Chapter 6 Lecture Part Two

Exposure Timers

Definition: The duration of time X-rays are produced, directly influencing the quantity of X-rays that reach the image receptor.Key Concepts:

• Determines the quantity of X-rays based largely on the control of tube current.

• Technologists initiate the exposure procedure, while timers assume the responsibility for terminating it at appropriate intervals.

• Various exposure time selections include options for both short and long durations to suit different imaging requirements.

Types of Exposure Timers:

1. Mass Timer:

   • Monitors mass in complex systems utilizing falling load or capacitor discharge methods; essential for optimizing the quality of X-ray images while minimizing unnecessary patient exposure.

2. Synchronous Timer:

   • While functional, it is not considered ideal for scenarios involving multiple exposures as it requires manual resetting before each exposure.

3. Electronic Timer:

   • The most accurate and sophisticated type, these timers are controlled by microprocessors, which allow for precise timing and enhanced automation in imaging procedures.

4. Automatic Exposure Control (AEC):

   • This system shuts off exposure after a predetermined radiation level is achieved. It employs an ionization chamber situated between the patient and the image receptor, effectively detecting radiation levels to terminate exposure at the optimal moment, thus minimizing patient dose. It includes a backup timer, which is usually set to 1.5 times the expected amount to prevent continuous exposure.

High Voltage Generator Components

Three Main Parts:

1. High Voltage Transformer:

   • Steps up voltage to the levels necessary for effective operation of the X-ray tube, ensuring sufficient energy is available for photon production.

2. Filament Transformer:

   • Steps down voltage to approximately 12 volts, which is crucial for increasing filament current and promoting electron emission that will enable X-ray generation.

3. Rectifiers:

   • Critical components that convert alternating current (AC) into direct current (DC), allowing the electric current to flow in one direction, essential for the functioning of the X-ray tube.

Rectifiers and Current Rectification:

• Purpose: The primary function of rectification is to convert AC to DC, which is necessary for consistent X-ray production.

• Types of Rectification:

  1. Half Wave Rectification:

     • Utilizes 1 to 2 diodes, permitting only the positive current through, resulting in power loss and extended exposure times.

  2. Full Wave Rectification:

     • Requires a minimum of 4 diodes; harnesses both halves of the current wave, improving system efficiency and significantly reducing imaging technique time.

• Voltage Ripple:

  • Measured by the percentage of fluctuation in voltage, its reduction is crucial for achieving high-quality images with minimal artifacts.

Phases of Power Supply

• Single Phase:

  • Provides limited energy, which is generally insufficient for most diagnostic imaging applications due to a lack of consistency in energy provision.

• Three Phase:

  • Exhibits a more consistent energy output and minimizes gaps in current flow, resulting in better imaging quality and improved efficiency.

• High Frequency Generators:

  • Generate near-constant current, enhancing image quality significantly while also reducing patient radiation dose. They are often smaller, making them ideal for utilization in contemporary imaging systems such as CT scans and mammography, incorporating inverter circuits for improved efficiency.

Additional Generator Types

1. Capacitor Discharge Generator:

   • Notable for its dropping voltage during exposure, which can limit overall exposure quality; risk of ‘dose creep’ where higher settings are required arises with this type.

2. Falling Load Generator:

   • Designed for prioritizing short exposure times, particularly in high-capacity imaging contexts such as interventional radiology, this generator type improves workflow and efficiency in busy clinical settings.

Summary of Key Points

• Transformers: Typically rated in kilowatts, with most medical imaging systems operating in the range of 30-50 kW to ensure sufficient power delivery for X-ray generation.

• Purpose of Circuits:

  • Control Console (Primary Circuit): Initiates the current and manages various settings necessary for operation.

  • High Voltage Section (Secondary Circuit): Responsible for rectifying power and supplying it to the X-ray tube effectively.

  • Filament Circuit: Provides the essential electrons needed to generate X-rays during the exposure process.

• Ensure a comprehensive understanding of how each of these components interacts within the X-ray system to facilitate effective imaging procedures.