Principles of Radiographic Imaging: An Art and A Science: Chapter 18 Radiographic Imaging Flashcards


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1

1. When do you use a grid?

Body parts over 10 cm

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2. Purpose of a grid?

Used to improve the contrast of the radiographic image. It does this by absorbing scatter.

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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.

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4. As a general rule, a grid is employed when

Body thickness exceeds 10 cm and kVp is over 60.

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5. Grid errors

Off-level, Off- center, Off-focus, Upside - Down, Moire Effect

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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.

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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.

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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.

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9. As kVp increases

scatter increases and contrast is further impaired

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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

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11. The greater the atomic number of the tissue

the less will be the scatter produced

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12. amount of scatter radiation increases with

increases in pt thickness
larger field sizes
decreases in atomic # of the tissue and increases in kVp

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13. A grid's shape

thin, flat rectangular

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14. interspace material is

thicker and usually made of aluminum

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15. The encasing is

an aluminum cover to protect the lead strips and interspace material - to protect them from damage

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16. First grid

1913 - Gustav Bucky - wide strips of lead 2 cm apart and in 2 directions.

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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.

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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.

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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

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20. Higher grid ratios allow

less scatter to pass through their interspace material to reach the IR

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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

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22. Higher ratio grids require

Greater accuracy in their positioning and are more prone to grid errors

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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).

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24. Grids with higher grid frequencies have

thinner lead strips

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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.

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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.

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27. What grid positioning errors are possible with focused grids

off-level, off-center, off-focus and upside-down

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28. What is the only grid positioning error that can occur with parallel grids

off-level error

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29. Which grid is designed for a particular SID range

focused grids

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30. Which grid has a front and back

focused grids

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31. Errors are more common with

focused grids - due to the decreased positioning latitude

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32. K factor measures a

grid's ability to improve contrast
Grids with a high ratio have high K factors

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33. The higher the K factor

the greater the contrast improvement

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34. K factor

Contrast improvement ability

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35. GCF and mAs have an inverse or directly proportional relationship

directly proportional relationship

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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.

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37. Stationary grids are used for

portable exams
grid lines will be evident

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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.

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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.

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40. Parallel grids are best used at

long SIDs - when the most perpendicular portion of the beam is being used.