X-ray Tube Equipment (RAD 232) PDF - Elsevier 2025

X-ray Imaging Equipment RAD 232 [The X-ray tube] V2025 X-ray tube ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 1 General tube construction I. Housing The X-ray tube is situated in a protective housing....

X-ray Imaging Equipment RAD 232 [The X-ray tube] V2025 X-ray tube ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 1 General tube construction I. Housing The X-ray tube is situated in a protective housing. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 2 General tube construction I. Housing The housing is a lead-lined metal structure that: 1. serves as an electrical insulator and thermal cushion. 2. incorporates an oil bath and cooling fans to help dissipate heat away from the tube. 3. absorbs unwanted photons, which is called leakage radiation. 4. provides mechanical support to the tube ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 3 General tube construction II. X-ray tube ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 4 General tube construction II. X-ray tube General-purpose X-ray tube is an electronic vacuum tube that consists of an anode, a cathode, and an induction motor all encased in a glass or metal enclosure (envelope). The main purpose of the enclosure is to maintain a vacuum ----------------------------------------------------------------------------------------------- within the tube. ----------------------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------- ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 5 General tube construction II. X-ray tube The glass envelope variety is generally made of borosilicate glass, because it is very heat resistant. As these tubes age, vaporized tungsten from the filament deposits on the inside of the glass called “sun tanning”. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 6 General tube construction II. X-ray tube The metal envelope variety avoids the arcing problem and extends tube life. ---------------------------------------------------- ---------------------------------------------------- ---------------------------------------------------- ---------------------------------------------------- -------------------------------------------------- ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 7 General tube construction II. X-ray tube Glass and metal envelopes have a specially designed target window (the port in the figure) for the desired exit point of the X-rays produced. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 8 General tube construction II. X-ray tube Target window is a place on the enclosure that has been made thinner than the rest. ------------------------------------------------------------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------------------------------------------------------------ It is usually about 5 cm2. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 9 General tube construction III. Anode It is the target for electron interaction to produce X-rays. It is an electrical and thermal conductor. Some electrons interact with the target to produce X-rays and the rest continue as current flows through the X-ray circuit. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 10 General tube construction III. Anode A tremendous amount of heat is also generated during the production of X-rays. ---------------------------------------------------- ---------------------------------------------------- The anode is designed to dissipate heat. ---------------------------------------------------- ---------------------------------------------------- -------------------------------------------------- There are two designs for the anode: 1. Stationary anode 2. Rotating-anode ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 11 General tube construction III. Anode (stationary anodes) Stationary anodes were used in old tube designs. A tungsten button embedded in a copper rod. Tungsten (W, Z = 74) is a good heat conductor to dissipate heat and has a high fusion temperature (~3400°C). ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 12 General tube construction III. Anode (stationary anodes) The primary disadvantage of stationary anodes is that heat builds up rapidly and can damage the tube. ---------------------------------------------------- ---------------------------------------------------- ---------------------------------------------------- ---------------------------------------------------- -------------------------------------------------- This problem limits the exposure factors that can be used. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 13 General tube construction III. Anode (stationary anodes) Its main advantages: Compact Unit Less cost Stationary anodes can be found in dental x-ray sets, small portable and mobile x-ray units with limited output. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 14 General tube construction III. Anode (rotating-anode) Rotating-anode design is used in general- purpose tubes. It consists of a rotating disc made of molybdenum as a core material coated with tungsten and mounted on a copper shaft with a molybdenum core. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 15 General tube construction III. Anode (rotating-anode) Copper is used as part of the shaft, because it has excellent thermal and electrical conductive properties. Molybdenum is used as the disc base and core because it has a low thermal conductivity, and it is a light but strong alloy, making it easier to rotate the anode. The target material (coating) is made of tungsten. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 16 ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 17 General tube construction III. Anode (rotating-anode) The anode is rotated using an induction motor. Using an induction motor allows for the rotation of the anode in a vacuum without engineering a motor into the vacuum. These motors can rotate the anode at speeds of 3400 revolutions per minute (rpm) for general-purpose tubes. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 18 General tube construction III. Anode (rotating-anode) The purpose of rotating the anode is to spread the heat over a larger surface area. In rotating-anode, the electrons strike a small part of the total anode surface area at any one time and that area changes. The focal “spot” becomes a focal “track”. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 19 General tube construction III. Anode (line-focus principle) The face of the target is angled By angling the face of the anode target, a large actual focal spot size can be maintained, and a small effective focal spot size can be created. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 20 General tube construction III. Anode The actual focal spot is the area being bombarded by the filament electrons. The size of the electron stream depends on the size of the filament. The smaller this stream, the greater the heat generated in a small area. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 21 General tube construction III. Anode The effective focal spot is the origin of the X-ray beam and is the area as seen from the patient’s perspective. The smaller this area of origin, the sharper the image will be. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 22 General tube construction III. Anode When the angle of the target face is less than 45 degrees, the effective focal spot will be smaller than the actual focal spot. The target angles are 7 to 18 degrees for a general-purpose tube, with 12 degrees being the most common. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 23 General tube construction III. Anode The smaller the anode angle, the smaller the effective focal spot will be while maintaining a large actual focal spot area. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 24 General tube construction III. Anode Although the line-focus principle achieves this goal of balance between heat area and projected focal spot, it is not without tradeoffs. When the target angle becomes too small, the X-ray beam area may not be large enough to fully expose a 35*43 cm image receptor at a 100 cm SID. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 25 General tube construction III. Anode Another disadvantage of the line- focus principle is that the angle causes the intensity of the X-ray beam to be less on the anode side because the “heel” of the target is in the path of the beam. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 26 General tube construction III. Anode This phenomenon is called the anode heel effect. This lowering of intensity on the anode side of the beam can cause the image to be “lighter” on that end. This is less of an issue with digital technology, because these systems can record and display many shades of gray ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 27 General tube construction IV. Cathode Source of electrons needed for x-ray production. The cathode is made up of the filaments and the focusing cup. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 28 General tube construction IV. Cathode Most general-purpose tubes have two filaments and are referred to as dual-focus tubes. Each filament is a coil of wire usually 7 to 15 mm long and 1 to 2 mm wide. They are usually made of tungsten with 1% to 2% thorium added. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 29 X-ray Production Process Recall the principles of X-ray circuit operation (2nd lecture handout slides 43-48): We had a group, or cloud, of electrons created by the filament circuit (heating of the filament). The kilovoltage applied to the x-ray tube created a large positive charge on the anode and a large negative charge on the cathode (focusing cup). 30 Copyright © 2020 by Elsevier Inc. X-ray Production Process The large positive charge attracts the electrons boiled off the filament, giving them tremendous kinetic energy in the process. The large negative charge on the cathode keeps the electrons crowded together; otherwise, they would repel each other and scatter throughout the tube. The electrons travel across to the anode and interact there to produce x-rays until the timer circuit terminates the process. 31 Copyright © 2020 by Elsevier Inc. Quality control and extending tube life Several factors can shorten the life of an X-ray tube or damage it. Most have to do with the thermal characteristics of X-ray production. Most of the factors that can shorten X-ray tube life are within the radiographer’s control: Frequent use of very high or maximum exposure factors Use of lower but very long exposure factors Overloading the filament ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 32 Extending tube life Three processes of heat transfer are at play: Conduction of heat by heat-tolerant materials Radiation of heat energy from anode to oil bath Convection of heat into the room by cooling fans Protective circuits prevent the use of unsafe exposure techniques and heat overloads. Even with safety measures, the radiographer must understand anode thermal capacity and keep in mind that X-ray production is an inefficient process. 33 Copyright © 2020 by Elsevier Inc. Extending tube life There are many things that the professional radiographer can do to extend the life of an x-ray tube. Holding the rotor switch unnecessarily should be avoided. It increases the filament’s thermionic emission to exposure levels. Thermionic emission removes the electrons from the filament, deposits vaporized electrons on tube surfaces, and decreases the tube vacuum, all of which can cause tube failure. It causes stress to the rotor bearings. To avoid these problems, switches should be completely depressed in one motion. 34 Copyright © 2020 by Elsevier Inc. Extending tube life 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. Repeated exposures near tube loading limits should not be made, as total heat units may approach anode or housing loading limits. 35 Copyright © 2020 by Elsevier Inc. Extending tube life 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 tube should not be used when loud rotor bearings can be heard (unless it has been checked by a qualified service person) because a wobbling anode disk can cause tube failure. 36 Copyright © 2020 by Elsevier Inc. ©Elsevier: Essentials of Radiographic Physics and Imaging 3rd 37