X-Ray Imaging Circuit- X-Ray Tube

Department of Medical Imaging

  • Course: Radiographic Science I (IMG1260)

  • Lecture 7: X-Ray Imaging Circuit - X-Ray Tube

  • Instructor: Khalid Al Mahruqi (Contact: khalid.rad.ihs@gmail.com)

Objectives

  • Upon completion of this chapter, students will:

    • Understand the brief history of x-rays.

    • Describe the general design of an X-ray tube.

    • List external components that house and protect the X-ray tube.

    • Identify the purpose of glass or metal envelope.

    • Discuss the cathode and filament current.

    • Describe components of the cathode, anode, and induction motor.

    • Define the line-focus principle and the heel effect.

    • Identify causes of X-ray tube failure.

Discovery of X-Rays

  • Definition: X-rays are a form of electromagnetic radiation emitted when matter is bombarded with fast electrons.

  • Key Event: Discovered by Wilhelm Roentgen on November 5, 1895, while working on cathode rays.

  • First Medical x-ray image: Produced in early 1896.

  • Awards: Roentgen received the first Nobel Prize in Physics in 1901.

  • Examples: Illustrated by Crookes Tube and image of Mrs. Roentgen’s hand.

Development of X-Ray Unit

  • First Medical X-Ray Examination: Conducted in the United States in 1896.

  • Notable Figure: Thomas Edison viewed the first x-ray of a hand.

Important Figures in X-Ray Studies

  • Johan Hittorf (1824-1914): Co-invented the Crookes tube.

  • Ivan Pulyui (1845-1918): Assembled designs for vacuum discharge tubes.

  • Nikola Tesla (1856-1943): Studied x-rays using a specially designed single-electrode tube.

  • Fernando Sanford (1854-1948): Generated and detected x-rays in vacuum tubes.

  • Philipp Lenard (1862-1947): Won Nobel Prize for discovering properties of cathode rays.

  • Wilhelm Röntgen (1845-1923): Systematically studied x-rays.

  • Thomas Edison (1847-1931): Developed the medically effective fluoroscope.

Properties of X-Rays

  • Wavelength & Frequency: Extremely short wavelength (10−8 to 10−12 meters) and high frequency (1016 to 1020 Hz).

  • Ionization: Capable of adding or removing electrons in atoms/molecules.

  • Photographic Film: Affect film like visible light by turning it black.

  • Absorption: Absorbed by lead or metal, transmitted by healthy body tissue.

  • Biological Effects: They may cause biological effects (health effects).

Producing X-Rays

  • Requirements for Production:

    • A source of electrons.

    • A force to accelerate them rapidly.

    • A mechanism to suddenly stop them.

X-Ray Tube Overview

  • Definition: A device for generating X-rays by accelerating electrons to high energies that strike a metal target.

Construction of the X-Ray Tube

  • Main Components:

    • Cathode: Source of electrons.

    • Anode: Target that releases x-rays.

  • Additional Features:

    • Tube support, evacuated tube envelope, protective housing, cooling dielectric oil.

X-Ray Tube Support

  • Support Systems Needed:

    • Ceiling Support: Features perpendicular rails for transverse and longitudinal movements.

    • C-arm Support System: Tube and receptor attached at different ends.

Design & Functionality of X-Ray Tube

  • Emission Characteristics: X-rays are produced isotropically, but only those emitted through a small window (5 cm²) are useful as the primary beam.

  • Leakage Radiation: Decreased to 1 mGy/h at 1 meter to minimize exposure.

  • Thermal Management: Oil circulates around the tube for insulation and dissipating heat.

Tube Envelope Differences

  • Materials:

    • Pyrex Glass: Absorbs heat but can develop gas, affecting production efficiency and leading to tube failure.

    • Metal Envelopes: Often used in modern tubes; less likely to develop issues related to gas and filament vaporization.

Cathode Design

  • Components:

    • Tungsten Filament:

      • High melting point (3410°C).

      • 1–2% Thorium added to enhance tube life.

    • Functionality: Electrons ejected through thermionic emission.

Filament Current and Emission

  • Initial Heating: Low current warms the filament without x-ray production.

  • Thermionic Emission: High filament current leads to emission and increased tube current.

  • Space-Charge Effect: Limits additional electron emission due to negative charge build-up.

Types of Anodes

  • Stationary Anode: Low efficiency, used in dental and portable x-rays.

  • Rotating Anode: Enhanced heat capacity; suitable for longer scans with improved x-ray production.

Anode Functions

  • Primary Roles:

    • Convert electronic energy into x-radiation.

    • Conduct electricity and dissipate heat (99% heat, 1% x-rays).

Anode Heel Effect

  • Description: Higher x-ray intensity on the cathode side due to electron travel distance.

  • Usage Strategy: Position thicker body parts on the cathode side for optimal imaging.

Extrafocal Radiation

  • Challenge: Occurs when electrons interact outside the target area, leading to image quality reduction.

  • Mitigation: Use of diaphragms and metal enclosures to minimize stray radiation.

Tube Failure Causes

  • Heat Production: Excessive heat leads to several issues including anode surface melting and filament vaporization.

  • Preventative Measures:

    • Minimum exposure settings.

    • Efficient heat dissipation systems.

    • Warm-up the tube before heavy exposures.

Exposure Factors

  • Primary Factors:

    • Tube Voltage (kVp): Ranges from 45 to 120 kVp; influences penetration and image contrast.

    • Milliampere (mA): Ranges from 10 to 1200 mA; controls the amount of x-ray photons produced.

    • Time: Duration of exposure (0.001 to 6 seconds).

Summary of Key Concepts

  • Density & Contrast: Controlled primarily through kVp and mAs for optimal image quality.

References

  • Recommended Texts:

    • Bushong, C. (2013). Radiologic Science for Technologists.

    • Ball, J., Moore, A., & Turner, S. (2008). Essential Physics for Radiographers.

    • Carter, P.H., et al. (1996). Chesney’s Equipment for Student Radiographers.

Assignment Questions

  • Discuss properties of tungsten in the imaging system.

  • Explain why modern x-ray tube designs minimize issues like arcing and failure.

  • Describe the types and functions of anodes.

  • Define the heel effect and its advantageous use in x-ray imaging.