Grids Flashcards
How do we minimize and deal with scatter?
With beam restriction, we limit its production.
With grids, by cleaning it up.
What is grid ratio?
Height of the lead strips divided by distance of the interspace.
What is the function of a grid?
Clean up scatter radiation for more apparent or visible detail.
How much do we increase the technique with a grid?
At least 2 times more technique
What is the main disadvantage of using a parallel grid?
Grid cutoff because xRay beams are divergent. Even image forming X-rays get absorbed by the lead strips.
Define grid cut off
The undesirable absorption of primary x-Rays by the grid.
How can an RT limit grid cut off?
By centering the central Ray precisely to the grid.
Describe a focused grid
It is designed to minimize grid cutoff. By slanting the grid strips it mimics the divergent beam.
Define focus distance
The distance from focal point to the grid. If you draw lines from the lead strips, they meet up at a point. This is also our SID
What happens when the focal distance and our SID are not the same?
We get grid cutoff.
What is the main disadvantage of a focus grid?
We always need accurate focus distance, SID
What is a cross-hatch grid and a rhombic grid?
Linear grids stacked and perpendicular
And
Focused grids stacked
What are disadvantages of crosshatch grids?
- You cannot use an angle with the beam, bc you will get cutoff.
- CR of beam must coincide with center of grid.
- Requires higher technique, so higher pt dose.
What determines how much technique we need to use?
Grid ratio
Grid frequency
Focusing distance
Contrast improvement factor
What is the relationship of higher ratio grids and mAS?
Higher ratio grids need more mAs because there is more inadvertent absorption of the beam.
Disadvantages of higher ratio grids
Requires very accurate centering. They absorb more scatter and primary xray beams, so: Increase tech factors Increase pt dose Increase centering accuracy
Why do we use lower ratio grids on portables?
Because we have less centering accuracy.
Define grid frequency
The number of grid strips per unit of measure (cm or inches)
An advantage of higher grid frequencies
See less distinct grid lines on image
Disadvantage of higher frequency grids
Has more lead so needs more technique and increases patient dose.
What is the typical range of frequencies in most grids?
24-45 lines per cm
Why is frequency important?
We want skinny strips in small units of measure because the thinner the strips and closer together they are, the grid strips are less distinguishable.
When do you get moire or aliasing artifact?
If you put one grid on top of another.
In CR, if the reader laser scans too close to the strip frequencies .
How can we avoid the Moire artifact in CR
- Grid strips of cassette are perpendicular or opposite to the scan lines.
- We increase the grid frequencies to be greater than or equal to 70 lines per cm
Can we have a focusing distance on a parallel grid?
Never, BC there is never a point the lines meet
4 terms that all mean: point of convergence of grid lines when they meet up.
Focusing distance
Focal distance
Focal range
Grid radius
Focusing distance of a 40” SID is expressed in what range?
36 to 44”
Alignment is especially important in focus grids
What is the equation for contrast improvement factor
K = contrast with grid/ contrast without grid
What is the contrast improvement factor generally related to?
The lead content
More lead = more contrast
Contrast improvement factor (K) increases with higher ratio grids.
What is the equation for Bucky Factor?
B = incident remnant xrays/ transmitted image forming xrays = pt dose with grid / pt dose without grid
What are the grid ratios?
5: 1
6: 1
8: 1
12: 1
16: 1
Why don’t we use grids with peds and pregos?
We want to decrease patient dose
What is the purpose of moving grids?
Moves to blur out the objectionable grid lines, because one of the main problems of original grids was the lines were visible.
2 types of moving grids
Reciprocating
Oscillating
How does the reciprocating moving grid work?
Goes back and forth, about 2cms of total movement. If the exposure is too fast you can still get grid lines
Besides the moving grid, what else helps us see less distinguishable grid lines?
Compared to low frequency grids, high frequency grids show less distinguishable grid lines
What are the 4 types of grid cut off?
- Off center- central Ray is not centered to grid and grid strips absorb beam
- Off Level - grid needs to be parallel to floor and or perpendicular to part
- Off Focus - when SID is outside of focal range
- Upside Down Grid- only the very center will get exposure to IR
Why patient does increases with use of grids
- The lead strips absorb the photons of image forming and scatter radiation
- Scatter contributes to density in a negative way, so we need to increase technical factors
When you have more lead, higher ratio grids; what is required
You need more accuracy and you have less room for error.
With peds and pregos, what do we change in technical factors?
Increase kvp 15% instead of increasing mAs
How does the Air Gap Technique work?
Inherent OID functions as effectively as an 8:1 grid
So we need to increase technique
Scatter radiation escapes into the air space between pt and IR
Why does the use of a grid contribute to increased patient dose?
because there is some absorption of the primary beam by the lead strips.
Scatter, although a noise factor, does contribute to density
Why should a radiograph of a lumbar vertebrae be well collimated?
To reduce scatter radiation reaching the IR. This improves contrast.
Compared with contact radiography, why does air gap technique increase patient dose?
If SID remains constant, then patient dose is increased due to the decrease in SOD (the patient will be closer to the primary beam).
Also, the use of an air gap generates magnification. In some exams, a larger receptor size is required to obtain the same patient area coverage. If the air gap is obtained by increasing the tube-to-receptor distance, the technique must increase (higher dose to the patient) to obtain adequate exposure.
