Automatic Exposure Control (AEC)

Automatic Exposure Control (AEC)

• An AEC determines the exposure time based on the body part of interest.
• Ionization chambers activate when selected at the console.
• These chambers are located in the Bucky, between the patient and the image receptor.
• X-rays interacting with the ionization chamber ionize atoms inside, creating an electrical charge.
• This charge then signals the termination of the exposure.
• Chambers are calibrated during installation using phantoms, ensuring appropriate timing.
• AEC helps control exposure and adjusts for varying patient sizes.
• Larger anatomical parts require longer chamber ionization times.
• Smaller anatomical parts require shorter chamber ionization times.
• Exposure length is determined by the thickness and density of the imaged anatomical part.
• Accurate centering and collimation is a must.

Experiments

• Increasing the SID from standard when using an AEC causes the device to take longer to terminate the exposure.
• Increasing the SID allows for increased collimation, resulting in less beam divergence.
• Increasing mAs alone leads to an increase in dose.
• DR equipment prefers exposures with higher kVp and lower mAs due to sensitivity to radiation levels.

Dynamic Range

• Digital detectors have better dynamic range.

Understanding Grids

• Grids absorb scatter radiation.
• They are used for body parts thicker than 10cm.
• kVp values higher than 60 require the use of grids.
• Removing scatter increases contrast in the image.

DR and Scatter Considerations

• Digital image receptors are sensitive to scatter radiation due to their ability to detect low radiation levels.
• Scatter is displayed as fog over the image, reducing contrast.

Factors Influencing Scatter

• Increasing kVp or mAs increases scatter.
• Higher energy photons are more likely to transmit without Compton interactions.
• Increasing both kVp and mAs increases the number of lower energy photons, making Compton interactions more likely.
• Thicker body parts necessitate higher kVp and mAs.
• Scatter is a result of Compton interactions

Ways to Reduce Scatter

• Increase collimation.
• Reduce the number of photons reaching the patient.
• Decrease the size/thickness of the body part angled.
• Add a grid.

Grid Usage

• Balancing patient protection and image quality is key when deciding to use a grid.
• Reducing scatter improves image quality but requires an increase in mAs.
• Consider using a grid for body parts more than 10cm thick, such as a knee and shoulder.

Anti-Scatter Grids

• When grids are used, mAs needs adjustment to maintain IR exposure.
• More efficient grids require greater mAs adjustment.
• Exposure values at the control panel are designed for with or without grid usage.
• It is essential to understand when to use/not use a grid.
• AEC and grid are normally used together but it is non essential.
• Grids can be used without the AEC and vice versa.
• Mobile/trolley work has loose grids

Grid Ratios

• Grids have lead strips/lines with precise height, thickness, and space between them.
• Grid frequency is the number of lines per unit length (lines/cm).
• Grid ratio is the ratio of the height of lead strips to the distance between them.
• A high grid ratio removes more scatter compared to a low grid ratio but requires higher mAs.
• Grid Ratio = h/D, where h = height of lead strips and D = distance between them.
• Grid ratios range from 4:1 to 16:1.

Grid Types

• Linear grid.
• Crossed grid / cross-hatched grid.
• Focused grid – focal distance.
• Non-focused grid.

Stationary and Oscillating/Reciprocating Grids

• Moving the grid slightly during the exposure removes the grid lines.
• Grids in the Bucky are oscillated during exposure to blur the grid lines.

Grid Calculations

• Multiply the non-grid mAs value by the grid conversion factor.
  • No grid is a factor of 1
  • 5:1 is a factor of 2
  • 6:1 is a factor of 3
  • 8:1 is a factor of 4
  • 12:1 is a factor of 5
  • 16:1 is a factor of 6

Grid Cutoff

• Grid cutoff can happen if the grid is misaligned with the x-ray beam.
• Using a grid with an AEC may reach the maximum cut-off due to grid cutoff which can increase patient dose.
• Lacking the AEC, grid cutoff leads to a noisy image and does not allow enough photons to reach the IR.
• Errors leading to potential for grid cutoff:
  • Upside down focused grid.
  • Off-level - beam angled across the strips.
  • Off-center - beam not aligned with the center of a focused grid.
  • Off-focus - beam not at the correct distance for a focused grid.

Grid Guidance

• Parts thicker than 10cm may warrant grid use.
• Grids may be needed when kVp values are higher than 60.
• Consider a grid when enough scatter is produced to improve image quality.
• A high grid ratio removes more scatter but will require a higher mAs.
• Increasing collimation will reduce scatter.