Allan M. Cormack and Godfrey N. Hounsfield

COMPUTED MEDICAL IMAGING
Nobel Lecture, 8 December, 1979
BY
GODFREY N. HOUNSFIELD
The Medical Systems Department of Central Research Laboratories EMI,
London, England

In preparing this paper I realised that I would be speaking to a general
audience and have therefore included a description of computed tomography
(CT) and some of my early experiments that led up to the development
of the new technique. I have concluded with an overall picture of the
CT scene and of projected developments in both CT and other types of
systems, such as Nuclear Magnetic Resonance (NMR).

Although it is barely 8 years since the first brain scanner was constructed,
computed tomography is now relatively widely used and has been
extensively demonstrated. At the present time this new system is operating
in some 1000 hospitals throughout the world. The technique has succesfully
overcome many of the limitations which are inherent in conventional X-ray
technology.

When we consider the capabilities of conventional X-ray methods, three
main limitations become obvious. Firstly, it is impossible to display within
the framework of a two-dimensional X-ray picture all the information
contained in the three-dimensional scene under view. Objects situated in
depth, i. e. in the third dimension, superimpose, causing confusion to the
viewer.

Secondly, conventional X-rays cannot distinguish between soft tissues. In
general, a radiogram differentiates only between bone and air, as in the
lungs. Variations in soft tissues such as the liver and pancreas are not
discernible at all and certain other organs may be rendered visible only
through the use of radio-opaque dyes.

Thirdly, when conventional X-ray methods are used, it is not possible to
measure in a quantitative way the separate densities of the individual
substances through which the X-ray has passed. The radiogram records
the mean absorption by all the various tissues which the X-ray has penetrated.
This is of little use for quantitative measurement.

Computed tomography, on the other hand, measures the attenuation of
X-ray beams passing through sections of the body from hundreds of
different angles, and then , from the evidence of these measurements, a
computer is able to reconstruct pictures of the body’s interior.

Pictures are based on the separate examination of a series of contiguous
cross sections, as though we looked at the body separated into a series of
thin “slices”. By doing so, we virtually obtain total three-dimensional information
about the body.

[Fig. 1. CT scan taken through the kidneys]

However, the technique’s most important feature is its enormously
greater sensitivity. It allows soft tissue such as the liver and kidneys to be
clearly differentiated, which radiographs cannot do. An example is shown
in Fig. 1.

It can also very accurately measure the values of X-ray absorption of
tissues, thus enabling the nature of tissue to be studied.

These capabilities are of great benefit in the diagnosis of disease, but CT
additionally plays a role in the field of therapy by accurately locating, for
example, a tumour so indicating the areas of the body to be irradiated and
by monitorig the progress of the treatment afterwards.

It may be of interest if I describe some of the early experiments that led
up to the development of CT.

Some time ago I investigated the possibility that a computer might be
able to reconstruct a picture from sets of very accurate X-ray measurements
taken through the body at a multitude of different angles. Many
hundreds of thousands of measurements would have to be taken, and
reconstructing a picture from them seemed to be a mammoth task as it
appeared at the time that it would require an equal number of many
hundreds of thousands of simultaneous equations to be solved.

When I investigated the advantages over conventional X-ray techniques
however, it became apparent that the conventional methods were not
making full use of all the information the X-rays could give.

On the other hand, calculations showed that the new system used the
data very efficiently and would be two orders of magnitude more sensitive
than conventional X-rays. For this reason I hoped that it would be possible
to distinguish between the various tissues of the body, although I could not
find any literature which suggested that such X-ray absorption differences
existed.

[...]

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