Key Concepts in Radiologic Physics

Basic Circuitry of X-ray Equipment

• X-ray Machine Circuits:

  • Primary circuit: Low voltage, includes autotransformer, timing circuit, etc.

  • High voltage circuit: Generates x-ray producing voltage.

  • Filament circuit: Heats filament to enable thermionic emission.

Primary Circuit Components

• Supplies to:

  • Autotransformer, high-voltage transformer, timing circuits, and other low-voltage requirements.

• Circuit Breaker:

  • Protects against overloads, easily reset.

• Timing Devices:

  • Various types include synchronous (1/20 sec to 20 sec), impulse, and electronic for precise control of exposure times.

High Voltage Circuit

• High-Voltage transformer:

  • Induces a high voltage essential for x-ray production, requires rectification for unidirectional flow into the x-ray tube.

• Exposure Switch:

  • Automatic control of secondary voltage and current.

• Milliammeter:

  • Measures and registers tube current during exposure.

Filament Circuit and Transformer Basics

• Heating Circuit:

  • Induced by a filament transformer to create thermionic emission from the x-ray tube’s cathode.

• Transformers:

  • Change voltage levels through electromagnetic induction; can be step-up, step-down, or isolation types.

Types of Transformers

• Step-Up Transformer:

  • Increases voltage for high-energy x-ray production.

• Step-Down Transformer:

  • Decreases voltage for heating the filament.

• Autotransformer:

  • Used for small adjustments in voltage, operates on self-induction principles.

Efficiency and Losses in Transformers

• Electrical Losses:

  • Copper losses (due to resistance), eddy currents, and hysteresis losses (due to magnetic domain changes).

  • Mitigated using larger wires, laminated cores, and higher permeability materials.

X-ray Tube Structure and Function

• X-ray Tube:

  • Contains cathode (electron source) and anode (target for x-ray production).

  • Various types including stationary and rotating anodes; rotating anodes provide enhanced heat dissipation.

Cathode and Anode Details

• Cathode:

  • Comprises the filament and focusing cup to direct emitted electrons.

• Anode:

  • Stationary or rotating, with tungsten target favorable for x-ray production due to its high atomic number and melting point.

  • Both types serve to dissipate heat generated during x-ray production.

Grid-Biased and Grid-Controlled Tubes

• Grid-Controlled Tubes:

  • Enable rapid switching of x-ray exposure beneficial for fluoroscopy and angiography; reduces motion artifacts.

X-ray Tube Rating Charts

• Tube Rating:

  • Indicates maximum safe exposure values to protect the tube from excessive heat and ensure operational integrity.

  • Includes considerations for effective focal spot and heat unit calculations based on voltage, current, and exposure time.