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Thread: Grids

  1. #1
    function is offline Senior Member Post Graduate function has a reputation beyond repute function has a reputation beyond repute function has a reputation beyond repute function has a reputation beyond repute function has a reputation beyond repute function has a reputation beyond repute function has a reputation beyond repute function has a reputation beyond repute function has a reputation beyond repute function has a reputation beyond repute function has a reputation beyond repute
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    Default Grids



    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.



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  2. #2
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    Default Grids


    Grids


    Question Answer
    What causes scatter? kVp, field size, patient thickness
    what are some ways to reduce scatter? grid, beam restricting devices
    Extremely effective in reducing the amount of scatter reaching the IR grid
    Grids are made of ________. Alternating radiopaque material (lead- grid strips) and radiolucent material (aluminum or plastic - interspace material.
    what is the purpose of the interspace material? to maintain a precise separation between the delicate lead strips of the grid.
    What is the purpose of the grid strips? absorbs scatter
    Encases the grid completely by a thin cover of aluminum and provides rigidity and helps seal out moisture. Grid casing
    Grids are designed to transmit x rays whose direction is a ________ _____ from source to IR. straight line
    High quality grids can clean up how much scatter? 80-90% of scatter
    Grid Ratio height of the grid divided by the interspace width
    what is the formula for grid ratio? Grid Ratio= H/D
    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
    what is contrast? density differences
    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
    Do you use more or less dose with higher ratio grids? more dose
    The number of grid strips or grid lines per ich or centimeter Grid Frequency
    If a grid has a high frequency will it show more or less grid lines? less grid lines
    With increased frequency do you get more or less dose to patient? Do you have to use more or less technique? more dosemore technique
    what is the range of grid frequencies 25-45 lines per centimeter (60-110 lines per inch)
    What is the formula for grid frequency? grid frequency= 10,000 um/cm /(T+D) um/line pair
    What is the purpose of the interspace material? To maintain a precise separation between the delicate lead strips of the grid.
    What two materials can the interspace be made of? aluminum or plastic
    What material is mostly used for the grid strips? lead
    What is the principle function of grids? To improve image contrast
    Name 3 factors that affect grid performance. Contrast Improvement FactorBucky FactorSelectivity
    What is the contrast improvement factor? it detects the ability of the grid to improve radiographic contrast
    what is the formula for the contrast improvement factor (k)? k= radiographic contrast with grid / radiographic contrast without a grid
    what is the k range? 1.5-2.5 (radiographic contrast is doubled when grids are used; a k of 1 indicates no improvement)
    Is the k higher for higher ratio grids? yes
    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
    what is the formula for Bucky factor? incident remnan radiation/ transmitted image - forming radiation = patient dose with grid/ patient dose without grid
    The higher the grid ratio the ________the bucky factor. higher
    A increase in ________ will _______ the bucky factor. kVp, increase
    With an increase with bucky factor what will hapen to patient dose? increase
    Ratio of transmitted primary radiation and transmitted scatter radiation. Selectivity
    What is the formula for Selectivity? Greek sigma = primary radiation transmitted through grid/ scatter radiation tansmitted through grid
    The more lead a grid has the __________the selectivity and more efficient it is at cleaning up _______. higher, scatter
    What are some types of grids? Parallel (linear), Crossed, Focused, Moving
    Parallel (linear) grid simplest; all lead grid strips are parallel; clean up scatter radiation in only one direction
    Undesirable absorption of primary x rays by the grid. Grid cutoff
    With Linear grids if you do not use the proper ______ you will get grid cutoff. SID (source to image receptor distance)
    Lead grid strips running parallel to both the long and short axes of the grid Crossed grid
    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
    Which grid is designed to minimize grid cutoff? Focused grids
    How do the focused grid lead strips lie? They coincide with the divergence of the beam.
    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.
    What are grid lines? They are images made when primary x rays are absorbed in the grid strips.
    Moving grids movement of the grid while the x-ray exposure is being made. the grid lines disappear and less increase of technique.
    What is another name for the moving grids? Potter-bucky diaphragm (named after Hollis E. Potter who developed this idea in 1920)
    _______________ are usually used as moving grids. Focused grids
    What are the two types of moving grids? oscillating and reciprocating
    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
    Reciprocating grids moving grid that is motor driven back and forth several times during exposure
    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
    What are four types of problems that can occur with focused grids? off-level, off-centered, off-focus, upside-down
    Off-level grid caused by angle of tube or central ray; cuts across or perpendicular to grid lines; cutoffon entire image; most common
    Off-center grid cutoff on one side than the other due to central ray not centered on grid
    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)
    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
    What are some common grid ratios used? 5:1, 6:1, 8:1, 10:1-12:1, 16:1
    What are some disadvantages to using a grid? Increase in patient dose and increased technique
    What technique can we use in place of grids? Air gap technique
    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)
    what are some disadvantages of the air gap technique? increased dose for pt. and image magnification with associated focal spot blur
    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.
    For diagnostic kVp range the highest quantity is at ______ to patient. (backscatter) 180 degrees
    For the Diagnostidc level, the highest intensity or stregngth of scatter is at _____ to patient. (occupational exposure) 90 degrees
    In the chest, what percentage of the useful density is due to scatter? Abdoment? 50%; 90%
    When do we use a grid? when the part is greater than 10-12 cm thick and the kVp is greater than 60
    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
    what is grid cutoff? undesirable absorption of primary x rays by the grid
    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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