New trends in physics teaching, v.4; The ... - unesdoc - Unesco

Optics regained
is not taken, and one must always remember the dictum that, when we focus, we make the object
look as we think it ought to look - which is not necessarily as it really looks.
Now let us turn for a few moments to medical imaging. The developments since the early days
of the simple ‘shadow’ radiograph are enormous. I shall pick just three to illustrate the point. The
first is the use of ultrasonics. The advantage is that the waves do not themselves cause any tissue
damage and so can be used safely, for example, during pregnancy. The object is scanned with
a beam of ultrasonic radiation and the scattered radiation is detected with a suitable transducer
and an image is built up on a television picture tube. The various boundaries between bone, soft
tissue, fluids, etc. show up clearly and, for example, it is possible.to see the outline of many
organs within the body. The detection of multiple pregnancies is particularly easy as the foetal
skulls can be recognized clearly within the amniotic fluid.
Infra-red imaging - or thermography as it is often called - provides an example of an imaging
system in which the object is self luminous. The human body radiates in the infra-red region and
the nature and intensity of the radiation depends largely on the surface temperature. It is well
known that changes in the surface temperature of the body may be symptoms of abnormalities
such as, for example, malignant growths. Infra-red scanning devices which lead to the production
of a map of isotherms for the body, or part of it, can therefore help in the diagnosis of various
disorders.
One of the most recent and perhaps most revolutionary imaging systems is the computer
tomograph. A series of X-ray beams is used to produce radiographs through a particular section
of the body in a considerable number of directions. The resulting pictures are stored in digital
form and a computer program processes the information in such a way that it is re-assembled to
give a sectional view of the part of the body being considered. The effect is just as though the
body had been sawn through and then the section photographed. The results are not only dramatic
but extremely valuable as a diagnostic tool. A much simplified demonstration of the principle of
the tomographic imaging system is given in Images [ 71 .
Finally, we shall consider just two of the problems of the transmission of images back from
space probes as examples of an application of image-processing. The first is relatively straightforward
and involves removing the raster from television images so that they are more acceptable
as pictures. The image can be thought of as the product of two intensity distribution functions.
The first is the picture required; the second is the function representing the stripes. If a transparency
made from the television image is placed in an optical diffraction apparatus (i.e. we
explore the first stage of the formation of an image of it in coherent light), its diffraction pattern
wil be related to the diffraction patterns of the two functions. The diffraction pattern of the
‘stripe function’ is merely that of a diffraction grating - i.e. a set of regularly spaced points on
either side of the central beam. Fourier transform theory tells us that the multiplication of two
functions in one of the related Fourier planes corresponds to convolution in the other. In the
diffraction pattern of the product, therefore, the pattern corresponding to the picture required
is convoluted with (i.e. distributed to each point of) the set of diffraction grating peaks. All
information about the picture therefore exists at each of these diffraction peaks. So, if we
filter by eliminating all but one peak, and then recombine the image, the stripe information has
been eliminated and we have an unstriped picture (see figure 11). This is an example of what one
might call ‘cosmetic’ filtering. Clearly no information can be revealed about any detail of the
picture that happens to be in a ‘dark’ stripe. However, if the object being imaged is static and the
raster is made to move about relative to it, the resulting pictures can be integrated into a more
useful whole. In considering whether a filtered image is likely to reveal additional detail, rather
than merely looking more attractive, it is important to consider the operation from an information
standpoint. If the process enables us to transform more information, then the chances are the
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