Chapter 6 The X-ray Tube The electrical production of x-rays is only possible under very special conditions including a source of electrons, an appropriate target material, a high voltage, and a vacuum. The x-ray tube is the device that permits these conditions to exist. The tube consists of a cathode and an anode enclosed within an envelope and then encased in a protective housing. The cathode is the negative side of the x-ray tube. The function of the cathode is to: produce a thermionic cloud conduct the high voltage to the gap between cathode and anode and focus the electron stream as it heads for the anode The cathode assembly consists of the filaments, focusing cup, and associated wiring. The filament The filament is a small coil of thin thoriated tungsten wire set in the cathode assembly within the focusing cup. Tungsten is the material of choice because of its high melting point (3,370*) and because it is difficult to vaporize. The high melting point permits the filament to operate at the high temperatures requited of an x-ray tube. Vaporization produces particles that deposit on other surfaces and reduce the vacuum within the tube. The length and width of the filament have a great effect on the ability of the particular xray tube to image details. Most diagnostic x-ray tubes have dual filaments called a dual focus arrangement. The function of the filament is to provide sufficient resistance to the flow of electrons so that the heat produced will cause thermionic emission to occur. Thermionic emission causes electrons to leave the surface of the filament wire and form a thermionic cloud. Then the high voltage is released at exposure the entire cloud is available to be driven toward the anode target where x-ray photons will be produced. Vaporized tungsten deposited on the inner surface of the glass envelope causes a mirrored appearance and eventually high voltage arcing when sufficient current is attracted to a deposit during an exposure. Arcing immediately destroys the tube. Evaporization deposits on the glass envelope also cause increased filtration of the primary beam and this decreased tube efficiency. (grounded metal envelopes reduce this) Another major cause of tube failure is the breaking of the filament. Filaments become increasingly thin as vaporization (10% subject to breakage) continues. When the x-ray machine is first turned on, a mild current is sent to the filament. The filament remains in this preheated mode until immediately prior to an exposure. The activation of the rotor switch causes the rotor to turn and a higher current is sent to the filament to bring the thermionic cloud to the proper size for the mA selected. This increase in filament heating is what causes most of the vaporization of the filament. (Rotor switch = filament heat) Filament life is 6 – 9 hours (10,000 – 20,000 exposures) All units have electronic interlocks that will not permit the exposure to occur until the rotor has brought the anode up to the proper speed. The focusing cup The focusing cup is a shallow depression in the cathode assembly designed to house the filament. It is made of nickel and its purpose is to marrow the thermionic cloud as it is driven toward the anode. The focusing cup is provided with a low negative potential, which focuses the electrons toward one another in a convergence pattern. A biased focusing cup is used to decrease the size of the focal spot. It maintains the cup at a more negative voltage than the filament. This causes the exiting electron beam to be focused into a narrower stream as it heads toward the anode. As more and more electrons build up in the area of the filament, their negative charges begin to oppose the emission of additional electrons, called the space charge effect, it limits x-ray tubes to maximum mA ranges of 1,000-1,200. The filament saturation current has been achieved when there are no further thermionic electrons to be driven toward the anode. An increase in kVp will not increase the tube mA- must increase the filament mA. The addition of a more negative potential difference (approximately 2000 volts) at the focusing cup causes the cup to attract the thermionic cloud. The anode The anode is the positive side of the x-ray tube and has three functions: it serves as a target surface for the high voltage electrons from the filament, thereby becoming the source of the x-ray photons it conducts the high voltage from the cathode back into the x-ray generator circuitry and it serves as the primary thermal conductor. The anode target surface is where the high-speed electrons from the filament are suddenly stopped, resulting in the production of stray photons. The entire anode is a complex device referred to as the anode assembly. This assembly consists of the anode, stator and rotor and serves as the path for the high voltage flow during exposure. Rotating anodes turn during the exposure, presenting a much larger target area. The faster the anode rotates, the better the heat is dissipated. Nearly all units designed for diagnostic radiography utilize rotating anodes because of their greater efficiency. Stationary anodes are composed of rhenium-alloyed tungsten (rhenium has greater elasticity) imbedded in a 45* angled end of a copper rod. The anode’s function as the source of x-ray photons as the primary thermal conducting device is enhanced by the use of rhenium-alloyed tungsten as the target focal track material. Tungsten is the metal of choice for the source of x-ray photons for the following reasons: High atomic number: Tungsten’s atomic number (74) enhances the production of diagnostic range photons. High melting point (1,000 – 2,000* C during normal use) Heat conducting ability Mammography units use molybdenum as target material due to ability to emit a more uniform range of lower energy photons. The portion of the anode where the high voltage electron steam will impact is called the target, focus, focal point, focal spot, or the focal track. The target is considered to be a point source of x-ray photons and it is from this point that all tube to object and image receptor distances are measured. The focal track is used to represent the circular path that will be impacted by the electron beam. The terms target, focus, focal point, and focal spot refer to the area of the focal track that is impacted by the electron beam at one time. The actual focal spot is used to describe the physical area of the focal track that is impacted. The effective focal spot is used to describe the area of the focal spot that is projected out of the tube toward the object being radiographed. The line focus principle is used to reduce the effective area of the focal spot. This permits the best resolution of detail while permitting as large an actual area as possible to increase thermal conductivity. The effective focal spot is controlled by the size of the actual focal spot (which is controlled by the length of the filament) and the anode target angle. When the target angle is less than 45*, the effective focal spot is smaller than the actual focal spot. When the angle is decreased, smaller focal spots can be achieved. The effective focal spots vertical dimension is the one that is stated as the focal spot size. The anode heel effect is caused by the geometry of an angled anode target and results in the radiation intensity being greater on the cathode side. The induction motor electromagnets comprise the stator that turns the anode. The anode is composed of a hollow copper cylinder or cuff that is attached to the anode disk by a molybdenum shaft. Common rotating rotors revolve at 3,200 – 3,600 rpm. High-speed rotating anodes that operate at 10,000 – 12,000 rpm are also available to assist in dissipating heat. Any rough or extremely fast movement of the housing is not healthy for an x-ray tube and should be avoided. Ball bearings eventually become imperfectly round. This leads to a grinding noise and wobbling throws the focal track off center and tube efficiency drops dramatically. Superheated melted tungsten that drips onto the envelope will destroy the tube. Metal envelopes are increasingly becoming more common. They prolong tube life because they eliminate the problem of tungsten vaporization. The envelope must be sealed tight to maintain a high vacuum. At the point where the primary x-ray beam exits the envelope a window segment is constructed. The removal of the air permits electrons to flow from cathode to anode without encountering the gas atoms of air. The housing controls leakage and scatter radiation, isolates the high voltages and provides a means to cool the tube. When x-ray photons are produced at the anode, they are emitted isotropically. The primary beam consists of photons emitted through the window. The protective housing is composed of cast steel and is capable of absorbing most of the unwanted photons. Any photons that escape from the housing except at the port are leakage radiation. Leakage radiation must not exceed 100mR/hr at 1 meter. The housing also serves to cushion the x-ray tube from rough handling by operators. The oil insulates the high voltage components from the tube housing, which is handled by the radiographer, and absorbs much of the heat that is produced by x-ray production. Under no circumstances should anyone be in contact with a tube housing during exposure. Off-focus, or extrafocal radiation is composed of photons that were not produced at the focal spot and may contribute as much as 25 – 30% of the total primary beam. Tube rating charts are the most valuable because they provide a guide regarding the maximum technical factor combinations that can be used without overloading the tube. Any combination of factors at or under the curve is safe. Each filament of each tube has a unique radiographic tube-rating chart. Anode cooling charts permit the calculation of the time necessary for the anode to cool enough for additional exposures to be made. A heat unit is calculated as kVp x mA x rectification constant. To find the length of time for the anode to cool can be calculated by: Find the total heat units applied on the vertical scale. Read from the heat units over to the cooling curve and then down to read the corresponding time. Calculate the time necessary for the anode to cool to any desired level and subtract the corresponding time of the initial exposure. The anode will usually reach its limits ling before the housing. Warming up the anode according to the manufacturers recommendations prevents thermal shock (cracking of the anode). Holding the rotor switch unnecessarily should be avoided. The rotor switch increases the filaments thermionic emission to exposure levels. Thermionic emission removes the electrons from the filament; deposits vaporized electrons in tube surfaces and decrease the vacuum. The rotor causes stress to the rotor bearings, double press switches should be completely depressed in one motion and dual switched should have the exposure switch depressed first, followed by the rotor switch. Lower mA stations should be used when possible because high mA increases filament thermionic emission. The lower speed rotor should be used when possible because the high-speed rotor increases rotor-bearing wear. Rotating the tube housing rapidly from one position to another should be avoided because the gyroscopic effect may crack or otherwise damage the rotor. A wobbling anode disk can cause tube failure.