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115 Cards in this Set
- Front
- Back
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Definition of Grid
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A device used for filtering out the unwanted scatter radiation.
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Functions of a Grid
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To improve contrast
Increased scatter will cause a decrease in contrast The grid removes scatter |
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Grid Ratio Formula
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GR= h/D
h= height of Grid D= thickness of interspace material |
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Results of Increased Grid Ratio
(6 things) |
1. Decreased scatter - increased cleanup
2. Increased patient dose 3. Increased contrast 4. Increase Grid Factor 5. Decreased Positioning Latitude 6. Decrease Density |
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What is Positioning Latitude?
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It is the range in which one can position the grid without producing grid cutoff.
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What is contrast?
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The degree of difference between dark and light.
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What is density?
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The overall blackness.
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What is a grid made of?
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It is made of radiopaque material (grid material) alternating with sections of radiolucent material (interspace material)
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What is a grid made to do?
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Designed to transmit only those xrays traveling in the direction of the interspace. Xrays that travel obliquely are absorbed in the grid material.
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How much scatter do high quality grids usually cleanup?
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They absorb or weaken 80-90% of the incident scatter. This would exhibit good cleanup.
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Grid Ratio Range
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5:1 - 16:1
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What does the first number in GR represent?
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First number - always the ratio
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What does the second number in GR represent?
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Second number - always "1"
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16:1 ratio grids area used in what kind of radiography?
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They are used in high kVp radiography. Such as chests.
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What size grid is usually put in tables and chest bucky's?
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12:1
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How are high ratio grids made?
(St. Catherine's Question) |
They are made by reducing the width of the interspace or increasing the height of the grid material or a combo of both.
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What is grid frequency?
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Number of grid strips or grid lines per inch or per centimeter.
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The higher the grid frequency...
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The higher the technique required, and the greater patient dose because there is more grid material to absorb radiotion
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Most grids have what frequency?
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60-110 lines per inch
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What is the normal frequency for a grid?
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103 lines/ inch (lp/in)
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What is grid efficiency?
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The amount of scatter pickup (grid efficiency)
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What makes grid efficiency incraese?
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It will increase as the grid ratio increase because of more lead strips and the frequency decreases.
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The (higher or lower) the grid ratio, the greater the reduction of scatter radiation.
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Higher the grid ratio
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The efficiency of a grid or its ability to reduce scatter is affected by: (2 things)
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Grid ratio - highest
Grid frequency - lowest More is not always better |
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Purpose of interspace material?
(2 things) |
To maintain a precise separation between the lead strips.
Produce a visible image on radiograph |
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What is the interspace material made of?
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Either aluminum or plastic fiber
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What are 4 advantages to using aluminum interspace material?
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Provides some filtration of scatter not absorbed by lead strips
Produces less visible grid lines on film Does not absorb moisture like plastic fiber Easier to manufacture |
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What does nonhygroscopic mean?
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does not absorb water
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What are 2 disadvantages to using aluminum interspace material?
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Increased absorption of primary photons at low kVp which results in higher patient dose.
Above 100 kVp this property is unimportant but at low kVp, aluminum causes a 20% increase in pt dose. |
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What is the most widely used grid material?
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Lead
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3 Reasons why lead is the most widely used interspace material.
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1. Easy to shape
2. Inexpensive 3. High atomic number and high density |
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What is the grid encased in and why?
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It is encased completely by a thin cover of aluminum.
Reasons: 1. Provides rigidity for the grid 2. Seals out moisture |
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How do you measure grid performance?
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Contrast Improvement Factor
Bucky Factor Selectivity (Lead Content) |
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What is the contrast improvement factor?
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A measurement to determine the degree to which contrast is improved with the use of a grid.
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Most grids have a contrast improvement factor between?
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1.5 and 2.5
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What letter is the contrast improvement factor represented by?
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the letter "K"
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What kVp is contrast improvement factor usually measured at?
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100 kVp
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What is the contrast improvement factor formula?
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K= radiographic contrast with grid/radiographic contrast without grid
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What does Average Gradient mean?
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Only refers to film contrast. The patient has nothing to do with this.
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As grid ratio increases, the contrast improvement factor ______....
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Increases
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Contrast improvement factor is dependent of 3 things. What are they?
Know this! |
1. kVp
2. field size 3. patient thickness Know these! |
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Bucky factor is sometimes called?
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Grid factor
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Bucky factor is named after?
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Gustave Bucky, the inventor of the grid
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Technique must be (increased or decreased) when using a grid in order to produce the same...
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increased, density
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Bucky Factor Formula
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B= incident remnant radiation/ transmitted image-forming radiation
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What does remnant radiation mean?
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X-rays exiting the backside of the patient. Includes both primary and scatter.
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What does image-forming radiation mean?
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Only that which hit the film.
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As the grid ratio goes higher, what happens to the Bucky factor?
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It goes higher
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As kVp increases, what happens to Bucky factor?
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It increases
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What happens to radiographic technique and pt dose as the Bucky factor increases?
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They increase proportionately
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What influences selectivity?
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Lead
Increased Lead = Increased selectivity |
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What does selectivity describe?
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It describes how well the grid will differentiate primary from scatter radiation
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What are the 4 types of grids?
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Linear - Parallel
Linear - Focused Crossed Moving |
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What is the simplest type of grid?
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Linear - Parallel
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How are the lead strips positioned in a Parallel grid?
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Lead strips are parallel to each other and are aligned with the long axis of the grid.
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Which grid is the easiest to manufacture?
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Linear - Parallel
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What is grid cutoff?
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(undesirable absorption of primary-beam xrays in the grid) It is most common with parallel grids and when used with a short SID or with a large image receptor (14x17)
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How are the lead strips positioned in a Linear - Focused Grid?
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Lead strips are tilted or slanted so they lie as near parallel to the diverging remnant radiation.
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Which grid has the least amount of cutoff?
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Linear - Focused
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What is the selectivity (lead content) formula?
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Σ=Primary radiation transmitted through the grid / scatter radiation transmitted through the grid
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What type of grid has a set SID to avoid grid cutoff?
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Linear - Focused Grid
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Do high ratio grids have lower positioning latitude than low-ratio grids?
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Yes
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When answering questions about grids, what are some key words for linear - focused grids?
Know this! |
Diverging, divergent, etc...
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What grid is produced by placing two linear grids on top of each other but with leads strips aligned perpendicular to each other?
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Crossed Grid (Rhombic)
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What grid can be parallel or focused?
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Crossed Grid
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Which grid is the least commonly employed?
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Crossed grid
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Angling the tube horizontally will produce grid cutoff in which type of grid?
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Crossed grid
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What is a moving grid?
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It moves during exposure to eliminate grid lines (images made when the primary-beam xrays are absorbed in the grid strips)
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Who invented moving grids and when?
Know this! |
Dr. Hollis Potter in 1920
Know this! |
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The "Potter-Bucky Diaphragm" or just "Bucky" is another name for which type of grid?
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The moving grid
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What are the three types of moving grids?
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1. Single-stroke
2. Reciprocating 3. Oscillating |
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Which type of moving grid moves continuously across the film while the exposure is made?
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Single-stroke
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Which type of grid require manual cocking before each exposure?
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Single-stroke
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Are single-stroke grids currently employed in radiography?
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No
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Which type of moving grid is motor driven back and forth several times during the exposure?
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Reciprocating
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Do reciprocating grids require resetting after each exposure?
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No
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How is the motion of a reciprocating grid?
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Motion is to and fro
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How is the motion of an oscillating grid?
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It oscillates in a circular fashion around the grid frame, coming to rest after 20-30 seconds.
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Are oscillating grids used very often?
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No
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What are the 4 most frequent errors in the use of grids through positioning?
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1. Off Level
2. Off Center 3. Off Focus 4. Upside-Down |
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When does an off level grid occur?
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It occurs when the central oxis is not perpendicular to the grid
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Where does grid cutoff occur on the film when the grid is off-level?
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Over the entire film, resulting in reduced density
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When does an off centered grid occur?
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It occurs when the center of the grid in not positioned directly under the central ray.
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Where does grid cutoff occur on the film when the grid is off-centered?
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Any lateral shifting will result in grid cutoff across the entire grid. This error in positioning is called "lateral decentering."
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When does an off-focused grid occur?
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It occurs when radiographs are taken at SIDs unspecified for that grid. The farther the grid is from the specified SID, the more severe the cutoff will be.
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When is grid cutoff more severe with off-focused grids?
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On the periphery.
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When does an upside-down grid occur?
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When the grid is placed upside-down.
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Where does the grid cutoff occur on an upside-down grid?
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On either side of the central axis.
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When selecting a grid, what are 4 things that you need to take into consideration?
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1. How much kVp will I be using
2. Degree of cleanup I will need 3. Positioning Latitude 4. Patient Dose |
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If I were going to be using 70-89 kVp, what is the minimum grid ratio I would need?
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8:1
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If you were going to be using 90-100 kVp, what is the minimum grid ratio that you could use?
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12:1
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If you were going to be using a high kVp (chest), what is the minimum grid ratio that you could use?
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16:1
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Quality
Penetration Contrast Scatter |
Keywords for kVp
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Quantity
Number Density Dose |
Keywords for mAs
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What does attenuation mean?
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absorption, cleanup of scatter radiation.
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Attenuation of scatter increases as ____________ also increases?
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Grid ratio
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Which of these has a smaller pt dose?
1. High kVp, Low mAs 2. Low kVp, High mAs |
1. High kVp, Low mAs
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What is positioning latitude?
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Range in which one can position the grid without producing grid cutoff.
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True of False
High ratio grids have greater positioning latitude than low ratio grids |
False. They have less.
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True or False
Patient dose is inversely proportional to mAs. |
False. It is directly proportional.
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Does patient dose increase as grid ratio increases?
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Yes
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To maintain density from the use of a grid, what needs to happen to mAs?
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It must be increased, which leads to increased patient dose.
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What technique requires a gap between the patient and the film of approx. 10-15 cm?
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Air Gap Technique
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What is the purpose of the Air Gap Technique?
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It serves as an alternative to the use of grids.
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Is kVp inversely or directly proportional to patient dose?
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Inversely
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What is the disadvantage to the Air Gap Technique?
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Image magnification.
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What types of radiography is the Air Gap Technique used for?
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Angiography, chest radiography, and lateral cervical spines.
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True or False about Air Gap Technique
The air acts as a filter of low-energy scattered xrays. |
False.
It does NOT act as a filter of scatter radiation. Rather, the distance between the patient adn the film permit the scattered xrays to escape |
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When should you use a grid?
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When the body thickness is 10cm or more.
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Does scatter production have to do with the grid or the patient?
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Patient.
Scatter absorption has to do with the grid. |
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Where does scatter production occur?
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In the patient.
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Where does scatter absorption occur?
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In the grid.
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In a modern 8:1 grid, where does the majority of scatter reduction result from?
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The lead strips.
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What are the Morgan Bucky Factors?
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No grid - 1
5:1 - 2 6:1 - 3 8:1 - 4 12:1 - 5 16:1 - 6 |
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What is the new mAs Formula?
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New mAs= Original mAs X (GF2/GF1)
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