Grids

Question 1

Why are grids used

Answer 1

When higher voltage used for thick tissues some x-rays undergo changes and emerge as scatter
Results in image blur

Used with areas >10cm

Question 2

Describe grid construction (general)

Answer 2

Flat plate with lead strips separated by radiolucent material
Lead strips absorb scatter

Question 3

Describe parallel grid construction

Answer 3

Vertical parallel strips

Edge of beam may also be absorbed along with scatter resulting in cut-off

Question 4

Describe focussed grid construction

Answer 4

Strips angled to correspond with divergence of x-rays

Prevents cut-off

Question 5

Describe pseudo-focussed grid construction

Answer 5

Vertical strips shorten towards edge

Reduced absorption and cut-off

Question 6

Describe moving grid construction

Answer 6

Strips move slightly during exposure
Prevents parallel lines on final image
Req. connection between machine & grid (PBS)

Question 7

Describe crossed grid construction

Answer 7

Strips running in both directions

Greater scatter absorption

Question 8

How do grids affect exposure settings

Answer 8

Exposure time must be increased due to absorption by grid

mAs must be multiplied by grid factor to counteract potential under-exposure