Grids

Question 1

True or false: grids are the most effective means for limiting scatter radiation that reaches the IR

Answer 1

True

Question 2

Grid

Answer 2

• has very thin lead strips inside with radiolucent interspaces
• purpose is to absorb scatter from the patient
• goes in between the patient and IR to improve image contrast (reduce image fog)

Question 3

Grid construction

Answer 3

To describe a grid we would use grid frequency and grid ratio

• grid frequency: number of lead lines per unit length (inches/cm/both)
• grid ratio: ratio of the height of the lead strips to the distance between them

Question 4

Grid frequency

Answer 4

• when using a stationary high frequency grid, the resultant images will show less distinct grid lines
• grids with higher frequency have thinner lead strips

Question 5

Grid ratio

Answer 5

-ratios range from 4:1 to 16:1
-high ratio grids remove/clean up more scatter than lower ratio grids
🔹increase radiographic contrast
-the angle of scatter allowed by high ratio grids is less than that of low ratio grids

Question 6

Grid ratio

Answer 6

-the higher the grid ratio, the straighter the scatter must be in order to pass through to the IR
-scatter must have alignment very close to primary to be able to pass through interspaces
-meaning higher ratio grids:
🔹remove more scatter
🔹require more accurate positioning of tube
🔹prone to increasing grid errors

Question 7

Grid cut off

Answer 7

What is it?
-absorption of the primary beam (transmitted x-rays) by the lead strips
  🔹'cut off' from reaching IR
-undesirable 
-loss of radiographic density
-can be partial/complete
-can occur with any type of grid
-causes 'grid errors'

Question 8

Grid patterns

Answer 8

• refers to the pattern of the lead lines of a grid
• two types of patterns:
  1. Linear
• grid lines run in one direction
• most popular
  2. Crossed or cross hatched
• grid lines run in two directions

Question 9

Crossed or cross hatched pattern

Answer 9

-lead lines run at right angles to each other
-removes more scatter than linear because there are more lead strips oriented in two directions
-limited application as the tube cannot be angled
-very precise positioning required
Advantages: much more efficient at cleaning up scatter
Disadvantages: positioning of grid is critical and CR is critical, must be perfectly cantered, exposure technique is substantial, higher patient dose

Question 10

Linear pattern

Answer 10

-most commonly used because it allows for angulation along the length of the lines
-can be:
🔹parallel
🔹focused

Question 11

Parallel grid

Answer 11

• simplest type of grid
• has lead strips that run parallel to each other
• easiest to manufacture
• main disadvantage: cannot avoid grid cut off of the image
• more attenuation occurs as the x-rays approach the edge of the IR
• would notice greater radiographic density along center line of IR, but decreasing density towards the sides
• greater grid cut off will occur with shorter SID

Question 12

Focused grid

Answer 12

• designed to minimize grid cut off
• lead strips are angled to approximately match the angles of the divergent x-ray beam
• allows more transmitted photons to reach IR
• transmitted photons are more likely to pass through a focused grid to reach the IR than they are to pass through a parallel grid
• less grid cut off
• even density throughout image when positioned correctly

Question 13

Convergent point

Answer 13

-imaginary lines drawn from each of the lead lines, would meet to form an imaginary point
🔹called the convergent point
-if all points were connected along the length of the grid
🔹called convergent line