1 / 24

Principles of CT

Principles of CT. Limitations of Radiography. Inefficient x-ray absorption: typically ~25% for par speed cassette (prior to rare earth technology) High Scatter-to-Primary Ratios: may have >50% scatter at receptor with large beams even with high ratio grid

simeon
Download Presentation

Principles of CT

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. Principles of CT

  2. Limitations of Radiography • Inefficient x-ray absorption: typically ~25% for par speed cassette (prior to rare earth technology) • High Scatter-to-Primary Ratios: may have >50% scatter at receptor with large beams even with high ratio grid • Receptor Contrast vs latitude: required film dynamic range limits film contrast • Superposition/Conspicuity: overlapping structures with 3D anatomy rendered on 2D image

  3. Focal Plane Tomography: Bocage 1921

  4. Early Attempts at CT • Gabriel Frank: 1940 Patent: described CT principles using optical backprojection reconstr (but no filter) • Takahashi (Japan, ‘40s, published 1956): describes equipment to image slices by backprojection • Tetel’baum et al (Russia, 1957): Accurate formulation of inverse Radon Transform; TV-based reconstruct • Kuhl & Edwards: (1963): cross-sectional NM images by back-projecting transmission data on oscilloscope • Alan Cormack: built simple CT to measure densities for radiotherapy. Shared Nobel Prize.

  5. Godfrey Hounsfield and EMI: 1967 • Considered areas where much information available but inefficiently used: radiography • Estimated that if efficient detection/analysis, attenuation coefficients measurable within 0.5% from transmission measurements ---> sufficient to distinguish soft tissue differences • Invisioned “slice” divided in small “voxels” • Experiments using Americium source (9-day acquisition) verified 0.5% accuracy achievable

  6. Pixels and Voxels

  7. 1st Generation Data Collection

  8. Hounsfield’s CT Formulation • Measurement Ni written as sum of attenuation of pixel along path • Solve simultan-eous equations from data at many positions and angles • Experiments achieved 0.5% accuracy.

  9. Hounsfield’s Experimental CT

  10. Specimen Scan with Lab Device

  11. 1st Generation Data Collection • 1 Pencil Beam and 1 NaI detector • 160 samples/traverse • 1o increms over 180o • 28,800 samples • Solved simultaneous equations (Fortran) • 1602 image matrix but reduced to 802 for practical clinical use

  12. EMI Mark 1

  13. Image Reconstuction

  14. Image Reconstuction

  15. Backprojection

  16. Backprojection (con’t)

  17. Convolution

  18. Filtered Backprojection

  19. CT Numbers: Hounsfield Units • Example 1: voxel contains water (up= uw): • CT# = 1000 x (uw - uw)/ uw = 0 • Example: voxel contains air (up≈ 0): • CT# = 1000 x (0 - uw)/ uw = 1000 x (-1) = -1000

  20. CT Numbers

More Related