Week 10: Grids

Question 1

Grid

Answer 1

Device that improves image quality by removing scatter radiation before it reaches the image receptor. It requires an increase in technical factors that will increase patient dose and occupational exposure. Placed after patient but before IR.

Question 2

When to use a grid?

Answer 2

Body part over 10cm, technique over 60 kv.

Question 3

Linear Focused Grid

Answer 3

Strips are tilted progressively as they move away from the centre to match the beam divergence. The strips converge at a point known as the grid focus.

Question 4

Grid Ratio

Answer 4

The ratio of the height off the lead strips to the distance between the strips. Higher grid ratio = more effective at removing scatter but requires greater accuracy in positioning and more technical factors so there is a greater probability of having grid error due to misalignment.

Question 5

Grid Frequency

Answer 5

The number of grid lines per cm. Most common is 33-41 line/cm but can be from 25-80 lines/cm.

Question 6

Grid Radius

Answer 6

For a linear focused grid there is a line that can be extrapolated from each lead strips towards the tube that meet at the focal point. The grid radius is the distance from the grid to the focal point which coincides with the recommended SID. The x-ray tube must be located at the convergence lien to eliminate lateral grid cut off.

Question 7

What affects a grids effectiveness?

Answer 7

1. High selectivity: does the grid absorb a high degree of scatter while allowing most of the primary beam through.
2. High K Factor: is the grid actually good at removing scatter, how does it compare before and after the grid is used.
3. High Total Lead Content: higher lead content means it will reduce more scatter.

Question 8

Off Center Grid Error

Answer 8

Occurs when a tech does not align the CR with the centre of the grid. The greater the degree of decentering = the great the cut off. Decrease in exposure across the entire image (under exposed)

Question 9

Off Level Grid Error

Answer 9

Occurs when the tech angles the cassette or the CR across the grid. Results in undesirable absorption of the primary be a causing a decrease in exposure across the enter image, seen as quantum mottle.

Question 10

SID Grid Error

Answer 10

Occurs when a tech does not measure th SID which means they are imaging beyond o under the focal range of the grid. Results in grid cut off on only the peripheral edges of the image.

Question 11

Upside Down Grid Error

Answer 11

Occurs when tech places the grid upside down. Results in severe grid cut off on the peripheral edges of the image but the centre of the IR will have optimal exposure.