PHYSICS 2 LEC 7: X-Ray Tube: Design and Function

X-Ray Tube Design

• An x-ray tube's general design includes both external and internal components.

External Components

• Support systems for x-ray tubes include ceiling, floor-to-ceiling, and C-arm configurations.
• The protective housing guards against electric shock.
• The enclosure consists of glass or metal.

Internal Components

• The cathode has a negative charge and includes filament(s) and a focusing cup.
• The anode has a positive charge and can be rotating or stationary.

Cathode Function

• X-ray tube current is adjusted by managing the filament current.
• Thermionic emission at low kVp and high mA can be space charge limited.

Anode Composition and Function

• The anode conducts electricity and radiates heat and x-rays from the target.
• Tungsten's high atomic number, thermal conductivity, and melting point make it ideal for use in anodes.
• Molybdenum and graphite provide lower mass density and high melting points.
• The focal spot on the anode acts as the actual x-ray source.

Line-Focus Principle

• The line-focus principle is an important aspect of anode design.

Anode Heel Effect

• The smaller the anode angle, the larger the heel effect.

X-Ray Tube Failure

• Excessive heat reduces the x-ray tube lifespan.
• Maximum radiographic techniques should not be applied to a cold anode.
• The most frequent cause of abrupt tube failure is electron arcing from the filament to the enclosure due to vaporized tungsten.

Rating Charts

• Most x-ray imaging systems have a microprocessor control that prevents unsafe exposures using a radiographic rating chart.
• For a single-phase anode cooling chart: HU = kVp × mA × s = 0.7 J
• For a three-phase/high-frequency anode cooling chart: HU = 1.4 × kVp × mA × s = 1 J
• There is also a housing cooling chart

X-Ray Production Summary

• Technologists set the technique.
• Electrical current flows through the tube.
• Selected voltage partly determines the energy and flow rate.
• X-rays are produced.

Main X-Ray Tube Parts

• A lead casing absorbs scatter radiation.
• A glass envelope is vacuum sealed.
• The anode is positively charged.
• The cathode is negatively charged.
• Oil cools the tube.

Cathode Assembly

• The cathode assembly includes a focusing cup and a filament made of tungsten and thorium.

Thermionic Emission

• Thermionic emission involves electrons emitted from a heated source.
• The focusing cup is more negatively charged than the filament.

Anode Characteristics

• The anode is made of tungsten, rhenium, molybdenum, and/or graphite.
• The focal spot is where electrons bombard the anode.
• The focal spot size is determined by the cathode filament.
• A rotating anode increases heat dissipation.

Line-Focus Principle and Anode Heel Effect

• The line-focus principle describes how the angled, beveled edge impacts focal spot size.
• The anode-heel effect occurs when part of the beveled edge absorbs parts of the beam.

Line-Focus Principle Specifics

• The effective focal spot decreases angle acuteness.

Anode Rotation

• Anodes spin using an induction motor.

Essential Conditions for X-Ray Production and Tube Parts

• Essential conditions are needed for x-ray production.
• X-ray tubes include various parts.

Thermionic Emission and Cathode Assembly

• Thermionic emission must be defined.
• The cathode assembly must be remembered

Focal Spots

• Actual and effective focal spots differentiate.

Motor Function

• The rotor and stator combine to form a specific kind of motor and serve a particular purpose.

Tube Heat

• Almost all energy put into an x-ray tube converts to heat, and less than 1% converts into x-rays.
• Heat is dissipated through conduction, convection, and radiation.

Tube Damage Causes

• The most common causes of tube damage include vaporized filament and anode.
• Tube arcing is also a common cause of tube damage.

Tube Life Extension

• Tube life should be extended by warming up the tube, calibrating, and avoiding excessive rotoring.
• If something is seen or heard something, then it should be reported.
• Don't use a suspected damaged unit.