Laser imaging

Published on 12/06/2015 by admin

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Last modified 12/06/2015

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6 Laser imaging

Light and the Laser	White light • Made up of varying wavelengths • Ranges from ultraviolet in the short wave, through visible light, to infrared in the long wavelengths • Very short coherence lengths, due to the number of different wavelengths and the anti-phase components cancelling one another out Fig. 6.1 White light – a cacophony in vision.
Light and the Laser	Laser light • Only one wavelength Fig. 6.2 Laser light – coherent light of only one wavelength.
The First Laser	Pulsed laser In 1960, Theodore Maiman, working at the Hughes Aircraft Electronic Research Laboratory, built the first laser • It was basically a photographer’s electronic flashgun wound around a crystal of synthetic ruby (aluminium oxide mixed with a small amount of chromium) • The synthetic ruby was in the form of a tube with a mirror at each end of the tube • One mirror was a partial reflector • The distance between the mirrors was precisely defined • It was tuned to make the light produced bounce back and forth in a regular and reinforcing pattern to create standing waves of light in the rod which had a pulse of about three thousandths of a second Fig. 6.3 Maiman’s original synthetic ruby laser.

The Operation of a Laser

The Basic Physics	• If a photon is fired at an atom, the atom can absorb the photon and thus be raised to an excited state • The only way that the atom can return to stability is by releasing this absorbed energy in the form of a photon of light (spontaneous emission) • But if a photon is fired at an already excited atom, the atom will release TWO light photons and then return to its stable state (stimulated emission) (Fig. 6.4)
Inside a Laser	• The atoms are stimulated by an external power source to produce stimulated emission • When a stage is reached, whereby half the atoms are in an excited state, population inversion occurs (Fig. 6.5) • If more photons are then fired, stimulated emission takes place. Stimulated emission occurs because it is more likely that the photons will strike an already excited atom rather than a stable atom • If the above events occur in a tube, the tube can be ‘tuned’ to encourage the light produced to oscillate in a regular pattern • The light then breaks out as an extremely coherent, parallel beam of light which contains a considerable amount of energy – laser light
Continuous Wave Gas Laser	• Uses helium and neon as the lasing material • The light is very coherent and uniform and can be switched off and on like an electric light • They are used in laser imagers Fig. 6.6 Continuous wave gas laser.

Fig. 6.4 Stimulated emission.

Fig. 6.5 Population inversion.

The Original Laser Imagers

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	Fig. 6.7 Principles of a laser imager.
Operator Keypad	This allows control of the parameters for imaging at the time of the examination These can include: • Film size • Format, i.e. the number of images recorded on the film • Image-store sequence • Density • Contrast • Exposure • Image polarity • Clear or black border background • Number of copies required • Patient information • Automatic hospital name, date recording • Printing of the image
Exposure

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