2Limitations 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 gridReceptor Contrast vs latitude: required film dynamic range limits film contrastSuperposition/Conspicuity: overlapping structures with 3D anatomy rendered on 2D image
4Early Attempts at CTGabriel Frank: 1940 Patent: described CT principles using optical backprojection reconstr (but no filter)Takahashi (Japan, ‘40s, published 1956): describes equipment to image slices by backprojectionTetel’baum et al (Russia, 1957): Accurate formulation of inverse Radon Transform; TV-based reconstructKuhl & Edwards: (1963): cross-sectional NM images by back-projecting transmission data on oscilloscopeAlan Cormack: built simple CT to measure densities for radiotherapy. Shared Nobel Prize.
5Godfrey Hounsfield and EMI: 1967 Considered areas where much information available but inefficiently used: radiographyEstimated that if efficient detection/analysis, attenuation coefficients measurable within 0.5% from transmission measurements ---> sufficient to distinguish soft tissue differencesInvisioned “slice” divided in small “voxels”Experiments using Americium source (9-day acquisition) verified 0.5% accuracy achievable
111st Generation Data Collection 1 Pencil Beam and 1 NaI detector160 samples/traverse1o increms over 180o28,800 samplesSolved simultaneous equations (Fortran)1602 image matrix but reduced to 802 for practical clinical use