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Grids

QuestionAnswer
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
Created by: sr4095 on 2008-11-20



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