Chapter 18 - The Grid
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| Why does a grid improve contrast? | by absorbing scatter radiation before it can reach the image receptor
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| When should a grid be used? | when body part thickness exceeds 10 cm and when kVp is above 60
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| A thin, flat, rectangular device made by placing a series of radiopaque lead strips side by side separated by a radiolucent interspace material, the interspace material is thicker and usually made of aluminum. | a grid
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| What type of grid pattern has lead strips running only in one direction? | linear grids
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| Grids designed so that the central grid strips are parallel and as the strips move away from the central axis they become more and more inclined to match the divergence of the x-ray beam. | focused grids
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| As the ability of a grid to clean up scatter increases, what is the effect on patient dose and image receptor exposure? | The more effective the grid is at absorbing scatter, the greater the patient dose to receive adequate exposure and the less exposure will be received by the image receptor
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| The two criteria for measuring a grids performance? | selectivity and contrast improvement ability
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| How is the contrast improvement of a grid measured? | The contrast improvement factor (K):
K=Radiographic contrast with the grid/Radiographic contrast without the grid
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| When the tube is angled across the long axis of the grid strips, what error can occur? | an off level grid error
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| placing the patient at a greater object-to-image distance, creating an air gap between the patient and the image receptor, reducing the amount of scatter reaching the image receptor, improving contrast without the use of a grid. | air-gap technique
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| A post-processing operation that is capable of producing an image that looks "grid-like" , when in fact a grid was not used. | grid replacement software
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| All of the following are a cause of an increase in scatter radiation, except: a. reduction in atomic number of tissue b. larger patient size c. larger field size d. decrease in kVp | c. decrease in kVp
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| What would the grid ratio be if the lead strips are 4 mm high and are separated by an interspace of 0.50 mm? | 8:1
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| What happens to the grid ratio when the distance between the lead strips decreases, but the height of the grid remains the same? | it increases
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| The number of grid lines per inch or centimeters | grid frequency
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| What happens to the selectivity when a grid has a higher lead content? | it increases
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| What happens to the contrast improvement as the K factor increases? | it increases
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| What type of error occurs when the tube is angled across the long axis of the grid strips? | off-level error
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| What type of error occurs with the digital image receptor systems when the grid lines are captured and scanned parallel to the scan lines in the imaging plate readers? | the Moire effect
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| When the lead content of the grid increases, what happens to the ability of the grid to remove scatter and improve contrast? | It increases
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| Two linear grids placed on top of one another so that the lead strips form a criss-cross pattern | criss-cross grid
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| A grid created with the central grid strips parallel with the strips and becoming more inclined as they move away from the central axis; the lines would intersect along a point in space called the line of convergence | focused grid
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| A device used to improve contrast of the radiographic image | grid
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| The result of the primary beam being angled into the lead strips | grid cutoff
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| The number of grid lines per inch or centimeter | grid frequency
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| The ratio of the height of the lead strips to the distance between the strips | grid ratio
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| A grid with lead strips running in only one direction | linear grid
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| A grid created with the lead and interspace strips running parallel to one another; the lines will never intersect | parallel grid
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| Exposure Factors Chapter 18 - The Grid |
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