Grids

• Grids use thin lead strips to absorb scatter radiation before it strikes the image receptor (IR), improving image quality.
• They enhance image contrast.
• Lead strips in grids are spaced out with aluminum or plastic in between.

Grid Classification

• Parallel Grids:

  • Lead strips (grid lines) are in parallel formation.
  • The distance between strips is equal at the top and bottom.
  • A problem with parallel grids is that while grid lines are parallel, x-ray beams are diverging.
  • Photons on the outer edges of the beam can be obstructed by outer grid lines, creating an artifact called grid cutoff.
  • Grid cutoff results in portions of the IR receiving lower exposure because grid lines absorb portions of the useful beam.

• Focused Grids:

  • Feature angled lead strips that match the divergent beam.
  • Strips are focused at the focal spot of the x-ray tube.
  • Focused grids eliminate grid cutoff.
  • Photons are not obstructed by outer grid lines.
  • Designed for a specific Source-to-Image Distance (SID).
  • The degree of angulation depends on the distance the grid is designed for, either 40 or 72 inches.
  • Using the wrong focused grid results in cutoff at the edges of the image.

• Linear Grids:

  • Have one set of lead lines that run in the same direction through the grid.
  • Absorb some scatter but not all.
  • If a scattered photon is deflected on a path parallel to the gridlines, it can pass through and strike the IR.
  • Linear grids only capture scatter in one direction.

• Crossed Grids:

  • There are two sets of lead strips perpendicular to each other.
  • Highly effective at scatter absorption.
  • Must be perfectly positioned to avoid grid lines.
  • Cannot be used when the x-ray beam needs to be angled.
  • Not a common grid used in radiography.

• Stationary Grids:

  • Do not move during exposure.
  • Used on tabletop or portables.
  • Absorb scatter radiation and some of the useful beam.
  • Lead lines can appear, creating a grid line artifact.

• Moving Grids:

  • An x-ray machine rapidly oscillates or vibrates the grid during exposure.
  • Grid lines are blurred out.
  • Very common in radiography and only used with x-ray bucky systems (table/wall).

• Virtual Grids:

  • Are not physical grids; they have no lead strips.
  • No need to increase patient dose due to absorbing lead.
  • Uses a processing algorithm used by the computer to remove the effect of scatter from the image.
  • Results in less image noise, increased image contrast, and increased image quality.

• Grids must be used correctly to avoid errors in the image.

• All physical grids absorb a portion of the useful beam.

• The operator must increase patient dose to maintain appropriate receptor exposure.

Introduction to Grids

• Grids use thin lead strips to prevent scatter from striking the IR, improving image quality by increasing image contrast.
• Grids are necessary to manage scatter radiation.
• Absorbed and transmitted photons are good for image quality.
• Scattered photons decrease image contrast and create noise, obstructing visibility in the image of the patient's anatomy and pathologies.
• Grids remove scatter from the remnant beam before it strikes the IR.
• Constructed from very thin vertical lead strips spaced apart by radiolucent material (aluminum or plastic).

Grid Ratio

• Grid efficiency is judged by grid ratio.
• Grid ratio is the height of the lead strip compared to the distance between them.
• For example, lead strips 2 mm high with 1 mm of distance between each strip have a grid ratio of 2:1.
• There is a direct relationship between grid ratio and grid efficiency.
• As grid ratio increases, scatter reduction increases, and contrast improves.
• The choice of grid ratio depends on body part size, patient size, kVp setting, and patient dose.

Grid Usage

• Using a grid requires an increase in exposure technique.
• Grids reduce receptor exposure.
• mAs must be increased to maintain appropriate receptor exposure when using a grid.

Grid Conversions

• The larger the grid ratio, the larger the increase to technique.