Grids
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| What causes scatter? | kVp, field size, patient thickness
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| what are some ways to reduce scatter? | grid, beam restricting devices
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| Extremely effective in reducing the amount of scatter reaching the IR | grid
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| Grids are made of ________. | Alternating radiopaque material (lead- grid strips) and radiolucent material (aluminum or plastic - interspace material.
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| what is the purpose of the interspace material? | to maintain a precise separation between the delicate lead strips of the grid.
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| What is the purpose of the grid strips? | absorbs scatter
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| Encases the grid completely by a thin cover of aluminum and provides rigidity and helps seal out moisture. | Grid casing
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| Grids are designed to transmit x rays whose direction is a ________ _____ from source to IR. | straight line
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| High quality grids can clean up how much scatter? | 80-90% of scatter
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| Grid Ratio | height of the grid divided by the interspace width
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| what is the formula for grid ratio? | Grid Ratio= H/D
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| what are the three most important dimensions on a grid? | T = the thickness of the grid stripD= the width of the interspace materialH = the height of the grid
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| what is contrast? | density differences
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| Higher ratio grids are not as effective as low ratio grids in cleaning up of scatter. T/F | False - Higher ratio grids are better at cleaning up scatter due to the angle of deviation being smaller
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| Do you use more or less dose with higher ratio grids? | more dose
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| The number of grid strips or grid lines per ich or centimeter | Grid Frequency
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| If a grid has a high frequency will it show more or less grid lines? | less grid lines
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| With increased frequency do you get more or less dose to patient? Do you have to use more or less technique? | more dosemore technique
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| what is the range of grid frequencies | 25-45 lines per centimeter (60-110 lines per inch)
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| What is the formula for grid frequency? | grid frequency= 10,000 um/cm /(T+D) um/line pair
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| What is the purpose of the interspace material? | To maintain a precise separation between the delicate lead strips of the grid.
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| What two materials can the interspace be made of? | aluminum or plastic
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| What material is mostly used for the grid strips? | lead
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| What is the principle function of grids? | To improve image contrast
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| Name 3 factors that affect grid performance. | Contrast Improvement FactorBucky FactorSelectivity
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| What is the contrast improvement factor? | it detects the ability of the grid to improve radiographic contrast
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| what is the formula for the contrast improvement factor (k)? | k= radiographic contrast with grid / radiographic contrast without a grid
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| what is the k range? | 1.5-2.5 (radiographic contrast is doubled when grids are used; a k of 1 indicates no improvement)
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| Is the k higher for higher ratio grids? | yes
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| Increase of technique to produce the same optical density. Attempt to measure the penetration of both primary and scatter radiation through the grid. | Bucky factor
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| what is the formula for Bucky factor? | incident remnan radiation/ transmitted image - forming radiation = patient dose with grid/ patient dose without grid
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| The higher the grid ratio the ________the bucky factor. | higher
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| A increase in ________ will _______ the bucky factor. | kVp, increase
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| With an increase with bucky factor what will hapen to patient dose? | increase
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| Ratio of transmitted primary radiation and transmitted scatter radiation. | Selectivity
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| What is the formula for Selectivity? | Greek sigma = primary radiation transmitted through grid/ scatter radiation tansmitted through grid
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| The more lead a grid has the __________the selectivity and more efficient it is at cleaning up _______. | higher, scatter
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| What are some types of grids? | Parallel (linear), Crossed, Focused, Moving
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| Parallel (linear) grid | simplest; all lead grid strips are parallel; clean up scatter radiation in only one direction
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| Undesirable absorption of primary x rays by the grid. | Grid cutoff
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| With Linear grids if you do not use the proper ______ you will get grid cutoff. | SID (source to image receptor distance)
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| Lead grid strips running parallel to both the long and short axes of the grid | Crossed grid
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| What are two disadvantages of using cross grids? | Must position correctly so the x ray beam coincides with the center of the grid and if table and tube are not aligned properly you will get grid cutoff
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| Which grid is designed to minimize grid cutoff? | Focused grids
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| How do the focused grid lead strips lie? | They coincide with the divergence of the beam.
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| It is okay to use a focused grid at any desirable SID. True/False | False - all focused grids are marked with intended focal distance. It must be used at the specified SID.
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| What are grid lines? | They are images made when primary x rays are absorbed in the grid strips.
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| Moving grids | movement of the grid while the x-ray exposure is being made. the grid lines disappear and less increase of technique.
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| What is another name for the moving grids? | Potter-bucky diaphragm (named after Hollis E. Potter who developed this idea in 1920)
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| _______________ are usually used as moving grids. | Focused grids
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| What are the two types of moving grids? | oscillating and reciprocating
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| Oscillating grids | positioned in a frame , a powerful electromagnet pulls the grid to one side and releases it at the beginning of the exposure, and oscillates in a circular fashion
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| Reciprocating grids | moving grid that is motor driven back and forth several times during exposure
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| What are some disadvantages to moving grids? | - requires a bulky mechanism - distance between patient and IR is increased due to mechanisms increase in magnification and image blur-exposure time is longer
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| What are four types of problems that can occur with focused grids? | off-level, off-centered, off-focus, upside-down
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| Off-level grid | caused by angle of tube or central ray; cuts across or perpendicular to grid lines; cutoffon entire image; most common
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| Off-center grid | cutoff on one side than the other due to central ray not centered on grid
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| off-focused grid | not using with proper SID; only occurs with focused grids; cutoff on periphery but not at severe as focused grid upside down (use a parallel grid if have no control over SID)
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| Upside down grid | least common;a radiographic image is taken with an upside down focused grid shows sever grid cutoff on either side of the central ray
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| What are some common grid ratios used? | 5:1, 6:1, 8:1, 10:1-12:1, 16:1
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| What are some disadvantages to using a grid? | Increase in patient dose and increased technique
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| What technique can we use in place of grids? | Air gap technique
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| What is the air gap technique? | IR is moved 10-15 cm from pt. Portion of x rays are scattered away from IR contrast (mAs is increased approx. 10% for every cm of air gap)
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| what are some disadvantages of the air gap technique? | increased dose for pt. and image magnification with associated focal spot blur
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| In the air gap technique does air act as a filter? | No, it does not act as a filter. The scattered x rays diverge from the IR due to the distance between the patient and IR.
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| For diagnostic kVp range the highest quantity is at ______ to patient. (backscatter) | 180 degrees
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| For the Diagnostidc level, the highest intensity or stregngth of scatter is at _____ to patient. (occupational exposure) | 90 degrees
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| In the chest, what percentage of the useful density is due to scatter? Abdoment? | 50%; 90%
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| When do we use a grid? | when the part is greater than 10-12 cm thick and the kVp is greater than 60
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| What is the moire effect? | It is a zebra pattern that shows when you use tooo low a frequency grid with electronic imaging or a grid is put into a bucky
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| what is grid cutoff? | undesirable absorption of primary x rays by the grid
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| Focused grids | lead lines lie on the imaginary radial lines of a circle centered at the focal spot so they coincide with divergence of x ray beam
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