As I started reading the article on computed tomography (CT) by John Boone and Cynthia McCollough (Physics Today, September 2021, page 34), I expected to see Allan Cormack mentioned early on. Instead, only in box 1 is it noted that he shared a 1979 Nobel Prize with Godfrey Hounsfield. Cormack was the first to demonstrate the feasibility of x-ray CT through mathematical derivation and experimental validation. His investigations in that area, done with little or no funding, began in 1956 in South Africa, where he was assigned to a Cape Town hospital to oversee their radioactive sources. Observing how crudely radiotherapy planning was done at that time, he wondered if it would be possible to determine the internal inhomogeneities of each patient to improve their individual treatment plans.
In his 1964 paper, Cormack experimentally demonstrated the CT principle.1 He built a hand-operated scanner to measure the attenuation of a cobalt-60 beam as it passed through an object along paths at various angles, referred to now as translate–rotate geometry and shown in figure 1a of Boone and McCollough’s article. Using data collected over a two-day period, he reconstructed the scanned object’s attenuation-coefficient profiles along several lines through the object and showed that, aside from some slight ringing artifacts, the reconstructed values matched the known values. Those profile plots demonstrated that he had achieved his goal of determining the attenuation values inside an object from its x-ray attenuation measurements.
Cormack, in his 1963 paper, presciently suggested the application of his work to two other modalities: positron emission tomography and single-photon emission computerized tomography, commonly referred to as PET and SPECT, respectively, which are frequently performed in the clinic today.2 Prompted by an earlier suggestion by Robert Wilson that protons could be useful in medicine,3 Cormack was especially interested in the promise of proton CT, which is currently being investigated for proton-therapy treatment planning.4
