Scatter Control & Grids

Collimation effect on scatter

Decrease

kVp effect on scatter

Increase

Grid effect on scatter

Decrease

What do the top collimator shutters do?

Decrease off-focus radiation

What do the bottom collimator shutters do?

Decrease penumbra

What is off focus radiation?

Photons produced outside of the focal spot

What is penumbra?

Geometric unsharpness around the periphery of the image

When collimation is increased, mAs must:

increase

What is the drawback of using a grid?

Must increase mAs = increase patient dose

When should a grid be used?

• Patient thickness greater than 10 cm

• kVp greater than 70

Grid frequency

Number of lead lines per unit length

Grid ratio

Lead line height to distance between them

Higher the grid ratio, ___ scatter absorption

Higher

Grid type

Focused

Grid type

Parallel

Focal range

The recommended range of SID compatible with a focused grid

Positioning lattitude

The range you can position the grid without cutoff

Grid conversion factor formula

GCF = mAs with grid/mAs without grid

Name the grid conversion factor for the following:

Name the 5 grid errors

• Upside-down focused grid error

• Off level error

• Off center error

• Off focus error

• Moire effect

What is this grid error?

Upside-down focused

What is this grid error?

Off-level

Describe the off level grid error and what causes it?

Overall decrease in exposure, tube and grid are not at the same angle

Describe the off center grid error and what causes it?

CR is not aligned with grid, loss of density across image

What grid error is this?

Off center

What grid error is this?

Off focus

Describe the off center focus error and what causes it?

SID is outside of recommended range, loss of exposure around periphery of image

Describe the moire effect and what causes it?

Zebra pattern when laser scanning lines are parallel to grid lines

What grid error is this?

Moire effect