Chapter 18 - The Grid PowerPoint

Increased kVp leads to

Increased compton scatter (reason for grid)

Causes of Scatter Radiation

• High kVp (increased compton interactions)

• Patient thickness

• increased field sizes

How to decrease field size

increase collimation

Radiographic Grid

device that reduces the amount of scatter radiation reaching the image receptor

Where is a radiographic placed?

on top of or in front of the image receptor

Main goal of using a grid

improve image contrast (limiting scatter to IR)

Grids are made of

Lead strips alternating with interspace material

Grid interspace material options

• carbon fiber

• aluminum

• plastic (never lead)

When are grids recommended?

• size of patient part is 10cm or thicker

• 60 kVp+

Disadvantages of grid use?

increased patient dose (grid requires increased mAs)

Moving Grid or Reciprocating Grid - Table or Wall Bucky

Motion blurs out lead strips shadows by using vibration for the length of the exposure

Interspatial material has the quality of being

radiolucent

lead strips have the quality of being

radiopaque

Grid ratio

H/D

Commonly used type of grid

Parallel (linear) Grid

Rarely used type of Grid

Crossed Grid

Focused Grids have a specific

focal distance (SID)

SID range for Focused Grids

60'' - 72''

Angled lead strips allow

grid to match the diverging x-ray beams, thus increasing the grid’s ability to catch scatter

The relationship between the height of the lead strips and the distance between them

Grid Ratio

Higher the Grid Ratio

• cleans up more scatter

• higher the grid efficiency

Grid Frequency

measure of the number of grid lines per inch (or centimeters)

Grid Ratio

• No Grid

Grid Factor

• 1

Grid Ratio

• 5:1

Grid Factor

• 2

Grid Ratio

• 6:1

Grid Factor

• 3

Grid Ratio

• 8:1

Grid Factor

• 4

Grid Ratio

• 10:1

• 12:1

Grid Factor

• 5

Grid Ratio

• 16:1

Grid Factor

• 6

Most common error in use of grids

improper positioning

Off-Level Grid Error

• Occurs with a straight beam but tilted grid

• Overall decrease in receptor exposure

Off-Focus Error

• Incorrect (Insufficient) SID used with a focused grid

• Grid cutoff toward the edge of image

Off-Center Error

• Central ray of beam is off center from the center of a focused grid

• uniform loss of exposure across entire image

Upside Down Grid Error

severe grid cutoff on both sides of image

Moire Pattern

• wavy artifact caused by grid lines running parallel to the laser scanning motion

• error with computed radiography (cr) cassettes

What causes a Moire Pattern?

wrong orientation use of grid

Air Gap Technique

alternative method of reducing scatter radiation reaching image receptor

When can air gap technique be used?

in place of grid if one is not available

How far away is image receptor placed from patient using air gap technique?

10 - 15cm