The radiographer is responsible for the safe operation and proper maintenance of the x-ray unit.
Appropriate operation and maintenance stems from the knowledge of how it works.
General Tube Construction: Housing
A protective housing provides solid mechanical support.
Lead-lined structure
Oil bath
Cooling fans
Electrical insulation
Large cables
Absorbs stray photons
Cautions:
The housing can become rather hot with continuous use.
The high-voltage cables should not be used as handles to maneuver the tube head assembly.
X-ray Tube
The general-purpose x-ray tube is an electronic vacuum tube that consists of:
An anode
A cathode
An induction motor
Encased in a glass or metal enclosure
The main purpose of the enclosure is to maintain a vacuum within the tube to prevent electrical arcing.
Two varieties of enclosures:
Glass envelope: Generally made of borosilicate glass because it is heat resistant.
Metal envelope: Provides a constant electric potential between the electron stream from the cathode and the enclosure, thereby avoiding the arcing problem and extending tube life.
Both types have a specially designed target window for the desired exit point of the x-rays produced.
Anode
The positive end of the tube.
Provides the target for electron interaction to produce x-rays.
Serves as an electrical and thermal conductor.
Some of the electrons interact with the target to produce x-rays and the rest continue on as current flows through the x-ray circuit.
Stationary Anode
A tungsten button embedded in a copper rod.
The target does not move.
Disadvantage: the rapidly building heat can damage the tube, which limits use.
Rotating Anode
A rotating tungsten-coated molybdenum disc mounted on a copper shaft with a molybdenum core.
Advantage: the rotating anode spreads the heat produced during x-ray production over a larger surface area.
Materials
Copper: Used as part of the shaft because it has excellent thermal and electrical conductive properties.
Molybdenum: Used as the disc base and core because it has a low thermal conductivity and it is a light but strong alloy.
Tungsten: Used because it has a very high melting point and a high atomic number (74), improving the efficiency of x-ray production.
Rhenium: May be added to the tungsten to increase thermal capacity and tensile strength.
Induction Motor
The anode is rotated using an induction motor.
The two major parts of this motor are the stator and the rotor.
The stator is made up of electromagnets arranged in pairs around the rotor.
The rotor is made of an iron core (iron bars embedded in the copper shaft) surrounded by coils.
The induction motor is operated through mutual induction.
The stators are energized in opposing pairs and induce an electric current and magnetic field.
This induced field opposes that of the stator pair and the rotor turns to correct that orientation.
Just as the two fields align, the next pair of stators is energized and a new electric current and magnetic field are induced, causing the rotor to turn again.
This process causes the rotor to turn continuously.
An induction motor allows for the rotation of the anode in a vacuum without engineering a motor into the vacuum.
Capable of speeds of 3400 revolutions per minute (rpm) for general-purpose tubes and 10,000 rpms for specialty tubes.
Line-Focus Principle
By angling the face of the anode target, a large actual focal spot size can be maintained and a small effective focal spot size can be created.
When the angle of the target face is less than 45 degrees, the effective focal spot will be smaller than the actual.
The target angles are 7 to 18 degrees for a general-purpose tube (12 degrees is the most common).
The smaller the anode angle, the smaller the effective focal spot will be while maintaining a large actual focal spot area.
The smaller the effective focal spot, the sharper the image will be.
Anode Heel Effect
The angle causes the intensity of the x -ray beam to be less on the anode side because some of the x-rays are absorbed in the target heel.
Cathode
The negative end of the tube.
Provides the source of electrons needed for x-ray production
Made up of the filaments and the focusing cup
Filaments
Dual-focus tubes are general-purpose tubes with two filaments
Each filament is a coil of wire usually 7 to15 mm long and 1 to 2 mm wide.
They are usually made of tungsten with 1% to 2% thorium added.
The filaments are parallel to one another in the focusing cup and share a common ground wire.
Focusing Cup
Made of nickel and surrounds the filaments’ back and sides, leaving the front open and facing the anode target
Receives a strong negative charge from the secondary circuit that forces the electrons together into a cloud as they are boiled off of the filament through electrostatic repulsion
Principles of Operation
At the operating console, the radiographer selects the desired exposure factors.
When the exposure switch is first pressed, some of the electricity is diverted to the induction motor of the x-ray tube to bring the rotor up to speed.
Inside the x-ray tube, the induction motor turns the anode at approximately 3400 rpm.
The selected filament is energized until the desired thermionic emission is achieved.
The second phase initiates the x-ray production process.
The process from rotor preparation to exposure lasts only a few seconds.
The actual exposure is measured in milliseconds.
When the exposure switch is pressed, the voltage from the autotransformer passes to the step-up transformer.
The voltage then passes through a rectifier bank before passing to the anode and cathode of the x-ray tube.
During the preparation phase, some power from the autotransformer was diverted to the filament circuit where it passes through a rheostat to a step-down transformer, then to the selected filament within the cathode focusing cup.
The current heats the filament to the point of incandescence and electrons are boiled off of the filament by thermionic emission.
The focusing cup forms them into a cloud called a space charge.
Once the space charge reaches a size commensurate with the current used, it becomes difficult for additional electrons to be emitted; this is called the space-charge effect.
The three things needed to produce x-rays are now present:
A large potential difference to give kinetic energy to the filament electrons (provided by kVp)
A vehicle on which kinetic energy can ride (a quantity of electrons provided by mAs)
A place for interaction (the target of the anode)
As they penetrate the target surface, filament electrons interact with the atoms of tungsten, generating heat and x-rays.
Quality Control and Extending Tube Life
Most of the factors that can shorten x-ray tube life are within the radiographer’s control:
Frequent use of very high or maximum exposure factors
Use of lower but very long exposure factors
Overloading the filament
Three processes of heat transfer are at play:
Conduction of heat by heat-tolerant materials
Radiation of heat energy from anode to oil bath
Convection of heat into the room by cooling fans
Protective circuits prevent the use of unsafe exposure techniques and heat overloads.
Even with safety measures, the radiographer must understand anode thermal capacity and keep in mind that x-ray production is an inefficient process.
Heat Units
A measure of the amount of heat stored in a particular device
Calculated by multiplying kVp ´ mA ´ s ´ c
If multiple exposures are made using a given technique, the answer from this formula is multiplied by the number of exposures
Heat Units = kVp \times mA \times s \times c
Extending Tube Life
To extend tube life, simple procedures and guidelines should be followed.
Follow the machine-specific warm-up steps completely and routinely.
Do not prep the rotor excessively.
Do not routinely use extremes of exposure factors.