X-Ray Tube Lecture Notes PDF

X-Ray tube Dr. Nahla Atallah  TABLE OF CONTENTS 1. X-Ray tube  Objective:  To familiarize the student with the X-Ray tube and how it works  X-Ray Tube THE X-RAY tube is a component of the x- ray imaging system rarely seen by radiologic technologists...

X-Ray tube Dr. Nahla Atallah  TABLE OF CONTENTS 1. X-Ray tube  Objective:  To familiarize the student with the X-Ray tube and how it works  X-Ray Tube 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. Figure 6-1 is a schematic diagram of a rotating anode diagnostic x-ray tube.  EXTERNAL COMPONENTS The x-ray tube and housing assembly are quite heavy; therefore, they require a support mechanism so the radiologic technologist can position them. Figure 6-2 illustrates two of the three main methods o x-ray tube support.  EXTERNAL COMPONENTS The x-ray tube and housing assembly are quite heavy; therefore, they require a support mechanism so the radiologic technologist can position them. Figure 6-2 illustrates two of the three main methods o x-ray tube support.  CEILING SUPPORT SYSTEM The ceiling support system is probably the most frequently used. It consists of two perpendicular sets of ceiling-mounted rails. This allows for both longitudinal and transverse travel of the x-ray tube.  FLOOR-TO-CEILING SUPPORT SYSTEM  The floor-to-ceiling support system has a single column with rollers at each end, one attached to a ceiling-mounted rail and the other attached to a floor- mounted rail.  The x-ray tube slides up and down the column as the column rotates.  A variation of this type of support system has the column positioned on a single floor support system with one or two floor-mounted rails.  C-ARM SUPPORT SYS TEM  Interventional radiology suites o ten are equipped with C-arm support systems, so called because the system is shaped like a C.  These systems are ceiling mounted and provide for very flexible x-ray tube positioning.  The image receptor is attached to the other end of the C-arm from the x-ray tube. Variations called L-arm or U-arm support are also common.  Protective Housing  When x-rays are produced, they are emitted isotropically, that is, with equal intensity in all directions.  We use only x-rays emitted through the special section of the x-ray tube called the window (Figure 6-3).  The x-rays emitted through the window are called the useful beam X-rays that escape through the protective housing are called leakage radiation; they contribute nothing in the way of diagnostic information and result in unnecessary exposure of the patient and the radiologic technologist. Properly designed protective housing reduces the level of leakage radiation to less than 1 mGya/h at 1 m when operated at maximum conditions. The protective housing also provides mechanical support for the x-ray tube and protects the tube from damage caused by rough handling The protective housing around some x-ray tubes contains oil that serves as both an insulator against electric shock and as a thermal cushion to dissipate heat.  Some protective housings have a cooling fan to air cool the tube or the oil in which the x-ray tube is immersed.  Glass or Metal Enclosure An x-ray tube is an electronic vacuum tube with components contained within a glass or metal enclosure. The glass enclosure is made of Pyrex glass to enable it to withstand the tremendous heat generated. The enclosure maintains a vacuum inside the tube. This vacuum allows for more efficient x-ray production and a longer tube life. When just a little gas is in the enclosure, the electron flow from cathode to anode is reduced, fewer x-rays are produced, and more heat is generated.  The modern x-ray tube, the Coolidge tube, is a vacuum tube. If it becomes gassy, x-ray production falls, and the tube can fail.  Metal enclosure tubes maintain a constant electric potential between the electrons of the tube current and the enclosure.  Therefore, they have a longer life and are less likely to fail. Virtually all high-capacity x-ray tubes now use metal enclosures. The x-ray tube window is an area of the glass or metal enclosure, approximately 5 cm2, that is thin and through which the useful beam of x-rays is emitted. Such a window allows maximum emission of x-rays with minimum absorption.  INTERNAL COMPONENTS Cathode Figure 6-4 shows a photograph of a dual- filament cathode and a schematic drawing of its electric supply. The two filaments supply separate electron beams to produce two focal spots.  The filament  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. An x-ray tube filament emits electrons when it is heated. When the current through the filament is sufficiently high, the outer-shell electrons of the filament atoms are “boiled of ” and ejected from the f lament. This phenomenon is known as thermionic emission. Filaments are usually made of thoriated tungsten. Tungsten provides for higher thermionic emission than other metals. The filament is embedded in a metal shroud called the focusing cup (Figure 6-5). Because all of the electrons accelerated from cathode to anode are electrically negative, the electron beam tends to spread out owing to electrostatic repulsion. Review of Radiation Oncology Physics: A Handbook for Teachers and Students - 1. The focusing cup is negatively charged so that it electrostatically confines the electron beam to a small area of the anode (Figure 6- 6). 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. Most rotating anode x-ray tubes have two filaments mounted in the cathode assemble “side by side,” creating large and small focal spot sizes When the x-ray imaging system is first turned on, a low current passes through the filament to warm it and prepare it for the thermal jolt necessary for x-ray production. At low filament current, there is no tube current because the filament does not get hot enough or thermionic emission.  When the filament current is high enough for thermionic emission, a small increase in filament current results in a large increase in x-ray tube current  This relationship between filament current and x-ray tube current depends on the tube voltage (Figure 6-7).  At any given filament current, say, 4.8 A (Figure 6- 8), the x-ray tube current rises with increasing voltage to a maximum value.  A further increase in kVp does not result in a higher mA because all of the available electrons have been used. This is the saturation current.  When an x-ray tube is operated at the saturation current, it is said to be emission limited.  Focal Spot Most diagnostic x-ray tubes have two focal spots— one large and the other small. The small focal spot is used when better spatial resolution is required. The large focal spot is used when large body parts are imaged and when other techniques that produce high heat are required.  Small focal spots range from 0.1 to 1 mm; large focal spots range from 0.3 to 2 mm.  Each filament of a dual filament cathode assembly is embedded in the focusing cup (Figure 6-9).  The small focal spot size is associated with the small filament and the large focal spot size with the large filament.  An electric current is directed through the appropriate filament.  Anode  The anode is the positive side of the x-ray tube.  There are two types of anodes, stationary and rotating (Figure 6-10).  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.  The anode serves three functions in an x-ray tube. 1. The anode is an electrical conductor. 2. It receives electrons emitted by the cathode and conducts them through the tube to the connecting cables and back to the high voltage generator. 3. The anode also provides mechanical support for the target. 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.