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 thicknesslarger field sizesdecreases 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.

inverse relationship

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

off-level error

29. Which grid is designed for a particular SID range

focused grids

30. Which grid has a front and back

focused grids

31. Errors are more common with

focused grids - due to the decreased positioning latitude

32. K factor measures a

grid's ability to improve contrastGrids 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 tablegrid
lines are blurred due to motion - moved at rt. angle to lines.

37. Stationary grids are used for

portable examsgrid 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.