Radiology Physics CH 1 : MODERN X-RAY TUBES
STATIONARY ANODE X-RAY TUBE
In earlier periods, gas tubes were used to produce X-rays. But, these tubes suffered with disadvantages. Hence, Coolidge proposed a prototype X-ray tube based on the thermionic emission principle. On the basis of Coolidge tube, several X-ray tubes have been designed. The stationary anode X-ray tube is one of the modern X-ray tubes in which the anode is stationary.
The stationary anode X-ray tube consists of a cathode and an anode which are kept in a evacuated glass envelope (Fig. 3.10). The cathode consists of a tungsten filament in the form of a coil placed in a shallow focusing cup. The filament is heated by passing an electric current through it from a low voltage supply.
The anode is made of copper block in which a small tungsten plate is embedded. The tungsten plate serves as a target. The target is positioned on line focus principle, in order to increase the ratio of the actual focal area to the effective focal area. The anode angle is usually 15–20o. A high voltage supply is applied between the cathode and anode to accelerate the electrons. A vacuum of the order of 10–5 mm Hg is maintained in the tube.
When the filament is heated to white light, it emits electrons. The focusing cup (made up of nickel) produces an electric field (negative) that focuses the electron to the focal area. The focusing cup also protects the adjacent parts of the tube wall from damage by electron bombardment. If the anode is made positive with respective to the filament, these electrons will be attracted to the anode. This will constitute an electron current around the circuit in the anti clock wise direction. The tube current is measured by the milliammeter (mA).
Since the space between anode and cathode is a high vacuum, the electrons do not collide with gas molecules in crossing the gap and so acquire very high velocities. The electrons which are accelerated by the applied voltage possess high kinetic energy. When they suddenly stopped in the target, X-rays are emitted in all directions. About one-half of these are absorbed in the target itself. The remaining portion emerges as a useful primary X-ray beam. During the production of X-rays, large amount of heat is produced in the target. The tube is also provided with suitable cooling system to remove the heat very quickly.
Stationary anode tubes have a small target area that limits the heat dissipation and this limit the X-ray output, but they are small in size and weight. Dental X-ray units (intraoral and orthopantomography), portable X-ray units, and portable fluoroscopy systems use stationary anode X-ray tubes .
ROTATING ANODE X-RAY TUBE
In 1933, the rotating anode X-ray tube was invented, in which the anode is made to rotate before the electron is emitted. It was developed to increase the heat loading with higher X-ray output. In these tubes, the electrons transfer their energy over a large area of a rotating target. Rotating anode tubes are larger in size, but the principle and function
are similar to that of stationary X-ray tube.
Principle
Consider a rotating anode of radius R and circumference L as shown in the Figure 3.11. The electrons bombard a region of height ab and width cd. The length may range up to the circumference (L = 2R) depending on the exposure time. But, the X-rays always appear to come from a focal spot of area cd × cd.
Thus, the rotating anode arrangement helps to increase the loading to a greater extent of the order 100. The construction of such a rotating anode is a remarkable technological development. The diameter of the tungsten disk determines the total length of the target track, and obviously affects the maximum permissible loading of the anode.
Cathode
Rotating anode X-ray tube consists of a cathode and an anode which are kept in a glass bulb (Fig. 3.12). The cathode is a tungsten filament which is offset from the long axis of the X-ray tube to face the target near the periphery of the anode disk. Usually, rotating anode tubes are fitted with two filaments (Fig. 3.13), one larger than the other set side by side in the cathode assembly. One filament is designed to focus the electrons on a larger area of the anode, which require heavy tube loading. The other filament is used to focus the electrons on a smaller area of the target. This type is used when high resolution is required. Both filaments should focus the electron on the same part of the anode, so that focal spot is at the same point for both modes of operation. Some tubes provide two target angles for two filaments, so that each filament will have a separate focal spot. Smaller angle is used with the smaller focal spot.
Focusing Cup
The focusing cup (cathode block) surrounds the filament, shapes the electron beam width. It is used to focus the electrons on a small area (focal spot) in the anode. There are two ways by which the focusing cup is energized, namely, unbiased and biased (Fig. 3.14). In unbiased setup, same voltage is applied to both focusing cup and filament. In this type, the electron spread is wider and the focal spot width is larger. In biased X-ray tubes, insulated focusing cups are used and it is given more negative supply (–100 V) than the filament.
This creates a tighter electric field around the electron, reduces the electron spread and gives smaller focal spot width. Thus, the focusing cup width determines focal spot width and the filament length determines the focal spot length .
Anode Stem
The anode disk is mounted on a stem, which is attached to the rotor. The anode assembly rotates with the help of bearings. The stem is made of molybdenum, which is having high melting point (2620°C) and poor heat conduction. The molybdenum stem prevents the flow of heat from the tungsten to the bearings of anode assembly, due its small
cross section. Thus, the bearings are protected from heat, which may cause them to expand and bind. Higher the length of the stem, higher the inertia of the tungsten disk, more will be the load on the bearings. Hence, it is desirable to keep the stem as short as possible.
Rotor
The anode disk is connected to a rotor, which is made up of copper bars arranged around a cylindrical iron core. There are electromagnets surrounding the rotor, outside the glass envelope is called stator (Fig. 3.15). Both the stator and rotor is called as an induction motor. When the stator coils are energized, a rotating magnetic field is produced, that induces current in the copper bars of the rotor. This induced current produces an opposing magnetic field that causes the rotor to spin.
HEEL EFFECT
The heel effect refers to the reduced intensity of the X-ray beam towards the anode side of the X-ray field (Fig. 3.16). The X-ray photons that are emitted on the anode side of the field must pass through a greater thickness of the anode than those directed toward the cathode side. This results in a reduced intensity on the anode side of the field. The magnitude of the heel effect depends on the anode angle, focus to film distance (FFD) and field size. The heel effect is less important at large FFD, because the film subtends a smaller beam angle. To reduce heel effect, anode angle should be increased and field size should be decreased. For better balance of the transmitted X-rays, the cathode side of the tube is oriented over the thicker parts and anode over the thinner parts of the patient.
OFF-FOCUS RADIATION
Off-focus radiation is produced by an X-ray tube when high speed electron interacts the anode surfaces, other than the focal spot area. The main source of off-focus radiation is scattered electrons at the target. They are accelerated back to the anode, outside the focal spot. They create a low intensity X-rays over the face of the anode. Off-focus radiation
increases the patient exposure, geometric blurring and background fog, resulting poor image quality. To reduce the off-focus radiation, small lead collimator may be placed very close to the X-ray tube port. Grounded anode X-ray tubes (anode and metal tube envelopes are given same electrical potential) reduce off focus radiation since the scattered electrons are attracted by the metal envelope. Tubes that are used in mammography also reduce off-focus radiation.
X-RAY TUBE AND HOUSING
The tube housing supports, insulates and protects the tube insert from the environment (Fig. 3.17). The tube housing is internally shielded with lead to attenuate X-rays emitted in other directions except through the window. The shield should perform four functions namely, (i) radiation protection, (ii) electrical protection, (iii) thermal protection and (iv)
physical protection. Steel casing is lined with lead to prevent radiation emerging in all directions. The Perspex/beryllium window is convex upwards to reduce filtration of the X-ray beam by oil. To prevent electrical shock, the shield is earthed. Wherever high tension cables enter the shield, insulated sockets are used. The shield is filled with mineral oil, which act as a electrical insulator and prevents sparking across the insert.
The oil also acts as a cooling medium and expands at higher temperatures. The oil expansion activates bellow to operate a micro switch, so that further use of the tube is prevented. The oil expansion also helps prevent the entry of air into the tube insert. The shield also protects the insert from accidental damage caused by knocks and bumps.
FILTERS
A filter is a metallic sheet introduced in the path of X-rays, in order to reduce the patient dose. Diagnostic X-rays consist of both low energy and high energy X-rays. When X-rays passes through a patient, only high energy X-rays penetrate through the patient and form the radiological image. Whereas, the low energy X-rays are absorbed in the first few
centimeter of tissue, thereby increasing the radiation dose. The introduction of filters absorb these low energy X-rays and reduce the patient dose. This process of removing the low energy X-rays, by introducing metallic sheets is called filtration.
SCATTERED RADIATIONS
There are three types of radiation involved in patient imaging, namely, primary, scattered and leakage radiation. Leakage radiation does not contribute to image formation and no discussion is required. However, primary and scattered radiations are responsible not only for image formation but also the degree of image quality. Two vital factors of image quality are spatial resolution and contrast resolution. Spatial resolution is greatly controlled by focal spot, whereas contrast resolution is controlled by scatter radiation or noise. Scatter radiation is produced by Compton interaction, resulting in noise. Hence, scatter radiation needs to be reduced to obtain good quality image. That is why collimators and grids are used in patient imaging.
Scattered radiation mainly depends on kVp, field size and patient thickness. As the kvp increases, the X-ray energy increases. As a result, Compton interaction increases, and photoelectric interaction decreases. Hence, increase of kVp, increases the scatter radiation and reduces image quality. Therefore, X-ray imaging should be done with minimum kVp, with lowest scatter. But, at low kVp, the percentage of transmission may be lesser, which can be compensated with increase of mAs. Increase of mAs may account higher patient dose, hence optimal selection of
kVp and mAs is required.
Scattered radiation increases with field size. As the field size increases, scatter radiation also increases, which reduces the contrast of the image. Smaller the field size, lesser the scatter radiation and lesser the optical density. To maintain the optical density, higher exposure techniques are required with smaller field size.
COLLIMATORS
An X-ray beam restrictor is a device that is attached to the X-ray tube housing, to
regulate the size and shape of an X-ray beam.
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