation
post-patient collimation
x-ray tube focal spot
___
Mx8000
Dual Slice
Dual Slice Detector Optimized for 2
Slice Acquisition
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24. Approximately 10%
more efficient than
matrix detectors
Variable Wide Area Detector
Asymmetrix™
Variable detector length Fixed detector length
Quad Detector Technology
• Philips patented variable wide area detector
• Variable slice thickness
– 4 x 1mm
– 4 x 5mm
– 4 x 2.5mm
– 2 x 0.5mm
– 2 x 8mm
– 2 x 10mm
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25. 8 Element
2-D array
4 Slices
Quad Technology
How it works
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26. 2x0.5mm
4x1mm
4x2.5mm
4x5mm
2x10mm
p-plane
fused fused
to FEE
Asymmetrix™ Technology
Variable slice thickness
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27. CT
• CT attenuation information
• CT image quality
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28. Attenuation
• X-ray beam passes through patient
• Each structure attenuates X-ray beam
differently
– According to individual densities
• Radiation received by detector varies
according to these densities
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29. Density information
• Transferred from detector to CT computer
(A to D converter)
• Reconstructed by computer into a cross-sectional image
– Displayed on screen
– Each pixel displayed on monitor has varying brightness
• The greater the attenuation, the brighter the pixel
• The less attenuation, the darker the pixel
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30. Density information
• Density values
correspond to a
range of numbers
– Hounsfield scale
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31. Window settings
• Window width
– Determines range of CT numbers displayed on an image
• Values above this range = white
• Values below this range = black
– Window level
• Sets the center CT number displayed on the monitor
• Determines the location on the Hounsfield scale about which the window
width will be centered
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32. CT image quality
• Spatial resolution
– Ability to resolve small
objects in an image
– Measured in lp/cm
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33. Isotropic Imaging
• True 0.5mm Isotropic imaging
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34. CT image quality
• Contrast resolution
– Ability to differentiate small density differences in an image
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35. Post Processing Options
• Visualization of
vasculature in
relation to pathology
– Show course of vessels
– Show stent placement
– Define vascular stricture
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36. Cervical Spine
Spiral Acquisition
Rotation – 0.75 sec
Coverage – 160 mm
Pitch – 0.875
Acq. Time – 36 sec
FOV – 250 mm
ST – 1.0 mm
Rec. Incr. – 0.6 mm
Std Res. – 8 lp/cm
120 kV, 200 mAs
CTDI100w – 39 mGy
CTDIFDAw – 17 mGy
Thin-Slice Spiral Neck
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37. Neuro-Angiography
Circle of Willis
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38. Thin-SliceSpiralLungs
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39. MIP 4D-Angio
MasterCut
Panoramic View
RenalArteries
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40. Ext. Spiral Acq.
Spiral Acquisition
UltraFast – 0.5 sec
Coverage – 1400mm
Pitch – 1.75
Acq. Time – 41.5 sec
FOV – 420 mm
ST –2.5 mm
Rec. Incr. – 1.6 mm
Std Res. – 8 lp/cm
120 kV, 96 mAs
CTDI100w – 7.3 mGy
CTDIFDAw – 4.9 mGy
Extended Spiral Acquisition
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41. CT Scanners
• Provide a window into the body
• Customer considerations
– How many patients
– Referring physicians
– Budget
– Upgrade expectations
• Philips has ALL the answers
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