Xray production

Chapter 5: The X-Ray Tube

Overview

  • Origin of content: Carlton/Adler/Balac, Principles of Radiographic Imaging: An Art and a Science, 6th Edition.

  • Focus: Structure, function, and operational principles of the x-ray tube.

Production of X-Rays

  • Source of Electrons: Cathode filament.

  • Target: Tungsten anode.

  • High-Voltage: Essential for electron acceleration.

  • Vacuum: X-ray tubes operate within a vacuum to facilitate electron flow.

Tube Components

  • Diagram of the typical rotating x-ray tube.

Cathode Assembly

  • Major Components:

    • Filament: Coiled tungsten wire helix.

    • Focusing Cup: Directs electrons towards the anode.

    • Associated Wiring: Ensures electrical connectivity.

    • Operates on the low voltage side of the x-ray circuit.

    • Serves as the source of electrons.

Filament Details

  • Shape and Size:

    • Coil of thoriated tungsten: 0.1–0.2 mm thick, 1-2 mm wide, 7-15 mm long.

  • Impact: Filament length and width affect recorded detail.

  • Material Properties: Tungsten chosen for its high melting point (3,370°C).

Filament Material

  • Preferred Materials:

    • Tungsten: Excellent properties for x-ray production.

    • Alternatives: Rhenium and molybdenum.

Thermionic Emission

  • Process: Filament heat release of electrons akin to incandescent bulbs.

  • State Before Exposure: Cathode filament is in a low temperature, pre-heat mode.

Tube Failure Types

  • Common Issues:

    • Tube arcing: Vaporized tungsten collecting on envelope.

    • Filament breakage from repeated boosting.

  • Longevity: Most modern x-ray tubes last 10,000-20,000 exposures.

Focusing Cup

  • Material: Nickel.

  • Function:

    • Low negative potential applied to compress thermionic cloud.

    • Biased to enhance focusing.

    • Space Charge Effect: Limits exposure milliamperes to 1000–1200.

    • Saturation Current: Typically managed by x-ray circuitry.

Anode Assembly (1 of 2)

  • Functions:

    • Target surface for x-ray production.

    • Conducts high voltage and maintains closed-circuit pathway.

    • Primary thermal conductor.

Anode Assembly (2 of 2)

  • Components:

    • Anode, Stator, Rotor.

Rotating Anode

  • Material: Tungsten–rhenium alloy, high atomic number (Z# 74).

  • Advantages:

    • High melting point and excellent heat conduction properties.

Anode Layering

  • Purpose: Assists with heat loading.

  • Construction:

    • Backing with molybdenum/graphite acts as a heat sink to increase thermal capacity.

Warm-Up Procedure

  • Importance: Gradually warms the anode to prevent cracking and maintain vacuum.

  • Recommendation: Perform if tube has been idle for over 2 hours, varying by manufacturer.

Target Area

  • Description: Portion of anode that the electron stream contacts, also called:

    • Target, Focus, Focal point, Focal spot, Focal track.

  • Function: Acts as the point source of x-ray photons.

Anode Heat Loading

  • Factors: Influenced by exposure factors and rotational speed of the anode.

    • The design can increase heat capacity by a factor of 300.

Line Focus Principle

  • Concepts:

    • Actual Focal Spot: Physical area hit by electrons.

    • Effective Focal Spot: Projected area of x-ray beam.

  • Controlled by size of actual focal spot and target angle.

Actual Versus Effective Focal Spot

  • Capacity: Small actual focal spot limits mA (typically 25–100 mA).

  • The effective focal spot is always smaller than the actual focal spot, affecting spatial resolution.

Anode Heel Effect

  • Definition: Variation in x-ray beam intensity across its axis due to anode design.

  • Intensity Variance: Can vary by as much as 45%, more intense under the cathode side.

  • Placement of body parts can mitigate the heel effect.

Stator

  • Composition: Copper windings and electromagnet setup outside the envelope.

  • Failure Consequences: Stator failure affects anode rotation speed.

Rotor

  • Design: Features ferromagnetic bars in a cylindrical pattern inside the tube.

  • Connection: Copper cylinder and molybdenum stem to the anode disk, enabling rotation of 3,000-10,000 rpm.

Anode Rotation Considerations

  • Types: Standard speed vs. high speed (3,200–12,000 rpm).

  • High-speed rotations add stress; observe for unusual sounds as an alert for potential failures.

Envelope

  • Material: Heat-tolerant Pyrex glass or metal, functions to support components and maintain vacuum.

  • Dimensions: Typically 10 inches long with varying diameters; the window allows x-ray exit.

Protective Housing

  • Functionality: Supports the x-ray tube, controls leakage/scatter radiation, provides cooling mechanisms including oil, fans, or water cooling.

Control of Leakage and Scatter Radiation

  • Material: Lead-lined cast steel.

  • Safety Limit: Exposure limit of 100 milliroentgens per hour (mR/hr) at 1 meter.

High-Voltage Isolation and Tube Cooling

  • Cooling Mechanisms: Dielectric oil for insulation and cooling, supplemented by air fans.

Off-Focus Radiation

  • Impact on Imaging: Undesired radiation contributes 25% of primary beam, affects image quality, producing​ ghosting.

Rating Charts and Cooling Curves

  • Purpose: Protect x-ray tube and extend its life through proper management of exposure.

Anode Cooling Curves

  • Cooling Behavior: Nonlinear, with initial rapid cooling slowing over time.

  • Calculation: Required for managing anode heat units, often integrated into generator controls.

Calculation of Heat Units

  • Formula: kilovoltage peak (kVp) × milliamperage (mA) × time × rectification constant (varies with phase).

  • Integration: Typically built into manufacturer's software to avoid detrimental exposure settings.

Closing

  • Thank you for your attention!