X-Ray Tube Components & Functions PDF

Luisa T, Cruz, RRT. 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. Ceiling Support System  3 Main Methods of X-ray Tube Sup...

Luisa T, Cruz, RRT. 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. Ceiling Support System  3 Main Methods of X-ray Tube Support. 1.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  3 Main Methods of X-ray Tube Support. 2.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 C-Arm Support System  3 Main Methods of X-ray Tube 3.C-Arm Support System Support. Interventional radiology suites often 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. PARTS OF XRAY TUBE The 4 major parts of the x-ray tube 1. Cathode 2. Anode 3. Envelope 4. Tube housing. X-ray Tube Housing X-ray Tube Housing Contains the x-ray tube. Its what the technologist sees. Made of metal. This housing is a lead-lined metal structure that provide solid stable mechanical support and also serves as an electrical insulator and thermal cushion for the tube itself X-ray Tube Housing We use only x-rays emitted through the special section of the x-ray tube called the The x-rays emitted through the window are Window 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 Its purposes: 1. Holds the x-ray tube. 2. Lead-lined to prevent leakage radiation from exiting (no more than 100mR/hr) 3. Insulated against electrical hazards from high voltages. 4. Cools with oil bath and cooling  Glass Envelope  Glass Envelope Enclosure that contains the cathode and anode. The purposes: 1. allows efficient flow of electrons from cathode to anode (no air molecules to bump into). 2. Provides insulation from electric shock (cathode and anode are electrical charges). 3. Dissipates heat by conducting it to 2 Types of Envelope 1. Glass made of pyrex. tolerates high heat. vacuum- contains no air so that nothing will interfere with the travel of the electrons towards anode. Immersed in oil- to disperse the heat created. Window (port) where the x-rays exit the envelope, designed to allow the primary  2 Types of Envelope 2. Metal More commonly used today. Longer tube life span than glass envelopes.  As a glass enclosure ages, some tungsten vaporizes and coats the inside of the glass enclosure. This alters the electrical properties of the tube, allowing tube current to stray and interact with the glass enclosure; the result is arcing and tube failure. 5cm^2 area of tube window. Cathod e Cathode Is a negatively charge electrode. It comprises a filament and a focusing cup.  2 Major Parts of 1.Filament Cathode a coil of tungsten wire embedded in the focusing cup. its purpose is that it is the source of electrons for x-ray production. Approximately 2mm in diameter 1 or 2 cm long Made of Thoriated tungsten  Why Tungsten? Filaments are usually made of Thoriated Tungsten providesTungsten. for higher thermionic emission than other metals. Tungsten does not vaporize easily The addition of 1% to 2% Thorium to the tungsten filament enhances the efficiency of thermionic emission and prolongs tube life. 3410 degree celcius melting point or 6000  2 Kinds of The larger Fillament The smaller filament is filament is used for used for imaging imaging of of larger small body body parts parts  Thermionic Emission Boiling of electrons  2 Major Parts of Cathode 2. Focusing cup made of nickel. Surrounds the filament. Its purpose is to focus the stream of electrons. It has a negative charge to keep the electron cloud spreading. it helps to focus the electrons towards  Grid-controlled x-ray tube the focusing cup is the grid and therefore the exposure switch. Designed to be turned on and off very rapidly Filament current the tube current is adjusted by controlling the filament current.  however, tungsten metal does vaporize and is deposited on internal components. This upsets some of the electric characteristics of the tube and can cause arcing and lead to tube failure. Such malfunction is usually abrupt. Space Charge Note : cloud of electrons, called a space This charge, makes it difficult for subsequent electrons to be emitted by the filament because of electrostatic repulsion. This phenomenon is called the space charge effect. A major obstacle in producing x-ray tubes with currents that exceed 1000 mA is the design of adequate space charge Note At a given filament current, tube current : reaches a maximum level called saturation current. Saturation current is not reached at a lower kVp because of space charge limitation. When an x-ray tube is operated at the saturated current, it is said to be emission limited. Anod e There are 2 types of anodes Stationa ry  Stationa ry 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  Rotatin g Rotating anode x-ray tubes are in diagnostic machines that requires high tube current and power.  The Anode serves 3 functions in an X- ray 1.The anode is an tube. conductor electrical It receives electrons emitted by the cathode and conducts them through the tube to the connecting cables and back to the high- voltage generator. 2.The anode also provides mechanical support for the target.  The Anode serves 3 functions in an X- ray tube. 3.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 dissipa- tion is 3 Major Parts of Anode 1.Target 2.Rotor 3.Stem 1.Target is the area of the anode struck by the electrons from the cathode. Tungsten, rhenium, molybdenum, graphite, rhodium (mammography) Target 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 (Figure 6-11, A). In rotating anode tubes, the entire rotating disc is the  Specialty x-ray tubes for mammography have molyb- denum or rhodium targets principally because of their low atomic number and low K- characteristic x-ray energy. Table 6-1 summarizes the properties of these target materials. Dual focus most diagnostic x-ray tube has two focal spots, one large and other small. The small focal spot Is used when better spatial resolution is required. SFS range from 0.1 to 1mm The large focal spot Is used when large body parts are imaged and when other techniques that produce high heat are required. 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- raytubes rotate at 10,000 rpm. The stem of the anode is the shaft between the anode and the rotor. It is narrow so as to reduce its thermal conductivity. The stem usually is made of molybdenum because it is a poor heat conductor. Occasionally, the rotor mechanism of a rotating Stem molybdenum, because it is poor heat conductor. Induction motor an electromagnetic induction motor is used to turn the anode. 2 parts (stator (outside) & rotor (inside). An electromagnetic induction motor is used to turn the anode. An induction motor consists of two principal parts separated from each other by the glass or metal enclosure (Figure 6- 15). The part outside the glass or metal enclosure, called the stator, consists of a series of electromagnets equally spaced around the neck of the tube. Inside the enclosure is a shaft made of bars of copper and soft iron fabricated into one mass. This part is Line-Focus Principle 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.  Actual Focal Spot Larger is where the electron interact with the anode (Target) Effective focal Spot Smaller is what exits the tube and interacts with the Anode angle can affect the size of the effective focal spot  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 Off-Focus Radiation X-ray tubes are designed so that projectile electrons from the cathode interact with the target only at the focal spot. However, some of the electrons bounce off the focal spot and then land on other areas of the target, causing x-rays to be produced from outside of the focal spot  These x-rays are called off- focus radiation. This is similar to squirting a water pistol at a concrete pave- ment: Some of the water splashes off the pavement and lands in a larger area. Off-focus radiation is undesirable because it extends the size of the focal spot. The additional x- ray beam area increases skin dose modestly but unnecessarily. Off-focus radiation can significantly reduce image contrast.  Off-focus radiation is reduced by designing a fixed diaphragm in the tube housing near the window of the x- ray tube (Figure 6- 24). This is a geometric solution.  X-RAY TUBE The length of x-ray tube life is primarily FAILURE under the control of radiologic technologists. Basically, x-ray tube life is extended by using the minimum radiographic factors of mA, kVp, and exposure time that are appropriate for each examination. The use of faster image receptors results in longer tube life.  Three modes of Heat 1.Radiation Transfer is the transfer of heat by the emission of infrared radiation. Heat lamps emit not only visible light but also infrared radiation. 2.Conduction is the transfer of energy from one area of an object to another. The handle of a heated iron skillet becomes hot because of 3. Convection is the transfer of heat by the movement of a heated substance from one place to another. Many homes and offices are heated by the convection of hot air.