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What causes scatter?
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what are some ways to reduce scatter?
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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 |
Created by:
sr4095
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