anti scatter grids

Question 1

what does scattering do ?

Answer 1

degrades images, reduces contrast and increases the noise of the image

Question 2

how can we reduce the amount of scatter produced ? (in terms of collimation and compression)

Answer 2

-Minimising volume of tissue in primary beam by utilizing Collimation, Compression

-for example, in mammography the breast is compressed to produce a much higher image quality.

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Question 3

how can we reduce the amount of scatter produced ? (in terms of kv, compton)

Answer 3

• Keeping kV low – less chance of Compton interactions
• however, this will Increase absorbed dose (remember we also need to make sure our kv is high enough to penetrate the patient)

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Question 4

how can we reduce the amount of scatter produced ? (in terms of absorbing scatter)

Answer 4

-Lead-glass screens & Lead rubber aprons* (radiation protection)

• anti scatter grid- absorbs scatter increasing image quality, without interfering with the main beam.

Question 5

what is the anti scactter grid and where is it paced ?

Answer 5

-x-ray tube > patient > SCATTER GRID > image receptor

• prevents scatter reaching the image receptor DOES NOT REDUCE SCATTER

Question 6

what is the scatter grid made out of ?

Answer 6

• anti sactter grids are made up of lead strips with interspace material
• lead is very dense so able absorb x-rays
• interspace material is very low density

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Question 7

how does the scatter grid work?

Answer 7

• x- ray photons from primary bean travel perpendicular to the scatter grid so travel straight through the interspace material
• scatter travels in a different direction to the primary bean, (different angle)
• ## as it travels at an angle, the photons find it much more difficult to get through the spaces between the lead strips as it will interact and get absorbed by the lead strips .

Question 8

what is the problem with parallel grids?

Answer 8

geometric cut off-
as you get towards the edges of the grid, the grid absorbs the primary beam

Question 9

what is the difference between a parallel grid and a focused grid ?

Answer 9

the lead strips on a focused grid are angled towards the centre

Question 10

why is focused grid favoured over parallel grid ?

Answer 10

due to the lead strips being angled it allows for the divergent beam to be let through all the way to the edge

Question 11

explain what the apparent/grid focus is ?

Answer 11

if you extend the all lead strip angles all the way they should all meet at the same point.

This is the point at which the x-ray beam should be placed.

Question 12

what does the SID need to correspond with ?

Answer 12

apparent focus

Question 13

what must the central beam be parallel to ?

Answer 13

vertical axis of the lead strips

Question 14

where do you usually find focus grids and give examples ?

Answer 14

• Usually found in fixed units
• Table and erect bucky
• C-arm intensifiers

Question 15

what can occur the central beam is not parallel to the vertical lead strips

Answer 15

gradient appearance:

-as some photons will get absorbed on one side of the grid by the lead strips the others will pass through

-causing a big exposure on one side and all small exposure on the other side.

Question 16

what is a weakness with stationery grids ?

Answer 16

can cast image of slats unto receptor

Question 17

advantages of moving grids

Answer 17

• Improved image quality
• Thicker lead slats
• More scatter removed
• Improved efficiency

Question 18

disadvantages of moving grid

Answer 18

• Increased object to image distance
• Unit not transportable
• More expensive

Question 19

name 2 grid like vendors

Answer 19

SkyFlow, SmartGrid

Question 20

what do these grid like vendors do

Answer 20

Create a grid-like image when no grid was used

Question 21

how do grid like vendors work (to our knowledge)

Answer 21

• Post-processing feature manipulates the image histogram
• Generally it increases the contrast of the image

Question 22

when do grid like vendors work better

Answer 22

Generally accepted that it works better on chest over abdominal imaging

Question 23

what doesnt grid like vendors do and why is this good (hint mas)

Answer 23

Doesn’t require an increase in mAs, therefore dose saving

Question 24

what is the most common ratio we use for grids

Answer 24

8:1

Question 25

which ratio requires first class alliangment

Answer 25

16:1

Question 26

what are the 4 things that affect the Ability to remove scattered radiation with minimal loss of useful primary beam ? (In terms of the grid)

Answer 26

• Grid ratio
• Thickness of lead slats
• Type of grid
• Nature of inter-space material

Question 27

why do we increase the Mas when a grid ussed

Answer 27

• Reduction in exposure at the image receptor
• Reduction in number of photons making up the image
• Reduces signal
• Underexposed image

Question 28

equation to work out grid factor ?

Answer 28

grid factor= mas with a grid/mas without a grid

Question 29

what is a disadvantage of increasing mas as a result of using a grid ?

explain why this is necessary

Answer 29

• Increase mAs, resulting in increase patient dose
• Justified as it produces a diagnostic image