Chapter 6 Lecture Notes
The X-Ray Tube
In your position you will need to know that parts of the X-Ray Tube. The main parts are shown in the above diagram. We will look at each of the pieces of the tube and what they do to help with the production of X-Rays. The first piece we will look at is the Cathode Assembly.
The Cathode Assembly
Cathode Assembly: Consists of the filament, focusing cup, and associated wiring.
Filament: A small coil of thin thoriated tungsten wire. The Filament is where the electrons for the production of X-Rays are emitted.
Focusing Cup: A shallow depression in the cathode assembly designed to house the filament.
Dual Focus: A two-filament arrangement within the x-ray tube. Some tubes have two filaments so that the tubes can have a greater variety of exposures. The diagram below is an example of a Dual Focus filament.
The diagram on the left below shows a very basic wiring for a dual focus filament. The blue lines are the wires for the small filament and is not operating in this diagram, because the switch open for the blue filament. The red lines are the wires for the large filament and the switch is closed for the red filament.
The diagram on the right above shows the wiring for a dual filament tube. The autotransformer is where you select the milliamps for the filament. The filament selector is where the different filaments are selected.
The focusing cup is a place where the electrons are accumulating so that there are enough electrons to produce X-Rays. The diagram below shows the focusing cup and the filament. The focusing cup is negatively charged so that when the electrons are emitted by the filament they will congregate by the filament and will surge across to the anode when the exposure begins.
Space Charge Effect: As more and more electrons build up in the area of the filament, their negative charges begin to oppose the emission of additional electrons.
Saturation Current: As kVp increases, a greater percentage of the thermionically emitted electrons are driven toward the anode.
The Anode Assembly
Anode Assembly: Consists of the anode, stator, and rotor. The anode is positively charged so that the electrons from the filament (cathode) are attracted to it to produce x-rays. The rest of the anode assembly are there so that the anode can rotate and have a bigger target for the electrons.
Stator: The stator is the stationary part of an electric motor or alternator. Depending on the configuration of the motor the stator may act as the field magnet, interacting with the armature to create motion. The stator may be either a permagnet or an electromagnet.
Rotor: The rotor is the non-stationary part of a rotary electric motor or alternator, which rotates because the wires and magnetic field of the motor are arranged so that a torque is developed about the rotor's axis.
Purpose of Anode:
1. serves as a target surface for the high-voltage electrons
2. conducts the high-voltage from the cathode back into the x-ray generator circuit
3. serves as the primary thermal conductor.
Stationary Anode: An anode assembly that is immobile.
Rotating Anode: An anode assembly that turns during exposure.
In the diagram below the left anode is a stationary anode because it does not move when an exposure occurs. The anode on the right is a rotating anode because the anode rotates during an exposure.
The target (place where the electrons interact with the anode to produce x-rays) part of the anode is made of tungsten or molybdenum.
Tungsten is material of choice for target because:
1. High Atomic Number (74) - High Atomic Number means that when an electron falls back into the L shell after ionization there is enough energy for an x-ray to be emitted.
2. High Melting Point (3422 C, 6192 F) - The anode will not melt easily under repeated exposures. (You can still damage an anode if you operate the machine incorrectly.)
3. Heat-conducting ability - The ability to conduct heat helps cool the anode and prevent melting.
Molybdenum (42) is used for soft tissue imaging.
In the diagram below the construction of a rotating anode is shown. The base is made of graphite for a solid base. The target (dual target for this example) is made of a tungsten-rhenium alloy. The parts holding the target are made of molybdenum.
When the anode gets old or from miss use the target can get pock marked as shown below.
Anode Warm-up Procedure: Need to follow manufacture's recommendations.