X-ray Tube Study Guide PDF

Study Guide- Chapter 2 ▪ Why is the x-ray tube considered a diode? It has polarity with a positive charge anode and a negative charge cathode. ▪ What are the two functions of the x-ray tube? X-ray production and circuit continuity. ▪ How are x-rays produced? Electrons are bomb...

Study Guide- Chapter 2 ▪ Why is the x-ray tube considered a diode? It has polarity with a positive charge anode and a negative charge cathode. ▪ What are the two functions of the x-ray tube? X-ray production and circuit continuity. ▪ How are x-rays produced? Electrons are bombarded from cathode to anode (target). The electrons are decelerated or stopped upon striking the target, and x-rays and heat are produced. ▪ Why are x-ray tubes referred to as dual focus tubes? They have two filaments, a small and a large one. ▪ What are the main components of an x-ray tube? Cathode, anode, rotor, stator. ▪ What is the source of electrons during x-ray production? The filament. ▪ How many filaments are located within the cathode? o What material is the filament wire made of? Tungsten. (74 atomic number, 3400 degrees Celsius melting point). o how many filaments can be energized at once? Only one, because they correspond to the focal spot. ▪ The focusing cup nearly surrounds the filament. What is its purpose? To keep the electron cloud from spreading apart. The negative charge in the focusing cup, helps create the space charge effect. o What material is the focusing cup made up of? Nickel. o What is thermionic emission? Boiling off electrons at the filament. (need to understand this process). ▪ What are the components of the anode? Target, rotating anode tubes, a stator and rotor. o What materials make up the anode? Tungsten, rhenium alloy, molybdenum, graphite, and copper. See parts breakdown below. ▪ Target made up of: Tungsten (90%), rhenium alloy (10%). ▪ The target of a mammography x-ray tube is made up of molybdenum. ▪ Base of the anode made up of: molybdenum and graphite. (light material that allows for easier rotation). ▪ Anode stem is made up of: Molybdenum. ▪ Rotor made up of: Copper. o How many types of anodes are there? Stationary and rotating. ▪ What are the differences? Why is a rotating anode better than a stationary? They withstand higher heat loads because the rotation causes a larger physical area, focal track to be exposed by the electrons. ▪ What is the average range of revolutions per minute (rpm) of a rotating anode? 3,000 to 10,000rpm. o Why is the face of the anode angled? To help x-rays exit the tube. ▪ What is the range of target angulation? 5-20 degrees. o The stator is an electric motor that turns the rotor at very high speed. o The rotor rotates the anode at high speed. Ball bearings in the rotor allow a smooth rotation at high speed. ▪ X-ray production is a very inefficient process. 99% of the process results in heat, and 1% in x-rays. Overheating can cause the tube to fail. For this reason, heat is transferred to the tube envelope and then to the dialectric oil. ▪ Dialectric means that it does not conduct electricity, it only insulates or dissipates heat. The oil is between the envelope and the metal housing. ▪ What is the actual focal spot? Physical area of the target that is bombarded by electrons. o In a stationary anode, the focal spot is a fixed area that is constantly bombarded. This type of anode is used in dental, because small body parts such as the teeth are being exposed. o In a rotating anode it is represented by a focal track. It does not change the size of the physical area, but instead this area changes on the track, allowing for the dissipation of heat. This is used  in modern x-ray systems due to the number of frequent exposures and need for a wider range of techniques. ▪ The components of an x-ray tube are housed within a glass or metal envelope. The metal envelope is mostly used because of superior electrical properties and ability to reduce off focus radiation. ▪ What is off focus radiation? The result of electrons that strike any area on the anode other than the actual focal spot. This type of radiation diminishes image quality. The metal envelope is able to collect the electrons and conduct them away from the anode. ▪ What are the functions of the envelope: o House x-ray tube components o Allows for air to be completely evacuated from the tube, which helps to keep electrons flowing in one direction from cathode to anode. o Provides some insulation from electrical shock o Provides heat dissipation by routing heat to the insulating oil. ▪ What is arcing? Tungsten deposits or builds up over time, mostly in middle section of the envelope, and once it evaporates and interacts with electrons, it interrupts the flow of current and creates a spark or short circuit. This may cause tube failure. ▪ The metal housing encloses all of the components of the tube listed above on all sides, except the window, which allows the x-rays to exit the tube. ▪ What is leakage radiation? Any x-rays other than the desired primary beam that exits the tube housing. Leakage radiation cannot exceed 1mGya/h(100mR/h) when measured at 1 meter from the source when the tube is operating at maximum output. ▪ What is the spacing between the cathode and anode? 1 to 2cm. The X-ray Circuit ▪ Electronic circuits are designed to produce a predictable x-ray beam. The x-ray circuit is intended to raise the intended voltage momentarily at precise milliamperage for x-ray exposure. ▪ What is a circuit? A fixed path that controls the flow and intensity of electrons as they travel through its components. ▪ All electrical devices behave according to the electrical power formula. o Power (W) is expressed in Watts. ▪ X-ray machine power is expressed in Kilowatts (kW). o The formula is: Power (W)= Amps (I) X Volts (V) ▪ Power (W): This is the amount of energy used or produced by an electrical device over time. ▪ Amps (I): This refers to current (measured in amperes or "amps"). It represents the flow of electric charge through a conductor (like a wire) per unit of time. ▪ Volts (V): This represents voltage, which is the electrical potential difference between two points. It is the force that pushes the electric charge through a conductor. o X-ray production is a high voltage process that range from 50,000 to 150,000 Volts or (50-150 kV). This is referring to the voltage potential in the process. ▪ 1000 volts= 1 kV. o X-ray machine power ranges from 30 to 100 kW. The higher the power, the higher you may raise mA and kVp technical factors, which also means lower exposure times. There are 2 main divisions to the x-ray circuit: a high voltage side (main), and a low voltage filament side (secondary).  ▪ Main Power Switch: electrical power source. It supplies incoming power to the autotransformer. Power comes in the form of alternating current. This type of current changes direction, and it is the type of current that we use in our homes. The incoming power ranges from 220V to 480V. o Circuit Breaker: protects the circuit from power surges. If the power coming in goes up excessively, the breaker will trip and stop the flow of electrons. o Autotransformer: The autotransformer adjusts and regulates the input voltage before it reaches the step- up transformer in the X-ray circuit, which then increases the voltage to the Kilovolts, which are required for the X-ray tube as a higher voltage is needed to accelerate the electrons. o The autotransformer supplies the induced voltage to the primary windings of the step-up transformer. It allows for the kVp selection by the operator. o The autotransformer works on the principle of mutual induction, which means that an electrical current coming into a set of coils on the input side, will induce (create) a current and voltage on the secondary coil on the output side. o If voltage is increased, the current (amperes) is decreased proportionately. Since x-ray is considered a high voltage process, the amperes drop to milliampere (mA) levels. (see example below to understand why this is the case). Example: Imagine you're using a water hose as an analogy: Voltage is like the water pressure in the hose. The higher the pressure, the harder the water is pushed out of the nozzle. Current is like the flow of water—how much water comes out of the hose. When you increase the voltage in the X-ray machine (like increasing the water pressure in the hose), the electricity is "pushed" harder, meaning more energy is available to accelerate the electrons toward the anode (the target inside the X-ray tube). However, because the voltage is so high, the current needs to be low to keep the circuit balanced. If the current were too high, it could overload the system. So, in the X-ray machine, instead of using a large flow of electricity (high amperage), it uses a very small amount of current, measured in milliampere (mA). High Voltage Circuit (Main) ▪ Exposure timer: The exposure timer controls how long the X-ray tube is activated and producing X-rays. o The timer is set by the operator (usually in milliseconds or seconds) to control the duration of the X-ray exposure. o Once the timer reaches the preset value, it turns off the flow of current to the X-ray tube, stopping the X-ray production. ▪ kVp Meter: The kVp meter measures and displays the kilovolt peak (kVp), which is the voltage applied to the X-ray tube to control the energy of the X-rays. ▪ Step up transformers take incoming voltage and raise it to kilovolts. These are highly efficient electrical components that have less windings on the primary side, and more windings on the secondary side. ▪ Rectification: Once the voltage is raised to kilovolts (still in AC current), diodes are strategically placed to allow for the current to flow in only one direction (hence Direct Current-DC). Once the current is converted to DC current, it is then sent to the x-ray tube. Rectifiers convert AC current to DC current, but with voltage ripple (voltage fluctuation). o Voltage ripple refers to the small fluctuations or variations in the DC voltage that occurs when converting AC (alternating current) to DC (direct current). These fluctuations are caused by the fact that the AC current doesn’t provide a constant flow of power. Instead, it changes direction, so when it’s converted into DC, you get "pulses" instead of a smooth, steady voltage. o This can cause problems because the X-ray tube needs steady, continuous voltage to work properly and generate consistent X-rays. o Voltage ripple in older (single-phase systems) was about 100%, but in newer (high frequency systems) ripple is about 1%. ▪ A capacitor is an electrical component that sits right after the rectifiers, they are used to store energy in the form of an electrical charge. Once the capacitor is fully charged, it can release the stored energy back into the circuit when needed, acting like a temporary energy reservoir. Capacitors help smooth out fluctuations in the electrical current. Note: brief explanation about single phase and three phase systems Difference between a single-phase x-ray system and a three-phase system. These systems differ by how power is supplied to them. In a three-phase system the power never drops to zero, which means you have less voltage ripple. Feature Single-Phase X-ray System Three-Phase X-ray System Power One wave of electricity Three waves of electricity Voltage High (voltage fluctuates up and down) Low (more stable, no drops to zero) Ripple Less stable, can affect image More stable, better and more consistent X- X-ray Quality consistency rays Low Voltage Filament Circuit The low voltage filament circuit heats up the filament (cathode) in the X-ray tube. The heat causes the filament to release electrons, which are needed to create X-rays. By controlling the current to the filament, the system regulates how many electrons are available to create X-rays, affecting the amount of radiation produced (controlled by the mA setting). ▪ Variable Resistor (or Rheostat): adjust the current that passes through the filament. mA selector. o Higher resistance means less current flows, causing the filament to produce fewer electrons (and thus lower X-ray output). o Lower resistance means more current flows, causing the filament to produce more electrons, which increases the X-ray output. o After the autotransformer receives power, it will adjust the voltage and lower it. This is because the filament is a thin wire that does not require much voltage to heat up. In fact, allowing too much voltage, will overheat the filament. ▪ Step-down transformer: will further step down the voltage to 10 to 15 volts. o Once stepped down, the voltage powers the filament, where the boiling off of electrons will happen. ▪ High frequency generators are a crucial part of the x-ray circuit. A high-frequency generator in X-ray systems is a power supply that converts alternating current (AC) into a stable, high-frequency electrical signal (typically 100Hz -4000 kHz) to drive the X-ray tube. This advanced technology allows for more compact components, more precision in controlling the output, and better monitoring of the system which aids in service and repair.