X-Ray Tube: Conventional Radiological Techniques Equipment (PDF)

‫جامعة العين‬ ‫كلية التقنيات الصحية والطبية‬ ‫قسم تقنيات االشعة والسونار‬ conventional Radiological techniques Equipment X-Ray tube Dr. Hussein A.Dakhild Ph.D. Medical Imaging Technology 1 The X-ra...

‫جامعة العين‬ ‫كلية التقنيات الصحية والطبية‬ ‫قسم تقنيات االشعة والسونار‬ conventional Radiological techniques Equipment X-Ray tube Dr. Hussein A.Dakhild Ph.D. Medical Imaging Technology 1 The X-ray tube is a component of the x-ray imaging system rarely seen by radiologic technologists. It is contained in a protective housing and therefore is inaccessible. Its components are considered separately, but it should be clear that there are two primary parts: the cathode and the anode. Each of these is an electrode, and any electronic tube with two electrodes is a diode. An x-ray tube is a special type of diode. The X Ray tube components : Glass envelope Cathode An X-ray tube is a vacuum tube that converts electrical input power into Anode X-rays. Protective housing 2  Glass envelope The glass made of Pyrex: Pyrex able to withstand tremendous heat. Tube maintains a vacuum. Tube window : Segment of glass that is thinner than the other glass envelope 3  Cathode The cathode is the negative side of the x-ray tube; it has two primary parts, a filament and a focusing cup. The filament is a coil of wire similar to that in a kitchen toaster, but it is much smaller. The filament is approximately 2 mm in diameter and 1 or 2 cm long. In the kitchen toaster, an electric current is conducted through the coil, causing it to glow and emit a large quantity of heat. Filaments are usually made of thoriated tungsten. Tungsten provides higher thermionic emissions than other metals. Its melting point is 3410°C; therefore, it is not likely to burn out like the filament of a light bulb. Also, tungsten does not vaporize easily. If it did, the tube would become gassy quickly, and its internal parts would be coated with tungsten. The addition of 1% to 2% thorium to the tungsten filament enhances the efficiency of thermionic emission and prolongs tube life. 4 The filament is embedded in a metal shroud called the focusing cup. Because all of the electrons accelerated from cathode to anode are electrically negative, the electron beam tends to spread out owing to electrostatic repulsion. Some electrons can even miss the anode completely. The focusing cup is negatively charged so that it electrostatically confines the electron beam to a small area of the anode. The effectiveness of the focusing cup is determined by its size and shape, its charge, the filament size and shape, and the position of the filament in the focusing cup. 5 6 7  Anode The anode is the positive side of the x-ray tube. There are two types of anodes, stationary(A) and rotating(B). Stationary anode x-ray tubes are used in dental x-ray imaging systems, some portable imaging systems, and other special-purpose units in which high tube current and power are not required. General-purpose x-ray tubes use the rotating anode because they must be capable of producing high-intensity x-ray beams in a short time. The anode serves three functions in an x-ray tube. The anode is an electrical conductor, radiates heat. The anode also provides mechanical support for the target. 8 9  The anode also must be a good thermal dissipater. When the projectile electrons from the cathode interact with the anode, more than 99% of their kinetic energy is converted into heat. This heat must be dissipated quickly. Copper, molybdenum, and graphite are the most common anode materials. Adequate heat dissipation is the major engineering hurdle in designing higher capacity x-ray tubes. (Most rotating anode x-ray tubes have two filaments mounted in the cathode assemble “side by side,” creating large and small focal spot sizes. Filaments in bi-angle x-ray tubes have to be placed “end to end,” with the small focus filament above the large filament). Element Chemical symbol Atomic number K x-ray energy (kev) Melting Tem© Tungsten W 74 69 3400 Molybdenum Mo 42 19 2600 Rhodium Rh 45 23 3200 10  High-capacity x-ray tubes have molybdenum or graphite layered under the tungsten target. Both molybdenum and graphite have lower mass density than tungsten, making the anode lighter and easier to rotate.  The target is the area of the anode struck by the electrons from the cathode. In stationary anode tubes, the target consists of a tungsten alloy embedded in the copper anode. In rotating anode tubes the entire rotating disc is the target. 11  Heat capacity can be further improved by increasing the speed of anode rotation. Most rotating anodes revolve at 3400 rpm (revolutions per minute). The anodes of high-capacity x-ray tubes rotate at 10,000 rpm.  The rotating anode x-ray tube allows the electron beam to interact with a much larger target area; therefore, the heating of the anode is not confined to one small spot, as in a stationary anode tube. The figure below compares the target areas of the typical stationary anode (4 mm2) and rotating anode (1800 mm2) x-ray tubes with 1-mm focal spots. Thus the rotating anode tube provides nearly 500 times more area to interact with the electron beam than is provided by a stationary anode tube. 12  The focal spot is the area of the target from which x-rays are emitted. Radiology requires small focal spots because the smaller the focal spot, the better the spatial resolution of the image. Unfortunately, as the size of the focal spot decreases, the heating of the target is concentrated onto a smaller area. This is the limiting factor to focal spot size. The effective target area, or effective focal spot size, is the area projected onto the patient and the image receptor. This is the value given when large or small focal spots are identified. When the target angle is made smaller, the effective focal spot size also is made smaller. Diagnostic x-ray tubes have target angles that vary from approximately 5 to 20 degrees. 13 Protective Housing Designed to enclose an X-ray tube and to Provides mechanical Some contain a cooling provide two types of support and prevents fan to air-cool the tube. protection: radiological damage. and electrical. Prevents electric shock They reduce the level to patient and of radiation leakage. exposure. 14 15  heel effect The x-rays that constitute the useful beam emitted toward the anode side must traverse a greater thickness of the target material than the x-rays emitted toward the cathode direction. The intensity of x-rays that are emitted through the “heel” of the target is reduced because they have a longer path through the target and therefore increased absorption. This is the heel effect. The difference in radiation intensity across the useful beam of an x-ray field can vary by as much as 45%. The central ray of the useful beam is the imaginary line generated by the centermost x- ray in the beam. If the radiation intensity along the central ray is designated as 100%, then the intensity on the cathode side may be as high as 120%, and that on the anode side may be as low as 75%. 16  The heel effect is important when one is imaging anatomical structures that differ greatly in thickness or mass density. In general, positioning the cathode side of the x-ray tube over the thicker part of the anatomy provides more uniform radiation exposure to the image receptor. The cathode and anode directions are usually indicated on the protective housing, sometimes near the cable connectors. 17  A B  Posteroanterior chest images demonstrate the heel effect. A, Image taken with the cathode up (superior). B, Image with cathode down (inferior). More uniform radiographic density is obtained with the cathode positioned to the thicker side of the anatomy, as in A. 18 19