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This allows a line image of a waveform or picture to be built up and retained on the face of a special cathode ray tube. Such devices are found in the electronics laboratory for examining the electrical signals found inside electronic apparatus, and they are also found in medical imaging apparatus such as in nuclear medicine and ultrasonic apparatus.
Lines drawn on the screen by the electron gun are retained as an electrostatic image on a mesh just behind the screen, and this can be retrieved by illuminating the mesh with a low- velocity electron beam from a flood gun. The phosphor layer on the CRT screen is activated in proportion to the electrostatic charge pattern retained on the mesh. New methods of retaining a stored image (such as digital storage mechanisms) are sought because it is difficult to obtain an image which has varying shades of brightness using the bi-stable CRT mechanism. This has particular disadvantages for ultrasonic B-scanners since the echo amplitude contains an important part of the information required for diagnosis. Bi-stable CRTs have been used for many years in ultrasound machines but have now been supplanted by newer techniques which can provide a true grey-scale image of the echo pattern.
Content and Design Copyright 2000 Dr. Malcolm C Brown. See Title Page for more details