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

18. Backprojection (con’t)

19. Convolution

20. Filtered Backprojection

23. 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

24. CT Numbers