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Exposure Ch 18

Grids

QuestionAnswer
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 on 2006-11-04



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