MOTION MOUNTAIN

176 4 images and the eye – optics
X-ray
tube
sample
computer
controlled
positioning
X-ray
detector
Vol. V, page 157
F I G U R E 126 A set-up for high-resolution X-ray tomography, and two examples of images produced
with it: a cross-section of a coffee bean (lower left) with a size of 8 mm, and a three-dimensional
reconstruction of the exoskeleton of a foraminiferan, with a diameter of only 0.5 mm (© Manuel Dierick).
beimaged;theradiation thatisscatteredand/ortransmittedisdetected,andwithsophisticated
computer programming, a cross section of the object is reconstructed. Threedimensional
reconstructionsare also possible.Tomography can be performedwith any
typeofradiation thatcan beemitted in sufficientlywell-definedbeams,suchasgamma
rays, X-rays, light, radio waves, electron beams, neutron beams, sound and even earthquakes.X-raytomographyisastandardmethodinhealthcare;
visible light tomography,
which has no side effects on humans, is being developed for breast tumour detection. Additionalspecializedtechniquesareelectricalresistivity
tomography, magnetic induction
tomography and cryo-electrontomography.
Inseveral types of tomography, the resolution achieved is breath-taking.Anexample
for modern high-resolution X-ray tomography of really small objects is shown in Figure126.AnexampleofX-raytomographyofalargeobjectisshowninFigure127.Building
a set-up that produces such images is a large project and an impressive feat. Also
magnetic resonance imaging, widely used in health care to image the interior of the human
body, is a type of tomography, based on radio waves; it will be presented later on
in our journey. Various types of tomographic systems – including opto-acoustic tomography
based on sound produced by pulsed light, positron emission tomography, optical
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net