Allan Macleod Cormack (1924-1998)

Lecturer in Physics, University of Cape Town,
1950 - 1957

written by ROBIN CHERRY Emeritus Professor of Physics, University of Cape Town. 24 May 1998

[This article appeared, in slightly abbrieviated form, in the Johannesburg Sunday Times of 24 May 1998]

Four South Africans have won the Nobel Prize for Peace and one has won the Literature Prize. Less well-known, perhaps, is that South Africa has nurtured three winners of the scientific Nobel Prizes: Theiler, Cormack and Klug. The week before last the second of this distinguished trio died in Massachusetts at the age of 74.

Allan Cormack was born in Johannesburg in February 1924. He matriculated from Rondebosch Boys High School in 1941 and received his undergraduate education at the University of Cape Town, where he graduated B.Sc. in 1944 and M.Sc. in Physics in 1945. In the Physics Department at UCT he studied under a man who was to have a major influence on him: the late Professor R.W. James, now legendary for having counted two Nobel laureates (Klug as well as Cormack!) among his students. James was an outstanding teacher and a first-class physicist, one of the pioneers of the technique of X-ray crystallography. Unsurprisingly, Cormack's master's thesis was in X-ray crystallography. After a spell as a junior lecturer at UCT he went to the Cavendish Laboratory, Cambridge, where he changed to nuclear physics. He returned to a lectureship in the Physics Dept. at UCT in 1950 and proved to be an inspiring lecturer. I was one of the small third-year Physics class in 1952 which had the extraordinary good fortune to be lectured to by both James and Cormack. An unforgettable combination they were, too: James a lecturer of outstanding clarity, shy and kindly, already an academic of stature, respected and revered by us all; Cormack lively and enthusiastic, amusing and irreverent, friendly and helpful.

The first half of 1956 was a seminal period for Cormack. The hospital physicist at Groote Schuur Hospital had resigned in 1955, and, as the only nuclear physicist in Cape Town, Cormack was asked to spend one-and-a-half days a week at the hospital supervising the use of radioactive isotopes. He worked under the radiotherapist Dr. J. Muir Grieve, whom he was to thank later "for pointing out the necessity for a solution of the absorption problem". Hugely oversimplified, this problem can be described as follows: how can one best measure and interpret the absorption of X- (or gamma-) radiation by a human body in such a way as to obtain a detailed picture of the organs and tissues through which the radiation passes?

Cormack started to think about it. In mid-1956 he left for Harvard on his first sabbatical leave from UCT. There he took enough time off from his research in experimental nuclear physics to develop the fundamentals of a mathematical theory of the absorption problem. Towards the end of his sabbatical he was offered a post in the Physics Department of Tufts University in Massachusetts. For a variety of reasons, a mixture of professional, personal and political factors, he accepted the Tufts job and resigned from UCT. He made an amicable deal with UCT which required him to return to Cape Town for the third quarter of 1957, and it was during these three months that he performed what appears to be the first "X-ray reconstruction" ever made. He did a simple experiment involving a "phantom" which had been constructed in the Physics workshop to his instructions: this "phantom" was simply a cylinder of aluminium surrounded by a cylindrical ring of wood. He measured the degree to which gamma-rays from a radioactive source were absorbed on passing through the "phantom", and found that the results agreed satisfactorily with the mathematical theory he had been developing. Then he went off to his new post in the USA.

He continued working on the subject intermittently over the next six years. By 1963 he had, by the ingenious use of mathematical techniques which he had probably first encountered in X-ray crystallography, generalised his theory substantially. He tested it using a more complicated "phantom", and published the results and the full theory in two remarkable papers in the "Journal of Applied Physics" in 1963 and 1964. These papers contained the essence of what is now known as "axial tomography", and merited the share in the Nobel Prize for Physiology and Medicine which Cormack received in 1979. But when they appeared in the early 'sixties they aroused virtually no response! Credit for the development of the first commercial instrument capable of obtaining high resolution images for medical purposes goes to G.N. Hounsfield, Cormack's co-laureate in 1979. Hounsfield worked at the EMI laboratories in England, and used technological advances to extend the procedure from laboratory "phantoms" to human bodies. By 1971 the first clinical machine had been installed in a London hospital. The "CAT scanner" ("computerised axial tomography", or "computer assisted tomography") was born. CAT scanners are now standard, albeit expensive, tools in the medical repertoire. Basically, they repeat the experiment which Cormack performed at UCT in 1957: part of a human being replaces the "phantom", the radiation detectors have changed out of all recognition, and Cormack's beautiful mathematics is hidden inside the computer.

After moving to Tufts University in 1957, Cormack remained there until retirement. He is survived by his wife Barbara and three children (Margaret, Jean and Robert) in the USA, and by his sister Amy and brother William in South Africa. There must be many like myself who have enjoyed correspondence and the occasional meeting with Allan Cormack during the last forty years. Both meetings and correspondence were invariably marked by the same humour and enthusiasm which I first appreciated in 1952, and will be sadly missed. Happy memories of a remarkable and inspiring man remain.