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PAM 2003 Lecture 4: Computed Tomography I

PAM 2003 Lecture 4: Computed Tomography I. Basic principles of CT. In this lecture. Understand… Limitations of conventional radiography What is CT Advantage of CT The measurement Equipment Spiral CT Multi-slice spiral CT. Limitations of Conventional X-ray. Superimposition

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PAM 2003 Lecture 4: Computed Tomography I

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  1. PAM 2003Lecture 4: Computed Tomography I Basic principles of CT

  2. In this lecture Understand… • Limitations of conventional radiography • What is CT • Advantage of CT • The measurement • Equipment • Spiral CT • Multi-slice spiral CT

  3. Limitations of Conventional X-ray • Superimposition • Poor contrast in soft tissue

  4. What is a CT Scanner • Doughnut shaped gantry • Moving patient table

  5. What is a CT Scanner • Cross sectional X-ray images - ‘slices’

  6. Advantages of CT • Differentiate overlying structures • Differentiate soft tissues with small differences in X-ray attenuation

  7. Sheet X-ray beam patient Array of detectors Principles of CT • Use a series of 2D views through an object to calculate its contents • Slice defined by ‘sheet’ of x-rays, produced by a fan beam (typically 1 cm thick) • Thin slice also serves to reduce scatter Rotate tube & detectors through 360º Computer reconstruction of 2D slices

  8. volume element (VOXEL) picture element (PIXEL) slice thickness CT Image • Slice subdivided into matrix of tissue voxels • Voxels correspond to locations in computer memory or pixels in image • Brightness of each pixel governed by x-ray attenuation in corresponding voxel

  9. Data Acquisition • Tube mounted with axis perpendicular to slice • Reduce any heal effect slice thickness

  10. What do we Measure? • Total attenuation between tube & detector • Sum of attenuation coefficients in all voxels beam has travelled through • A measure of how rapidly x-ray are absorbed along line within material • Goal: To calculate attenuation within each individual voxel attenuation

  11. The First Generation Scanner • Single pencil beam • Single detector • Translate ~200 steps • Rotate 1º • 3-5 min per slice

  12. The Second Generation Scanner • Narrow fan beam • Small curved detector array • Translate & Rotate (fewer steps) • 20 sec per slice

  13. The Third Generation Scanner • Wide fan beam • Larger detector array • Rotation only • 1 sec per slice • What happens if a detector fails?

  14. Ring Artefact • Ring artefact – occurs when a single detector has blown

  15. The Fourth Generation Scanner • Wide fan beam • Complete ring of detectors • Only tube rotates • No faster • Easier to calibrate • Tube closer to patient • Higher dose!

  16. Tube • High speed rotors • High anode heating capacity (~ 8 MHU) • Small focal spot • 4th generation scanners use continuous X-ray output

  17. Detectors • Requirements • High efficiency • Fast response (short afterglow) • Wide dynamic range • Linearity – signal proportional to x-ray intensity • Small – allow close packing density • Low cost – due to large number required • Two types • Scintillation • Gas-filled

  18. Detectors • Gas-filled • Large metallic chamber • Baffles spaced ~1mm • Grid • Inert gas with high Z • High pressure Xenon • Efficiency ~45% • Scintillation • Crystal: Cadmium tungstate (CdWO4) • 5 detectors per degree • 90% efficient • 50% area

  19. Image Acquisition • One slice acquired per revolution • Slice positioned by moving couch in ‘z-direction’ • Rotate gantry • Reposition patient & reverse gantry • Repeat • Slow process • 1 second per slice- several hundred • Spiral CT • Multi-slice

  20. Spiral CT • Continuous gantry motion (circular) • ‘slip-ring’ technology • Continuous couch motion (linear) • Slice at any z-position achieved by ‘interpolation’

  21. Multi-Slice Spiral CT • Faster • Whole body examination in single breath-hold • 4, 8, or 16 slices acquired in time previous required for one • Multi-detector Array 1 Ring of detectors 4 Rings of detectors

  22. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 • Combined detector output to give 4 thicker slices Multi-Slice Spiral CT • Control number of slices • Volume & Resolution • Typically 16 slice detector array • ( i.e. 16 rings of detectors) • More slices: Shorter image time • Thicker slices: More tissue imaged • Smaller detector size: Better spatial resolution

  23. Summary • Limitations of conventional radiography • What is CT • Advantage of CT • Measurement • Equipment • Spiral CT • Multi-slice spiral CT

  24. Next Lecture Computed Tomography II (Image Processing) • CT numbers (or Houndsfeild numbers) • Image processing • Back projection • Windowing

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