Grids Flashcards
Who built the first grid?
Gustave Bucky 1913. Built the first lead foil strips on the standing edge of the grid in order to trap scatter radiation.
Who is Hollis E. Potter?
Created the first wire grid in 1915-1917. Shares the credit with Bucky on creating the grid
When looking at the image, what does dark area represent?
Transmission of x-rays passing through the body and striking the IR.
What does light areas on the image represent?
Absorption of x-ray or absence of x-ray striking the IR.
As a general rule, use a grid when body parts exceed ___
10 cm
As a general rule, use a grid when kVp is above ___
60
Name the term that explains the height of the grid divided by interspace width.
Grid Ratio.
GR = (h/D)
What is radiopaque material made from?
inexpensive lead and has high atomic #
What does radiolucent material made from
interspaced materal, thicker than lead, either aluminum or plastic fiber, absorbs low energy scatter.
If height were to stay the same and width were to decrease, ____
the ratio would increase. Inverse relationship.
Grid ratios can range from ___ to ___.
4:1 to 16:1. The most common is the 8:1 to 10:1
Which type of grid ratio will remove most of the scatter radiation?
Higher grids
True or False: The higher the grid ratio, the higher the Bucky factor?
True
What is a bucky factor?
to measure the penetration of primary and scatter radiation through the grid
Which type of grid ratio will improve the contrast?
higher ratios
What is a grid frequency?
the # of lead strips per inch or cm. Most common lays between 60 to 110 lines
General rule: typically higher frequency grid has ___ lead strips.
thinner
How much scatter does 5:1 cleans up?
85%
How much scatter does 16:1 clean up?
97%
In grid frequency, as contrast increases, ___
scatter decreases
The simplest type of grid, easiest to manufacture, and absorbs scatter in one direction.
Parallel grid
The undesirable absorption of primary x-ray by the grid.
gird cutoff
More efficient than parallel grid in cleaning up scatter radiation, manufacturing is not excessively expensive.
crossed grids
Name the three disadvantages of crossed grids
1) positioning is critical
2) Tilt-table techniques are possible if tube and table are aligned
3) Exposure technique requires higher PT radiation dose.
Difficult to manufacture but has the same properties of parallel grids expect when positioned properly exhibit no grid cutoff.
Focused Grid
Three characteristics of focused linear grids
1) Proper centering
2) Preset SID
3) Careful alignment
What is a reciprocating grid?
a moving grid that is motor-driven. The total distance of the drive is 2 cm.
What is an oscillating grid?
A powerful electromagnet pulling the grid to one of the four sides. The gird moves (oscillates) in a circular motion for 20 to 30 seconds.
Disadvantages of moving grids
Distance between patient and image receptor is increased.
Magnification and image blur due to increasing distance.
True or False: Moving grids are the technique of choice and widely used
True
As a general rule, below 90 kVp use a grid of ___. About 90 kVp use a grid of ___.
below - 8:1
above - 10:1
As grid ratio increases, ______
radiographic density decreases. (inverse relationship)
What is a grid selectivity?
describe grid’s ability to allow primary radiation to reach image receptor and prevent scatter.
k = (contrast with grid/contrast without grid)
Ranges of 1.5 or 3.5 (times better with grids)
Commonly seen in mobile or beside radiography grid error.
Off-center and off-focus
Grid problems results in ____
underexposed image or underexposed edges of images
What is an off-level grid error?
Grid or tube was tilted. Cutoff happens around the image.
what is an off-center grid error?
A lateral detent problem, One side of the image was cut-off.
What will a off focus grid error look like?
Image appears blurry with some grid cutoff around the sides
What is an air technique?
IR is moved 10 to 15 cm away from the patient. A portion of the scatter radiation will never hit the IR.
What are the disadvantages with air technique?
increase mA 10% for every centimeter of air gap. Technique is about the same for 8:1 grid.
Higher dose of PT, increase SID and OID, image blur a bit.
