Cathode-Ray Tube Displays for Medical Imaging

DIGITAL IMAGING BASICS Cathode-Ray Tube Displays for Medical Imaging Peter A. Keller This paper will discuss the principles of cathode-ray crease the velocity of the electron beam for tube displays in medical imaging and the parameters increased light output from the screen; essential to the selection of displays for specific 4. a focusing section to bring the electron requirements. A discussion of cathode-ray tube fun- beam to a sharp focus at the screen; damentals and medical requirements is included. 9 1990bu W.B. Saunders Company. 5. a deflection system to position the electron beam to a desired location on the screen or KEY WORDS: displays, cathode ray tube, medical scan the beam in a repetitive pattern; and irnaging, high resolution. 6. a phosphor screen to convert the invisible electron beam to visible light. he cathode-ray tube (CRT) is the heart of The assembly of electrodes or elements mounted T almost every medical display and its single within the neck of the CRT is commonly known most costly component. Brightness, resolution, as the "electron gun" (Fig 2). This is a good color, contrast, life, cost, and viewer comfort are analogy, because it is the function of the electron gun to "shoot" a beam of electrons toward the all strongly influenced by the selection of a screen or target. The velocity of the electron particular CRT by the display designer. These beam is a function of the overall accelerating factors are especially important for displays used voltage applied to the tube. For a CRT operating for medical diagnosis in which patient safety and at an accelerating voltage of 20,000 V, the comfort hinge on the ability of the display to electron velocity at the screen is about present easily readable, high-resolution images 250,000,000 mph, or about 37% of the velocity of accurately and rapidly. light. Although the velocity of the electrons is The CRT dates back to 1897 in its present extremely high, their mass is very small, and form. In that year, Karl Ferdinand Braun, a normally, asa result, the phosphor screen lumi- German physicist and Nobel-prize recipient, dem- nesces where it is struck by the beam. However, onstrated a tube intended for the measurement of if the beam power (accelerating voltage times electrical waveforms. It was not until 1929 that beato current) is sufficiently high, intense local- the CRT was applied to the display of actual ized heating may occur, with a resultant phos- images for television by Vladimir K. Zworykin of phor burn or glass damage. Westinghouse Electric. Subsequently, television Each function will now be discussed briefly. provided much of the impetus for further refine- ments of the CRT, with imaging applications PHYSICAL STRUCTURE finally emerging rapidly during the 1980s with The glass envelope or bulb serves several increased performance demands placed on the purposes. These include maintaining the very CRT and associated electronics. Today, CRT high vacuum required to allow free movement of displays are used in almost every business, and the electron beam without colliding with residual much of society is dependent on them for low- gas atoms, providing electrical connection to the cost entertainment. electron beam-forming and -accelerating elec- The principles and operation of the CRT may trodes, and Ÿ these voltages, which may be divided into six functional groups (Fig 1). be as high as 30,000 V, from each other. The These are shape of the envelope required to meet the CRT 1. a mechanical structure known as the envelope to maintain a vacuum, provide electrical connections to the internal electrodes, From Tektronix, Inc, Beaverton, OR. and insulate them from each other; Address reprint requests to Peter A. Keller, Tektronix, lnc, Tektronix Industrial Park, PO Box 500, Beaverton, OR 2. the electron source and beam-intensity con- 97077. trol section; 9 1990 by W.B. Saunders Company. 3. one or more acceleration electrodes to in- 0897-1889/90/0301-0005503.00/0 Journa/of Digita//maging, Vol 3, No 1 (February), 1990: pp 15-25 15 16 PETER A. KELLER DEFLECTION NECK FOCUS Y~KE BASE ELECTROOE PHOSPHOR ~1 I H-J ~SCREEN ,,e-FACEPLATE ELECTRON '~ SOURCE ACCELERATORS ENVELOPE Fig 1. Basic CRT construction. performance objectives largely dictatr the physical configuration of the entire display. The envelope is divided into three distinct parts: the neck, which contains the electron gun and the Fig 3. Heater cathode assembly. base for making connections to the gun; the funnel, which literally contains nothing except an other electrodes. The oxide-coating purity, tem- electrically conductive coating to accelerate the peratures, and processing are critical to maintain electron beam; and the faceplate, which contains high emission and long cathode life. the luminescent phosphor screen. A second cup that surrounds the cathode cup is Glass has been the customary material for the control grid. The grid has a small aperture in CRT bulbs, the only exceptions being metal or front of the cathode coating that normally per- ceramic funnels for specific applications. Glass mits the electrons to flow through on their way to has the advantages of transparency, good insulat- the screen, if it is at the same voltage as the ing properties, strength under the compression of cathode. By applying a negative voltage to the the high vacuum, ah easy-to-clean, smooth, con- grid, the number of electrons is decreased until at tinuous surface to maintain the high vacuum a sufficiently high voltage, usually around 60 V, a integrity, and its separate pieces being easy to point is reached known as "cutoff," at which all seal together. electrons are repelled back to the cathode. By making the relationship of the grid-to-cathode ELECTRON SOURCE voltages synchronous to the scanning of the The source of the electrons to forma beam is electron beam across the screen, the brightness of the cathode (Fig 3). The cathode consists of a any point on the screen may be controllr to metal cup containing a small filament or heater create a picture with many shades of gray or to increase the temperature of an oxide-cathode "gray scale." The number of gray levels that may coating on the end of the cup to a sufficient be produced is primarily determined by the temperature for the emission of electrons, usually associated electronic circuits used to drive the around 800~ Electrons, which are negatively CRT. charged, are drawn away from the cathode and towards the screen by positive voltages on the ACCELERATION To produce a bright display, the electron beam must be accelerated to a high velocity before impinging on the phosphor screen. This is accom- plished by a two-step process. Another cup-shaped electrode with a central aperture, variously known as anode no. 1, grid no. 2 or G2, immediately follows the control grid. This electrode has a positive voltage of several hundred volts and forms a controlled field to pull CATHODE GRID GRID #2 FOCUS ACCELERATOR electrons from the cathode and to provide initial Fig 2. Electron gun proflle. acceleration prior to focusing the beam. DIGITAL IMAGING BASlCS 17 The final accelerating voltage of typically DEFLECTION 15,000 to 25,000 V positive is applied to the We have seen how the CRT produces a pencil- entire funnel and screen as well as to a cylindrical like beam of electrons. Without some means of electrode at the end of the electron gun. The positioning the beato to desired locations on the entire region is called the anode. Contact is made screen, or scanning it past all areas of the screen, through a metal connector sealed into the glass there would be merely a bright spot of light in the funnel and called the "anode button." The inte- center of the screen. Either electrostatic or mag- rior of the funnel is coated with a conductive netic fields may be used to bend the electron carbon paint called "dag" to form the major beam toward other locations on the screen. To- portion of the second anode. It connects with the day television, computer, and imaging displays screen, usually backed with conductive alumi- use magnetic deflection because ir produces num, and extends into the neck to make contact bright, sharply-focused displays over large screen with the end of the gun through "snubber areas with a minimum tube length. Magnetic springs." These provide mechanical centering of deflection apparatus consists of two sets of deflec- the gun within the neck as well as electrical tion coils, called a deflection yoke, over the CRT contact. neck at its juncture with the funnel. Each set of The high voltage applied to the anode is coils is mounted at right angles to the other to related roughly to screen size. For a given beam produce vertical and horizontal deflection when current (number of electrons), a tube having a energized. By applying a fast sawtooth current larger screen will spread the available electrons (Fig 4) to the horizontal coil, the spot is rapidly "thinner" by scanning the larger area. Conse- scanned across the screen to form a thin horizon- quently, the brightness of the image will be lower tal line repetitively. The horizontal sweep rate for a large screen than for a small one. This may be anywhere between 15,750 times per brightness may be gained back by the use of a second for television to 150,000 times per second higher accelerating voltage which increases the for ultra-high reso|ution imaging displays. By beam power to the screen. A good rule of thumb applying a considerably slower sawtooth current is 1,000 V/in of screen diagonal.

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