Computerized Transverse Axial Scanning (Tomography)

Computerized Transverse Axial Scanning (Tomography).
  1. This is Hounsfield’s most highly cited paper, with 4667 citations according to Google Scholar. That’s a respectable number (ten times more than any of my papers have), yet seems curiously small for a Nobel Prize-winning advance.
  2. Hounsfield’s paper is the first of a trilogy. Hounsfield is not a coauthor on the other two; they report clinical studies using the new technique.
  3. Hounsfield lists his institution as “Central Research Laboratories of EMI Limited”. EMI is famous in the music industry; it is the recording label responsible for the early hits of the Beatles.
  4. Hounsfield’s paper has only three references: two to his own preliminary reports and one to an article by Oldendorf. He didn’t cite Cormack’s papers.
  5. Hounsfield sounds most impressed not by recreating three-dimensional images from two-dimensional projections (which to me is the big advance) but instead by the increased sensitivity of the technique to small differences in x-ray absorption coefficient.
  6. Figure 3, illustrating the scanning device and sequence, is similar to Fig. 16.25 in IPMB.
Fig. 16.25 of IPMB.
  1. Hounsfield measured 160 points in each translation and performed 180 rotations. Each two-dimensional image was represented by an 80 × 80 grid of pixels.
  2. The reconstruction method was different from the two Russ and I analyze in Chapter 12 of IPMB: i) Fourier Transform Reconstruction and ii) Filtered Back-Projection. Instead, Hounsfield just fit his data using the least squares method (see Section 11.1 of IPMB). Hounsfield writes “Each beam path [in the CT scan], therefore, forms one of a series of 28,800 simultaneous equations, in which there are 6,400 variables and, providing that there are more equations than variables, the values of each [pixel] …. can be solved.”
  3. The Hounsfield unit was introduced in Fig. 9, but he did not, of course, call it that. Interestingly, his definition is different than what is used today. Equation 16.25 in IPMB gives the Hounsfield unit as H=1000(μtissue -μwater)/μwater), where μtissue and μwater are x-ray attenuation coefficients. In his paper, Hounsfield defines the unit the same way, except he replaces 1000 by 500.
  4. The article describes preliminary experiments using an iodine-containing contrast agent and digital subtraction, analogous to Fig. 16.23 in IPMB.
  5. The computer equipment pictured in Hounsfield’s paper look big and clunky today. I can only guess what paltry computer power he had available for these first reconstructions.
  6. I love the British Journal of Radiology, known as BJR. [What journal did you think that Bradley John Roth would like?]

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