Term
| _________ is one of the most important characteristics of image quality |
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Definition
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Term
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Definition
| Areas of light, dark, shades of gray, making up image |
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Term
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Definition
| Amount of blackness on film |
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Term
| What is contrast resolution? |
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Definition
| Ability to image adjacent similar tissues |
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Term
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Definition
| Grainy appearance due to insufficient number of x-rays of primary beam |
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Term
| Name two devices that control scatter |
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Definition
Beam-restricting devices and grids |
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Term
| List the various factors that influence scatter |
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Definition
| Kvp, field size, patient thickness |
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Term
| Why does lowering kVp reduce scatter and what disadvantage is associated with this decrease? |
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Definition
| Enhanced differential absorption, increased patient dose |
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Term
| ________ is one of the most important characteristics of film quality, and why |
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Definition
| Contrast; It's the ability to image and distinguish soft tissue |
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Term
| Define contrast Resolution |
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Definition
| Ability to image adjacent similar tissues |
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Term
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Definition
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Term
| What would happen if a radiograph was taken with only scatter radiation, how would this affect contrast and why? |
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Definition
| Image would be dull gray, contrast low |
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Term
| The more scattered x-rays, the ____ contrast |
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Definition
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Term
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Definition
| An extremely effective device for reducing level of scatter radiation reaching IR |
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Term
| The two parts of a grid are |
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Definition
| Grid strips, interspace material |
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Term
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Definition
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Term
| A grid transmits only those x-rays that |
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Definition
| are in a straight line from source to IR |
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Term
| In theory, what happens to scattered x-rays when in contact with a grid? |
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Definition
| They are absorbed and don't reach IR |
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Term
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Definition
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Term
| What happens to scattered x-rays that interact with a grid? What about primary x-rays? |
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Definition
| They are absorbed; primary beam x-rays incident on interspace material are transmitted to IR. Scattered x-rays may or may not be absorbed due to angle and grid characteristics |
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Term
| What does the text mean by scatter absorption depends on the angle? |
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Definition
| If angle of scattered x-ray great enough to intersect grid strips, it will be absorbed. Too slight, x-ray goes to IR |
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Term
| High quality grids can absorb ____% of scatter radiation |
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Definition
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Term
| The term used to describe a grid with high absorption is? |
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Definition
| said to have good Clean Up |
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Term
| The three aspects of grid construction are? |
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Definition
| thickness of grid strip, width of interspace material, height of grid |
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Term
| Grid ratio can be defined as |
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Definition
| height of grid divided by interspace width |
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Term
| What is the formula used to determine grid ratio? |
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Definition
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Term
| The higher the grid ratio the ____ effective the clean up and why? |
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Definition
| increased; angle of scatter allowed by high-ratio grids is less than permitted by low |
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Term
| What is the trade off when using a higher grid ratio? |
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Definition
| More difficult to manufacture, increased Radiation exposure |
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Term
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Definition
| Number of grid strips of grid lines per inch or cm |
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Term
| A grid with ____ frequency shows more grid line |
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Definition
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Term
| The higher the frequency, the thicker/thinner the strips |
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Definition
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Term
| What happens to patient exposure with increased grid frequency? |
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Definition
| increased radiation exposure |
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Term
| What two things happen when the width of the strips are reduced? |
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Definition
| Reduces grid ratio, reduces patient dose |
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Term
| Most grids have frequencies in the range of? |
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Definition
| 25-45 lines per cm (60-100 lines per inch) |
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Term
| The grids used in mammography are? |
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Definition
| 80 lines per cm (200 lines per inch) |
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Term
| _____ is the most commonly used material as grid strips (why?) |
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Definition
| Lead; easy to shape, relatively inexpensive |
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Term
| What other materials have been used and how effective are they? |
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Definition
| Tungsten, platinum, gold, uranium, not effective |
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Term
| What is the purpose of the interspace material and what are they made of? |
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Definition
| Maintain precise separation between lead strips. Aluminum, plastic fiber |
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Term
List the advantages and disadvantages of the two interspace materials |
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Definition
Aluminum has high atomic number, helps absorb scatter, less visible grid lines...increases absorption of primary beam, increase mas so increase patient dose Fiber absorbs moisture, can become warped, hard to get precise thickness |
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Term
| What material is used to encase grid and what does it provide? |
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Definition
| Thin cover of aluminum, rigidity of grid, helps seal out moisture |
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Term
| What is the principal function of a grid? |
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Definition
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Term
| Describe Contrast improvement factor (CIF) |
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Definition
| Ability of grid to improve radiographic contrast. |
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Term
| How much improvement would be gained by a CIF by 1, 1.5 or 2.5? |
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Definition
| 1- no improvement, 1.5-2.5 = approximately doubled |
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Term
| What factors contribute to CIF? |
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Definition
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Term
| What does Bucky Factor (BF) mean? Who is it named after? |
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Definition
| Amount of increase in dose needed to make up for grid; Gustave Bucky |
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Term
What generalization can be made by higher grid ratio compared to lower grid ratio? |
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Definition
| Higher grid ratio, higher bucky factor |
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Term
| What generalization can be made by increasing kVp (as far as BF) |
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Definition
| Increase kVp, increase BF |
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Term
| What happens to patient dose with an increase in BF? |
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Definition
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Term
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Definition
| Ratio of transmitted primary radiation to transmitted scatter radiation |
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Term
| How is it possible that two different grids can have the same ratio but have different selectivity? |
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Definition
| Total lead content in grid has primary influence on selectivity |
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Term
| List the three grid characteristics |
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Definition
1. High ratio grids, high constrast improvement. 2. High frequency, low contrast improvement. 3. Heavy grids, increase selectivity, increase contrast |
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Term
| How does a tech select a grid? |
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Definition
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Term
| Why would an engineer or physicist choose grids for a different reason? |
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Definition
| In order to set up for a radiological suite or procedure |
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Term
| List the three grid types by name |
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Definition
| Parallel, crossed, focused |
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Term
| Which type of grid is the smallest? |
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Definition
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Term
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Definition
| Undesirable absorption of primary x-rays by grid |
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Term
| Grid cut off can be ____ or ____ |
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Definition
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Term
| When is parallel grid cutoff most pronounced |
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Definition
| With short SID or large IR |
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Term
| What is the difference between parallel grids and crossed grids? |
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Definition
| Parallel grids only clean up scatter along axis |
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Term
| How are crossed grids designed? |
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Definition
| have lead strips running parallel to both long and short axes of grid |
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Term
| True or false: linear grids are more efficient than crossed grids |
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Definition
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Term
| What are the advantages and disadvantages of crossed grids? |
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Definition
| more efficient for clean up, high contrast improvement; high grid cut off, positioning grid crucial |
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Term
| How are focused grids designed? |
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Definition
| To minimize grid cut off. Lead strips lie on imaginary radial lines of circle at focal spot |
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Term
| True or false. Focused grids are easier to manufacture |
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Definition
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Term
| True or false. Both focues and unfocused have problems with grid cut off, even if they are used properly. |
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Definition
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Term
| What does the author mean when he states focused grids must be used with care because of their geometric limitations? |
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Definition
| Grid strips are parallel to primary x-ray path. Must coincide with divergence of x-ray beam |
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Term
| What are grid lines? Is it always true that a 5:1 grid would demonstrate more grid lines than a 12:1? |
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Definition
| Images made when primary x-rays are absorbed in the grid strips. No, visibility of grid lines directly related to width of grid strips. |
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Term
| Who is Hollis Potter? What name is now used for the device he developed? |
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Definition
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Term
| What are the two types of moving grids and describe how each works |
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Definition
Reciprocating-motor driven, moves back and forth several times. Oscillating-spring devices hold grid, electromagnet pulls grid, oscillates in circular fashion |
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Term
| What are the main differences between the two types of moving grids? |
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Definition
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Term
| What can occur if moving grids aren't designed properly? |
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Definition
| Can produce stroboscopic effect due to synchronization between x-ray pulsation and grid movement. Pronounced grid lines appear |
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Term
| Compare the advantages and disadvantages of moving grids vs. stationary grids |
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Definition
| Min. exposure time increased with moving grid. Moving grid blur undetectable. Require bulky mechanism. Increase OID, Increase magnification |
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Term
| Describe the places grids are mounted |
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Definition
| Moving mechanism below tabletop or behind chest board |
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Term
| What will happen if the grids are installed incorrectly? |
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Definition
| Grid lines will appear on radiograph |
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Term
| What is the most frequent error in grid usage? How must it be positioned to assure proper usage? |
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Definition
| Improper positioning. Must be precisely positioned relative to x-ray tube target and CR of beam |
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Term
| What are the different grid problems? |
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Definition
Off-level Grid Off Center Off focus Upside down |
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Term
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Definition
| Improperly positioned radiographic tube and not improperly positioned grid |
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Term
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Definition
| CR is shifted laterally on grid, results in lower OD due to grid cut off |
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Term
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Definition
| radiographs taken at wrong SID, farther grid from distance, more severe cutoff |
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Term
| What is upside down grid? |
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Definition
| A radiograph with uspide down focused grid shows severe grid cutoff on either side of CR |
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Term
| True or false. Stationary grids continue to be a problem. Defend your choice |
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Definition
| False; well manufactured. Radiologists can still diagnose. |
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Term
| ____ grids are generally less superior than ____ grids... |
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Definition
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Term
| What three factors must be considered when selecting a grid? |
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Definition
kvp degree of cleanup patient dose |
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Term
| As grid ratio _____ the amount of cleanup_____ |
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Definition
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Term
| Which grid ratio is not used as frequently and why? |
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Definition
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Term
| What is one of the major disadvantages of using grids? |
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Definition
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Term
| How much more radiation is needed with a moving vs. stationary grid? |
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Definition
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Term
| True or false. Mammo uses high grid ratio |
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Definition
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Term
| List the three factors to be used when selecting a grid |
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Definition
1. Patient dose increase with grid ratio 2. High ratio grids used for high kvp 3. patient dose at high kvp is less than at low kvp |
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Term
| Why must the technique be changed when using a grid? |
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Definition
| in order to make up for absorption from grid |
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Term
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Definition
| IR is positioned 10-15 cm from patient in order to reduce scatter radiation |
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Term
| How does air-gap enhance contrast? |
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Definition
| because fewer scattered x-rays interact with IR |
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Term
| How much must you increase mas when using air-gap? Does this increase of technical factors increase patient dosage? Why or why not? |
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Definition
| 10 % for every cm of gap. Patient dose higher than non-grid, less than grid. |
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Term
| Where is air gap commonly used? |
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Definition
| chest radiography, cerebral angiography |
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