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Exposure Ch 18
Grids
| Question | Answer |
|---|---|
| What is the purpose of the Grid? | To improve radiographic contrast, To absorb scattered radiation before it reaches the IR |
| What is Transmission responsable for? | the dark areas of a radiograph |
| What is Absorption responsable for? | the light areas of a radiograph |
| What does scatter do to an X-ray? | creates fog and lowers contrast |
| What increases scatter? | kV increases, Field size increases, thickness of part increases, atomic number decreases |
| What are the indications for Grid use? | when the part thickness is more than 10 cm, and when the kV is higher than 60 |
| Who created Grids, and when? | Dr. Gustav Bucky in 1913 |
| What are Grids constructed of? | Radiopaque lead strips that are separated by radiolucent interspace material, usually Aluminum |
| Who improved the design of Grids, and how? | Dr. Hollis Potter. He made the grid lines run in the same direction and made them move during exposure. |
| How do you find the Grid Ratio? | Grid Ratio = H/D |
| With a high grid ratio, what scale of contrast is there? | short scale of contrast |
| T\F: A higher grid ratio is better at removing scatter radiation. | True |
| What is the typical grid ratio? | 5:1 to 6:1 |
| What is Grid Frequency? | the number of lead strips in the grid per inch or cm |
| T\F: Lower frequency grids have thinner lead strips. | False : Higher frequency grids have thinner leas strips. |
| What type of grid contains the greatest amount of lead and is most effeciant in absorbing radiation? | High ratio, low frequency grids |
| T\F: As the lead content increases, the removal of scatter decreases, and the contrast decreases | False: As the lead increases, removal of scatter increases, and contrast increases. |
| What are some types of grid patterns? | Crosshatched, linear, Rombic, and consentric. |
| What does a crosshatched pattern look like? | It has both horizontal and vertical lead strips |
| What does a linear pattern look like? | lead strips run the length of the grid in one direction |
| What does a Rombic pattern look like and where is it used? | Looks like a wavy pattern and is used mostly in Europe |
| What does a Consentric pattern look like? | A circular pattern that is evergrowing and is used mostly in Japan |
| What type of grid has a higher margin of error? | the linear pattern |
| What are the types of linear grids? | focused and parallel |
| What do focused linear grids look like? | the lead strips are angled which matches the beam divergence |
| What is Canting? | When the lead strips are angled to match the beam divergence |
| What does improper centering on a linear grid result in? | Peripheral cut off |
| What does a parallel linear grid look like? | the lead strips are all parallel to one another |
| What is a downfall of using a parallel linear grid? | absorbs a large amount of the primary beam. |
| What is reciprocating grid movement? | the grid is moved back and fourth by a motor during exposure. |
| What is oscillating grid movement? | a electromagnet that pulls the grid to the side and releases it during exposure. |
| What is the grid conversion factor? | GCF = mAs with grid \ mAs without grid |
| What happens to the density of a radiograph when a grid is used? | decreases |
| What is the Potter-Bucky diaphragm, and what does it do? | it is the Bucky we use today, it moves the grid during exposure. |
| What formula is used when converting from one grid ratio ot another? | mAs of 1 over mAs of 2 = GCF of 1 over GCF of 2 |
| What does the ICRU do? | the International Commission of Radiologic Units and Measurements evaluate grid performance by two criteria; selectivity and contrast imporvement ability. |
| What does selectivity describe? | the ability of the grid to allow the primary radiation to reach the IR and prevent scatter |
| T\F: high lead content grids are more selective. | True |
| What is the "K" factor? | the contrast improvement ability which is a comparison of contrast of an image with a grid to that of an image without a grid |
| What are the typical ranges of the contrast improvement factor? | 1.5 an 3.5 |
| When the "K" factor is increased, what is incrased as well? | Contrast |
| What can you do to avoid grid errors? | proper aligmnent between tube and grid |
| What does improper alignment of the tube and the grid cause? | cut off |
| What are some types of grid errors? | off level, off center, off focus, upside down, morie effect |
| What is Moire effect? | the grid lines must be running in the same direction as the movement of the laser beam that is scanning the imaging plate. |
| What is the Air-Gap technique? | places a space between the part and the grid. a 10" gap has the same effect as a 15:1 grid |
| What happens when the grid is upside down? | severe peripheral grid cut off will occur |
| What does it mean when the grid is off focused? | the gird has a specified distance as the focal range. if a distance of 44 SID is required and a 72" SID is used, the result will be grid cut off on the peripheral edges of the image |
| What does it mean when the grid is off center? | the CR is off center and the result is a decrease in exposure across the entire image. |
| What does it mean when the grid is off level? | when the tube is angled, an off level grid error occurs with a focused grid and it is the only positioning error possible with a parallel grid. |
Created by:
aimee291020
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