X-ray Tubes

• An x-ray tube is a diode because it exhibits polarity with a positive anode and a negative cathode.
• The functions of an x-ray tube are to produce x-rays and maintain circuit continuity.
• X-rays are produced by bombarding electrons from the cathode to the anode (target).
• Electrons decelerate or stop upon hitting the target, generating x-rays and heat.
• Dual focus x-ray tubes have two filaments: one small and one large.
• The main components of an x-ray tube include the cathode, anode, rotor, and stator.
• The filament serves as the source of electrons during x-ray production.
• Typically, there are two filaments within the cathode.
• The filament wire is made of tungsten, which has an atomic number of 74 and a melting point of 3400 degrees Celsius.
• Only one filament can be energized at a time since each filament corresponds to a focal spot.
• The focusing cup, made of nickel, nearly surrounds the filament to keep the electron cloud from spreading apart.
• Its negative charge helps create the space charge effect.
• Thermionic emission constitutes boiling off electrons at the filament.
• Anode components are the target, rotating anode tubes, stator, and rotor.
• Anode materials: Tungsten, rhenium alloy, molybdenum, graphite, and copper.
• The target typically consists of 90% tungsten and 10% rhenium alloy.
• Molybdenum is used for the target in mammography x-ray tubes.
• Molybdenum and graphite form the base of the anode to allow for easier rotation.
• Molybdenum is used for the anode stem.
• Copper is used for the rotor.
• The two types of anodes are stationary and rotating.
• Rotating anodes withstand higher heat loads because the rotation causes a larger physical area/focal track to be exposed to electrons.
• Rotating anodes typically range from 3,000 to 10,000 rpm.
• The anode is angled to help x-rays exit the tube.
• Target angulation ranges from 5-20 degrees.
• An electric motor that turns the rotor at very high speed refers to the stator.
• The rotor rotates the anode at high speed.
• Ball bearings in the rotor allow a smooth rotation at high speed.
• X-ray production is an inefficient process: 99% heat and 1% x-rays.
• Overheating can cause the tube to fail, by transferring heat to the tube envelope and then to the dielectric oil.
• Dielectric oil does not conduct electricity; it insulates or dissipates heat, and sits between the envelope and the metal housing.
• The focal spot refers to the physical area of the target bombarded by electrons.
• In a stationary anode, the focal spot acts as a fixed area constantly bombarded, as used in dental x-ray tubes.
• In a rotating anode, the focal track represents the focal spot, and changes on the track allowing for the heat to dissipate.

X-Ray Tube Components

• The components of an x-ray tube are housed within a glass or metal envelope.
• The metal envelope is mostly used due to its superior electrical properties and ability to reduce off focus radiation.
• Off-focus radiation results from electrons striking areas on the anode other than the focal spot, diminishing image quality.
• The metal envelope can collect and conduct electrons away from the anode.
• The envelope houses x-ray tube components. -Also, it completely evacuates air from the tube, ensuring electrons flow in one direction from the cathode to the anode.
• The envelope provides some insulation from electrical shock.
• It dissipates heat by routing heat to the insulating oil.
• Arcing results from tungsten deposits building up over time, interrupting current flow and creating a short circuit.
• The metal housing encloses all tube components except the window, enabling x-rays to exit.
• Leakage radiation consists of any x-rays other than the desired primary beam exiting the tube housing.
• Leakage radiation cannot exceed 1mGya/h (100mR/h) when measured at 1 meter from the source when operating at max output.
• The spacing between the cathode and anode is 1 to 2cm.

X-Ray Circuit

• Electronic circuits produce a predictable x-ray beam, raising voltage momentarily at precise milliamperage for x-ray exposure.
• A circuit is a fixed path controlling the flow and intensity of electrons.
• All electrical devices follow the electrical power formula.
• Power (W) is expressed in Watts.
• An x-ray machine expresses power in kilowatts (kW).
• The formula is: Power (W) = Amps (I) x Volts (V).
• Power (W) is the amount of energy used/produced over time. -Amps (I) refers to current. -Volts (V) represents voltage, or the electrical potential difference.
• X-ray production uses high voltage. -1000 volts equals 1 kV.
• X-ray machine power ranges from 30 to 100 kW.
• The higher the power, the more the mA/kVp can be raised and exposure times lowered.
• There are 2 main divisions to the x-ray circuit: a high voltage side (main) and a low voltage filament side (secondary).

Electrical Components

• Main Power Switch: The electrical power source that supplies incoming power to the autotransformer. Power comes in alternating current (AC) that ranges from 220-480V.
• Circuit Breaker: Protects the circuit from power surges.
• Autotransformer: Adjusts and regulates the input voltage before it reaches the step-up transformer. It allows for kVp selection by the operator.
• The autotransformer works on mutual induction.
• If voltage increases, current (amperes) decreases.

Example of Electrical Components:

• Voltage is like water pressure, and current is like water flow.
• When you increase voltage in the x-ray machine, the electricity is "pushed" harder.
• Because voltage is high, the current must be low to avoid overloading the system.
• An x-ray machine uses milliamperage (mA).

High Voltage Circuit (Main)

• Exposure timer: Controls how long the x-ray tube is activated and producing x-rays, usually set in milliseconds or seconds.
• When the timer finishes, it turns off the flow of current to the x-ray tube.
• kVp Meter: Measures and displays the kilovolt peak (kVp), the voltage applied to the x-ray tube to regulate the energy of the x-rays.
• Step up transformers take incoming voltage and raise it to kilovolts.
• Rectification: Diodes allow current to flow in one direction.
• Rectifiers convert AC current to DC current, but with voltage ripple (fluctuation).
• Voltage ripple refers to the small fluctuations or variations in the DC.
• Voltage ripple in older (single-phase) systems was about 100%, but in newer (HF) systems, ripple is about 1%.
• A capacitor stores electrical charge/energy and releases it when needed.
• Capacitors smooth out voltage fluctuations.

Single vs. Three-Phase X-Ray System

• Single-Phase: One wave of electricity causing high voltage fluctuation, and is less stable with image consistency.
• Three-Phase: Three waves of electricity causing low voltage fluctuation, and results in more stable and consistent x-rays.

Low Voltage Filament Circuit

• This circuit heats the filament (cathode) to release electrons needed to create x-rays. By controlling the current to the filament, the system regulates how many electrons are available.
• Variable Resistor (Rheostat): Adjusts current through the mA selector.
• Higher resistance means less electrons, thus lower x-ray output.
• Lower resistance means more electrons, thus higher x-ray output.
• The autotransformer receives power, it will adjust the voltage and lower it since the filament is a thin wire that does not require much voltage to heat up.
• Step-down transformer: Further lowers the voltage to 10-15 volts to the filament, where the boiling off of electrons will happen.
• High Frequency Generators: Crucial for x-ray circuits as they convert AC into stable HF electrical signals that are typically 100Hz - 4000kHz to drive the x-ray tube.

Questions about the components of an X-ray tube, the function of different parts of the tube, and the principles behind their operation. Includes focusing, anodes, thermionic emission and transformers.