Ch. 5 X-Ray Tube RAD 145

Chapter 5: The X-Ray Tube

Overview

  • X-ray tubes are essential for radiographic imaging.

  • Work on the principle of electron flow through a vacuum.

Production of X-Rays

  • Components:

    • Source of Electrons: Cathode filament.

    • Target: Tungsten anode.

    • Operates under High Voltage conditions.

Tube Components

  • Typical Rotating X-Ray Tube:

    • Diagram with various components.

Cathode Assembly

  • Filament:

    • Coiled tungsten wire in a focused cup.

    • Low voltage side of the circuit.

    • Source of electrons; negative side of x-ray tube

Filament Details

  • Specifications:

    • Coil of thoriated tungsten dimensions:

      • 0.1-0.2 mm thick, 1-2 mm wide and 7-15 mm long.

    • Tungsten chosen for its high melting point (3,370°C) & difficult to vaporize.

Filament Material

  • Materials Used:

    • Primarily tungsten for its high melting point and low vaporization.

    • Alternatives include rhenium and molybdenum.

Thermionic Emission

  • Process:

    • Filament heating releases electrons similar to incandescent light bulbs.

    • Cathode filament is in low temperature, pre-heat mode until exposure is initiated.

Tube Failure

  • Common Issues:

    • Tube arcing

      • Happens when there is a short-circuit causing disruption; loss of exposure from x-ray output

    • Filament breakage

      • Repeated tube “boosting”; due to burn-out are caused by high operating temperatures;

    • X-ray Tube lifespan typically between 10,000-20,000 exposures.

Focusing Cup

  • Function:

    • Made of nickel with low negative potential.

    • Compresses the thermionic cloud.

    • Controls exposure by limiting current mA to 1000-1200 max.

Anode Assembly

  • Functions:

    • Target surface for x-ray production; positive side of x-ray tube.

    • Conducts high voltage; maintains closed circuit.

    • Acts as a primary thermal conductor (heat; high voltage)

    • Components: Anode, Stator & Rotor

Rotating Anode

  • Material:

    • Tungsten-rhenium alloy known for high atomic number (Z# 74).

    • Benefits include high melting point and efficient heat conduction.

    • High atomic number, High melting point & Heat-conducting ability

Anode Layering

  • Purpose:

    • Enhances heat loading capacity with molybdenum or graphite backing.

    • Increases thermal capacity of x-ray tube.; Warm-Up Procedure

  • Warm Up Procedure:

    • Warms the anode; Prevents cracking.

    • Specific routines depending on manufacturer (for idle periods > 2 hours).

Target Area

  • Definition:

    • Portion of anode contacted by electron stream (also called target, focus, focal point, spot or track).

    • Point source of x-ray photons

Anode Heat Loading

  • Definition: Heat loading a function of exposure factors.

  • Factors:

    • Influenced by exposure factors, anode rotations, and target materials.

    • Rotating anode: increase heat capacity significantly, revolutions per minute (rpm) & diameter of disk.

    • Larger the anode, the faster it spends; dissipating more heat.

Line Focus Principle

  • Understanding:

    • Actual Focal Spot: Physical area of focal track being hit by electrons.

      • Small focal point is mA limited; 25-100 mA

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

      • Controlled by Actual focal spot size & target angle

    • Important for image spatial resolution.

    • Smaller the focal spot; better the detail.

Anode Heel Effect

  • Mechanism:

    • Variation of x-ray beam intensity due to anode geometry.

    • More intensity under cathode side, with a possible variation of up to 45%.

    • Cathode to Anode axis (needing more thickness in certain area of the anode to compensate for the increased intensity on the cathode side, leading to better image quality and reduced patient exposure.)

Stator

  • Definition: Induction motor electromagnets comprise the stator that turns the anode at high speeds necessary for effective x-ray production.

  • Components:

    • Copper windings and electromagnets affecting anode speed.

    • Located outside envelope

Rotor

  • Definition: Rotates the anode, allows heat generation during production of X-rays; enhancing the tube's efficiency and longevity.

  • Composition:

    • Copper cylinder connected by molybdenum stem

    • Rotates at high speeds (3,000-10,000 rpm)

    • Inside x-ray tube glass envelope

Anode Rotation Considerations

  • Operation ranges: Standard (3,200-3,600 rpm) and High speed (10,000-12,000 rpm).

    • High-speed rotations induce stress; Gyroscopic effect

    • Abnormal sounds may indicate early warning of tube failure.

Envelope

  • Functions

    • Heat tolerant to Pyrex glass or metal

    • Supports anode/cathode assemblies

    • Maintain a vacuum

  • Tube Window

    • Area of x-ray beam exit to receptor

  • Vacuum

    • Near perfect and critical to efficient x-ray production

Protective Housing

  • Functionality:

    • Support for x-ray tube, controls leakage and scatter radiation.

    • Lead lining for safety and cooling mechanisms.

    • Isolates high voltages.

Control of Radiation

  • Housing Compliance:

    • Limit for leakage radiation set at 100 milliroentgens (mR/hr) at 1 meter.

High-Voltage Isolation and Tube Cooling

  • Methods:

    • Dielectric oil for insulation and cooling; air whisper fan aid in cooling.

Off-Focus Radiation

  • Impact:

    • Contributes to image ghosting and reduces quality; blocked by collimator.

    • Vacuum is causing the electrons to go from negative to positive.

    • Electrons or photons that didn’t follow the vacuum tube; causing off-focus radiation.

Rating Charts and Cooling Curves

  • Purpose:

    • Protect x-ray tube and lengthen tube life; prevents overheating via calculations.

    • Tube rating charts; Anode cooling curves; Housing cooling curves.

Calculation of Heat Units

  • Formula:

    • kilovoltage peak (kVp) × milliamperage (mA) × time × rectification constant.

    • Rectification constant takes electrical waveform

      • Single Phase

      • High Frequency