An X-ray tube is a vacuum tube that converts electrical input power into X-rays.[1] The availability of this controllable X-ray source has created the field of radiography, imaging partially opaque objects with penetrating radiation. Unlike other sources of ionizing radiation, X-rays are only produced when the X-ray tube is energized. X-ray tubes are also used in CT scanners, airport baggage scanners, X-ray crystallography, material and structural analysis, and industrial inspection. It is important to understand that mA, exposure time or equivalent mA all follow the same directly proportional rule in terms of exposure and dose. If one of these three factors is doubled, the exposure and dose double. If one of the three is halved, the exposure and dose are halved. The main advantage of the increased power density of the metal beam X-ray tube is the ability to work with a smaller focal point, say 5 μm, to increase the resolution of the image while capturing the image faster because the power is higher (15-30 W) than solid anode tubes with focal points of 10 μm. Alternatively, photons can be fully absorbed into the tissue. [8] The properties of physical tissues can determine whether energy is more easily absorbed, attenuated or dispersed. In particular, tissue density, thickness and atomic number alter the trajectory and absorption of X-rays.
[9] [10] Increased atomic number, thickness and density can lead to greater attenuation of photons, their absorption and dispersion. These characteristics create a contrast between the different tissues of the body, which makes it possible to separate intensity values and assess potential pathologies. The second milliampere (mAs) is a unit of X-rays determined by multiplying milliamperes by the time at which the X-ray tube produces X-rays. In many X-ray machines, this is a technical factor selected by the operator that, together with kRp, determines the exposure of the patient and the image receptor. [1] There are two versions: end window tubes and side window tubes. End window tubes usually have a “transmission target” thin enough to allow X-rays to pass through the target (X-rays are emitted in the same direction as electrons). In a common type of end window tube, the filament is around the anode (“ring or annular”), the electrons have a curved path (half of a toroid). • List the geometric factors that affect spatial resolution and explain why magnification affects resolution The focal point temperature can reach 2,500°C (4,530°F) during an exposure, and the anode layout can reach 1,000°C (1,830°F) after a series of large exposures. Typical anodes are a tungsten-rhenium target on a graphite-coated molybdenum core. Rhenium makes tungsten more ductile and resistant to wear caused by the impact of electron beams.
Molybdenum conducts heat from the target. Graphite provides heat storage of the anode and minimizes the rotating mass of the anode. The amount of exposure and dose to the patient are directly proportional to the mAs. For each of the four exposure techniques listed above, the volume of photons emitted and the dose received by the patient are the same. The density or blackening effect on the image is also the same. The mAs unit is the main controller of X-ray density. The kVp also affects the amount of IR exposure. When the kVp is increased, the electrons in the filament reach the anode with more energy. More interactions then occur in the anode and more X-rays are emitted. If kVp is increased, the density is increased; However, mAs is the primary density controller. Unlike the effects of mA, exposure time or mAs, changes in exposure are not directly proportional to kVp. The kVp is never doubled.
Doubling the kVp would result in four times more photons emitted! Conversely, kVp would never be halved because four times fewer photons would be produced. These would be extreme changes in exposure. Although kVp affects density, kVp should not be used to control X-ray density. There are two basic types of microfocused X-ray tubes: solid anode tubes and metal jet anode tubes. The heat unit (HU) has been used in the past as an alternative to Joule. It is a convenient unit when a single-phase power source is connected to the X-ray tube. [6] For a full-wave rectification of a sine wave, w {displaystyle w} = 1 2 ≈ 0.707 {displaystyle {frac {1}{sqrt {2}}}approx 0.707} , i.e. the unit of heat: This chapter explains the main radiographic exposure factors and their radiographic effects. Some of them have already been presented to you.
In addition, the four main factors of X-ray quality and the main control methods are presented. You will begin to observe the effects of exposure on X-rays and understand how the different factors controlled by the limited operator affect the final image. • The amount of exposure is directly proportional to mA The X-ray photon production effect is commonly referred to as Bremsstrahlungseffekt, a compound of the German brake for the brakes and radiation for the radiation. Crooke tubes were unreliable. Over time, the residual air was absorbed through the pipe walls, reducing pressure. This increased the voltage through the tube, producing “harder” X-rays until the tube finally stopped working. To avoid this, “softener” devices were used (see photo). A small pipe attached to the side of the main pipe contained a mica sleeve or chemical that, when heated, released a small amount of gas and restored proper pressure. The electrons in the cathode collide with the anode material, usually tungsten, molybdenum or copper, accelerating other electrons, ions and nuclei in the anode material. About 1% of the energy generated is emitted/radiated, usually perpendicular to the path of the electron beam, in the form of X-rays. The rest of the energy is released as heat.
Over time, tungsten settles from the target onto the inner surface of the tube, including the surface of the glass. This will slowly darken the tube and is thought to degrade the quality of the X-ray. The evaporated tungsten condenses inside the shell above the “window”, acting as an additional filter, reducing the tube`s ability to radiate heat. [4] Finally, tungsten deposition can become sufficiently conductive for arcing to occur at sufficiently high voltages.