Principles of Radiographic Imaging: An Art and A Science: Chapter 18 Radiographic Imaging Flashcards
1. When do you use a grid?
Body parts over 10 cm
2. Purpose of a grid?
Used to improve the contrast of the radiographic image. It does this by absorbing scatter.
3. The amount of scatter radiation increases with 3 things.
When the increase in patient thickness, larger field sizes and decrease in atomic number of tissue.
4. As a general rule, a grid is employed when
Body thickness exceeds 10 cm and kVp is over 60.
5. Grid errors
Off-level, Off- center, Off-focus, Upside - Down, Moire Effect
6. Moire effect
Occurs with the digital receptor system when the grid lines are captured and scanned parallel to the scan lines in the imaging plate readers.
7. Air gap technique
alternative to the use of a grid. Placing a patient at a greater OID thus creating an air gap between the patient and the IR.
8. Upside-Down error
Grid used upside meaning tube side was placed down causing the radiation will pass through the grid along the central axis where the grids strips are most perpendicular and radiation will be increasingly absorbed away from the center.
9. As kVp increases
scatter increases and contrast is further impaired
10. Scatter increases with
increases in the volume of the tissue irradiated and decreases with increased atomic number of the tissue, and increases when kVp is increased
11. The greater the atomic number of the tissue
the less will be the scatter produced
12. amount of scatter radiation increases with
increases in pt thickness
larger field sizes
decreases in atomic # of the tissue and increases in kVp
13. A grid's shape
thin, flat rectangular
14. interspace material is
thicker and usually made of aluminum
15. The encasing is
an aluminum cover to protect the lead strips and interspace material - to protect them from damage
16. First grid
1913 - Gustav Bucky - wide strips of lead 2 cm apart and in 2 directions.
17. 1920, Hollis Potter contribution
improved Bucky's grid design. Realigned the strips in just one direction, thinner strips, and then designed it to move during exposure.
18. Grid ratio
major influence on the ability of the grid to improve contrast.
It is the ratio of the height of the lead strips to the distance between the strips.
Grid ratio = h/d.
If height is constant, decreasing the distance b/w the strips results in an increase in the grid ratio.
19. Relationship b/w the distance between the lead strips and grid ratio when height of the grid is an inverse relationship or directly proportional.
20. Higher grid ratios allow
less scatter to pass through their interspace material to reach the IR
21. In order to pass through the interspace material in a grid with higher grid ratios
the scattered photon would have to be more closely aligned to the direction of the primary photon in order to reach the IR
22. Higher ratio grids require
Greater accuracy in their positioning and are more prone to grid errors
23. grid frequency
the number of grid lines per inch or cm. A range in frequency of 60-200 lines/inch (25-80 lines/cm).
24. Grids with higher grid frequencies have
thinner lead strips
25. Very high-frequency grids (103-200 lines/inch)
recommended for stationary grids used with digital IR systems to minimize the possibility of seeing the grid lines on the image.
26. Total quantity of lead in the grid
combo of the grid ratio and frequency - it is the grid's lead content that is most important in determining the grid's efficiency at cleaning up scatter.
27. What grid positioning errors are possible with focused grids
off-level, off-center, off-focus and upside-down
28. What is the only grid positioning error that can occur with parallel grids
29. Which grid is designed for a particular SID range
30. Which grid has a front and back
31. Errors are more common with
focused grids - due to the decreased positioning latitude
32. K factor measures a
grid's ability to improve contrast
Grids with a high ratio have high K factors
33. The higher the K factor
the greater the contrast improvement
34. K factor
Contrast improvement ability
35. GCF and mAs have an inverse or directly proportional relationship
directly proportional relationship
36. Mounted Bucky grids
move - reciprocating or oscillating - upon hitting exposure
grid lines run parallel to the long axis of the table
grid lines are blurred due to motion - moved at rt. angle to lines.
37. Stationary grids are used for
grid lines will be evident
38. Focused grids with high grid ratios require more what
more positioning accuracy to avoid grid cut-off. Lead strips are closer together or have taller strips - photons may be cut-off. Need more positioning accuracy.
39. Focused grids with low grid ratios allow
greater positioning latitude - means lead strips are further apart or are shorter - photons don't have to be perfectly aligned. Can be less accurate.
40. Parallel grids are best used at
long SIDs - when the most perpendicular portion of the beam is being used.