The X-Ray Tube

The X-Ray Tube

RADT 1010

Production of X-Rays

  • Source of Electrons: Electrons are crucial for the production of X-rays, primarily generated in the filament of the X-ray tube.

  • Target: The target is where the electrons hit, producing X-rays. It is typically made of tungsten or a tungsten alloy.

  • High-Voltage: A significant voltage is applied across the X-ray tube, which accelerates the electrons from the cathode to the anode.

  • Vacuum: The tube operates in a vacuum to facilitate the free flow of electrons and minimize scattering.

Tube Components

  • Stator

    • Composed of:

    • Electromagnets: These create the magnetic field that drives the rotation of the anode.

    • Armature: Part of the motor system that aids in the rotation of the anode.

    • Rotating Portion: The section of the tube that rotates to dissipate heat.

    • Bearings: Allow smooth rotation of the anode.

  • Tungsten Anode: Serves as the target for the electron beam.

  • Envelope: The structure housing the components, typically made of glass or metal.

  • Filament: Heated to produce electrons via thermionic emission.

  • Molybdenum Neck and Base: Provides structural integrity to the envelope.

  • X-ray Electron Beam: The directed flow of electrons toward the target that produces X-rays.

  • Filament Circuit: Electrical circuit that provides current to the filament.

Envelope Specifications

  • Material: Made of Pyrex glass or metal.

  • Size: Approximately 10 inches long with a 6-inch central diameter and a 2-inch peripheral diameter.

  • Window: Allows X-rays to exit the tube while maintaining the vacuum.

  • Vacuum: Essential for function and efficiency.

Protective Housing

  • Functions:

    • Controls leakage and scatter radiation.

    • Isolates high voltages, ensuring safety.

    • Provides cooling for the tube.

  • Components:

    • High-Voltage Connector: For electrical connections.

    • Leakage Radiation Shielding: Typically lead to contain unwanted radiation.

    • Fitting for Filters/Collimators: Helps shape the beam and reduce scatter.

    • Useful Beam Window: Enables the focused X-ray beam to penetrate materials.

Control of Leakage and Scatter Radiation

  • Housing Composition: Made from lead-lined cast steel.

  • Leakage Radiation Limit: Set to 100 milliRoentgens per hour (mR/hr) at a distance of 1 meter from the tube.

High-Voltage Isolation and Tube Cooling

  • Dielectric Oil:

    • Purpose: Insulates electrical components and aids in cooling.

    • Can be circulated through a heat exchanger to enhance cooling efficiency.

  • Air Fan: Additional cooling mechanism to prevent overheating by circulating air around the tube.

Cathode Assembly

  • Components:

    • Filament(s): Wire made of thoriated tungsten, which ensures proper thermionic emission.

    • Focusing Cup: A device used to direct and focus the electron beam towards the anode.

    • Associated Wiring: Connects components to the high-voltage circuit.

  • Filament Specifications:

    • Thickness: Between 0.1 - 0.2 mm.

    • Width: 1 - 2 mm.

    • Length: 7 - 15 mm.

    • Determines recorded detail based on focal spot size.

    • Must generate sufficient resistance for the thermionic emission at a temperature of 2200° C.

Filament Material

  • Tungsten: Preferred due to:

    • High Melting Point: Efficient in sustaining high temperatures.

    • Difficult to Vaporize: Ensures longevity of the filament.

  • Alternative Materials: Rhenium and molybdenum are also viable options because of similar high-temperature capabilities.

Dual Focus Arrangements

  • Focusing Cup: A component positioned to ensure accurate electron beam targeting toward the anode.

  • Filament for large focal spots and filaments designated for small focal spots are utilized based on operational requirements.

Thermionic Emission

  • Process: When the filament is heated to approximately 2200° C, it causes electrons to be released, known as thermionic emission.

  • After Emission: High voltage is applied, driving the electrons towards the anode for X-ray production.

Tube Failure Modes

  • Tube Arcing: A common failure mode, where vaporized tungsten deposits on the envelope leading to malfunction.

  • Filament Breakage: Typically occurs due to improper operation, such as holding down the exposure switch too long; users should press it fully down for accuracy.

Focusing Cup Specifications

  • Composition: Typically made of nickel or molybdenum; low negative potential is applied.

  • Function: Creates a biased environment that compresses the thermionic cloud of electrons.

  • Space Charge Effect: Maximum milliamperage (mA) output ranges from 1,000 to 1,200 mA, determined by saturation current and kV settings.

Anode Assembly

  • Functions of Anode:

    • Acts as a target surface for electron interactions.

    • Conducts high voltage required for operation.

    • Serves as the primary thermal conductor, dissipating heat generated during operation.

Stationary versus Rotating Anode

  • Stationary Anodes:

    • Made typically from copper with a tungsten target.

    • Utilized in systems for specific applications.

  • Rotating Anodes:

    • Constructed with a copper rotor to aid in rapid firing and cooling.

    • Focal Track Target: Typically a tungsten-rhenium alloy to sustain high instructional requirements.

Rotating Anode Assembly

  • Components:

    • Anode: Rotates to facilitate cooling.

    • Stator: Provides electromagnetic rotation force.

    • Composed of bearings for smooth operation.

    • Rotor: A copper cylinder that connects to the anode disk via a molybdenum stem, rotating at speeds between 3200-3600 rpm, up to 20000 rpm for high-speed anodes.

Stator Failure Points

  • Common Issues: Failure typically arises from malfunction or wear resulting in ineffective anode rotation.

Rotating Anode Properties

  • Material: Often a tungsten-rhenium alloy due to its high atomic number, excellent melting point, and superior heat-conducting properties.

  • Heat Generation: Focal track can reach temperatures of 1000-2000° C during regular use.

Anode Layering

  • Purpose: Aids with heat loading and transfer.

  • Construction: Typically backed with molybdenum or graphite to enhance thermal dynamics.

Normal Anode Wear

  • Monitoring: Operators need to inspect the anode for wear patterns to ensure efficient operation.

Warm-Up Procedure for Anodes

  • Objective: Gradually warms the anode to prevent cracking by relieving stress buildup and maintaining vacuum levels.

Anode Heat Loading Mechanism

  • Factors: Rotational speed in rpm directly influences heat dissipation efficiency.

  • Typical rpm ranges from 3200 - 3600 for standard use, with diameter of disk and material composition also affecting performance.

Target Area on Anode

  • Definition: The specific portion of the anode where the electron stream contacts, referred to variously as target, focus, focal point, focal spot, or focal track, which serves as the point source for X-ray photon generation.

Focal Spot Considerations

  • Impact on Anode Life: Larger focal spots enhance the anode's longevity but smaller focal spots are critical for achieving better image quality.

  • Resolution Strategy: The line focus principle is employed to reconcile the necessity of respective focal spot sizes.

Line Focus Principle

  • Definition: A technique used to control the effective focal spot size based on the geometry and angle of the actual focal spot.

  • Variables:

    • Actual Focal Spot Size: Defined by the target angle and electron beam size.

    • Effective Focal Spot Size: Controlled by these geometrical factors.

Anode Heel Effect

  • Description: Refers to the intensity variation of the X-ray beam emanating from the anode due to differences in thickness across the focal spot area.

  • Graphical Representation: 75%-100% of central ray intensity diminishes as it crosses the anode heel, showcasing the effectiveness of beam focus and alignment.

Important Reminders for Next Class

  • Date for Reference: Monday, Sept 22nd.

  • Books to Bring: Ensure to bring the 6th Edition of "Principles of Radiographic Imaging: An Art and a Science" by Richard R. Carlton, Arlene M. Adler, and Vesna Balac for in-depth review.