Ch. 18 Carlton

Grids

• Device to remove scatter radiation before it reaches the film, improving image contrast.

• Sequence of grids: X-ray beam, Patient, Grid, Image Receptor or photoelectric stimulated plate.

• Criteria for use:

  • Body part thickness more than 10cm.

  • kVp is above 60 (reality above 70).

  • Air gap = 90 kv or higher

  • Chest x-ray = 90 kv or higher

Grid Materials

• Radiopaque strips (lead) alternate with radiolucent interspace strips (aluminum or plastic fiber).

• Radiopaque strips absorb scatter radiation.

  • positive (barium, lead)

• Radiolucent interspaces allow radiation to pass through.

  • negative (air)

Grid Ratio

• Grid ratio = H/D (height divided by distance)

Grid Frequency

• Higher grid ratio means the more scatter is removed

• 8:1 grid is the thinner grid and is typically used for low to moderate scatter situations, making it suitable for general radiographic imaging.

• 16:1 grid is the thicker grid and is used in high scatter environments, such as in fluoroscopy or when imaging thicker body parts.

• Lead content of grid increases & the ability of the grid to remove scatter and improve contrast also increases

Grid Patterns

• Linear (parallel & focused)

• Criss-Cross

• Focused grids match x-ray beam divergence.

• Parallel grids best used with long SIDs.

Grid Errors

• Off-Level: Grid not aligned to bucky (cut-off).

• Off-Center: Grid not centered.

• Off-Focus: Grid lines are blurred or distorted.

• Upside-Down: Grid appears rotated 180 degrees from the intended orientation.

• Moire Effect: Occurs with digital IR systems when grid lines are captured parallel to scan lines in image plate reader

  • occurs with stationary grids that are not perfectly aligned with the imaging system, leading to interference patterns that can obscure the intended details in the image.

Tube/Grid Alignment

• Proper alignment prevents grid cutoff.

• Tube must be centered with focused grid at correct distance.

• All grids have tube and receptor sides.

• Decreases exposure and results in lighter, fogged films with lessened detail

Air Gap Technique

• Alternative to reduce scatter by increasing OID, allowing less amount of scatter to actually hit the IR be reduced. Also magnification can occur.

• Use of the grid increases patient dose and mAs.

Grid Factor Conversion Formula

• mAs1/mAs2 = Grid Factor1/Grid Factor2