Laser imaging

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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

	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	The patient is imaged
Memory Store	All the digital image information required for one ‘film’ must be put into the memory store, before the image recording can start • The image is stored in the RAM memory of the unit which has a basic memory capable of storing information to fill one 35 × 43 cm film from each modality at a time • When the memory is full, a film is automatically dispensed (by either gravity or rollers) and is positioned ready for scanning
An Interface	Transmits the digital information from the memory store to the laser
An Acoustic-optical Modulator	Modulates the laser beam in terms of brightness, grey levels, etc.
The Laser	• Lasers produce coherent beams of parallel light and therefore the scanning optics can be much simpler than if white light is used, which produces a divergent beam • Either a red, helium-neon laser, which emits light of 632 nm, or an infrared laser in the range of 750–850 nm, can be used • The information from the memory store is used to modulate the laser beam in terms of brightness, grey scales, etc. and therefore the intensity of the beam will be determined by the digital value of the input signal
An Optical System	A lens system is used to focus the laser beam onto a scanner
A (Mechanical) Scanner	A very accurate and fast Galvano scanner used to move the laser beam from side to side across the imaging field
A Mirror	Horizontal scanning • The beam is focused using an optical lens system onto a moving mirror system which moves the beam from side to side across the film, printing one line of information at a time • Each line is capable of containing between 3500 and 4100 pixels of information
A (Mechanical) Film Transport System	Vertical imaging • After each line has been printed, the film moves vertically down to allow the next line to be printed • Each film will have to move between 4250 and 5100 times before imaging has been completed
Imaging	Takes between 8 and 30 seconds
Film Characteristics	• The film is ‘written on’ by the laser, therefore only a single-sided, fine grain, silver halide emulsion is required • This is mounted on either a blue tinted or a clear polyester base with an antihalation layer to prevent curl
Film Characteristics	Note It is important that the correct film is purchased to match the type of laser used to obtain optimum results • Helium-neon lasers produce red light and therefore the film should have a sensitivity in the order of 633 nm and special green safelight filters may be purchased if required for darkroom film handling • Infrared lasers require film with a sensitivity of between 780 and 850 nm and should be handled in total darkness
Processing	• The film is packaged so that it can be loaded into the laser printer in daylight conditions and, from the printer, it is automatically fed into the automatic processing unit • Or, into a light-proof receiving magazine which is closed and taken to a separate automatic processor
Digital Input Signal	• Can either be transferred directly to the printer from the separate modalities • Or can be stored on computer disk and then taken to the laser imager • Or can be digitally stored and viewed only on a computer It is possible for the imagers to receive input signals at the same time as they are ‘writing’ an image and therefore there is no processing delay

Imaging Plates

Imaging Plate Construction

Fig. 6.8 Imaging plate.

• Protective layer – Thin, transparent film to protect the plate

• Phosphor layer – Barium fluorohalide contained in a binder provides a photostimulable phosphor

• Light reflection layer (may be absent) – Composed of light reflecting particles in a binder

• Conductive layer – Composed of conductive crystals in a binder. Its function is to reduce problems caused by electrostatic charges but in addition it absorbs light and therefore increases image sharpness

• Support (base) – Has a similar structure and function to that of a conventional intensifying screen

• Light shielding layer – Composed of carbon particles in a binder and its function is to prevent light leaking from the backing area

• Backing layer – A soft polymer layer to protect the plate during stacking of a number of imaging plates

• Barcode label – Used to provide a serial number and to identify a particular imaging plate which can then be linked with patient identification details, etc. The code is printed on a piece of paper measuring 25 × 61 mm

Imaging Plate Cassettes

• Protect the imaging plate from damage

• Hinged to allow for inspection and cleaning

• Aluminium back to absorb radiation and prevent back scatter

• Have a window to allow reading of the imaging barcode

Use of the Imaging Plate

Image Erasure	• The imaging plate is exposed to an erasing light to discharge any remaining energy from the previous exposure • The imaging plate is then ready for re-use
Exposure	The imaging plate can either: • Be loaded into the special cassette and used in exactly the same way as a conventional screen-film combination • Or can be used as part of a computed radiography system
Exposure	When the plate is exposed to radiation: • Electrons are excited to higher energy levels in the barium fluorohalide phosphor crystals • The excited electrons are trapped in colour centres (F centres) which are halogen ion vacancies in the crystals
Image Reading	• The cassette and imaging plate are then taken to the image recorder • If an imaging plate has been exposed but has not been read it should only be handled in conventional safelight conditions when the plate is being transferred to a magazine • If a computed radiography system is being used, the plate will be automatically transported into the image recording unit • A helium-neon laser scans the imaging plate and energy from the beam is absorbed by the plate • Therefore the previously trapped electrons are released and recombine, creating the phenomenon of photostimulated luminescence in the region of 400 nm Fig. 6.9 Principles of image reading.
Principle of Photostimulated Luminescence	• The barium fluorohalide crystals have gaps (colour centres or F centres) in the higher energy levels • When the crystal is excited by radiation, electrons receive energy and move from the low energy, valency band to the colour centres in the high energy, conduction band • When the crystal is scanned by the helium-neon laser, energy is absorbed and then is released in the form of photoluminescence • This release of energy causes the electron to drop back to its former position in the valency band • The luminescence is detected by the image detector (a photomultiplier) which converts it to timeseries image signals • These are, in turn, converted into electrical signals, digitised and automatically transferred to a computer or a laser printer where they provide the input signal for the laser beam • After printing, the process is similar to that of the laser imager, the film either passes directly and automatically to a conventional automatic processor or to a receiving magazine and is transported by hand Fig. 6.10 Photostimulated luminescence.

Dry Film Processing

	Technology is available which utilises direct laser printing to produce an image without the use of processing chemicals
Film Characteristics	• The 12 × 24 cm film can be handled in daylight conditions and is either a blue or clear polyesterbased film with a carbon coating • The carbon responds digitally to laser energy over a certain threshold value • Each pixel on the film contains in excess of 4000 sub-pixel elements called pels. The whole film has a scratch resistant ‘overcoat’
Imaging	• The digital signal from the appropriate modality is fed into solid state laser diodes which then scan the film in a way similar to that of other laser printers • As the signal is digital the pels are either exposed or not exposed and because of their very small size and large numbers shades of grey can be built up as a result • The film is ready for viewing in 90 seconds
Imaging System Characteristics	• Resolution of 4000 × 5000 pixels, with 256 grey levels • Transfer of data is in the order of 8–30 seconds • The image can be manipulated and the parameters changed prior to printing • The laser imager is very susceptible to dust and vibration, particularly when the laser beam transfer is via a mirror • Any vertical movement of the film by mechanical means must remain absolutely in register throughout the whole of the exposure. Any interruption of this travel, caused by vibration or mechanical movement, will produce gross artefacts on the image • As the optics for laser imagers are fairly simple there tends to be little image distortion

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