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Fosbinder Chap 12

Grids, Scatter, & Reduction

What is scatter? Unwanted density that does not contribute to diagnostic image.
Which interaction causes scatter? Compton interaction
What does scatter do too density? Increases
What does scatter do to contrast? Decreases, low contrast, long scale, lots of gray
How does kVp effect scatter? Directly. Increase kVp = increase in scatter
How does pat thickness effect scatter? Directly. Increase part thickness = increase scatter
How does field size effect scatter? Directly. Increase field size = increase scatter
What is an unrestricted beam? No collimator box. Beam is cone shaped and produces a round field on the patient and image receptor.
Purpose of beam restriction? Limits patient exposure and decreases scatter production
How does beam restriction effect contrast? Directly. Increase beam restriction, decreased field size, decreased scatter = increased contrast
How does beam restriction effect density? Inversely. Increase beam restriction, cleans up scatter = decrease density
What are the 4 types of Beam Restricting Devices? Aperture Diaphragm Cone/Cylinder Collimators Automatic Collimators/PBL
What is an Aperture Diaphragm? Flat piece of lead with a hole in the middle, stationary, projected field size is not adjustable. Not used often.
What is a Cone/Cylinder? An aperture diaphragm with an extended flange attached and produces a circular projected field. Cylinder is most effective.
What are Collimators? Made of 2 sets of adjustable lead shutters. Most sophisticated, considered to be the best beam restricting device.
What are Automatic Collimators/ PBL (Positive Beam Limitation)? Automatically limits size and shape of beam to size and shape of image receptor, decreases patient does, has an override feature.
What are Grids? A device that has very thin interspaced lead strips intended to absorb scatter emitted from the patient before hitting the image receptor.
What is grid frequency? Number of lead lines per unit length in inches/cm Ranges from 24-25lines/cm or 60-110lines/in
What is grid ratio? Ratio of height to distance
How does grid ration effect density? Increased ratio, more efficient, cleans up scatter/fog, increased contrast = decrease density
What are the 2 types of grid patterns? Linear and Crossed/Cross hatched
What is a linear grid? Lines run in one direction. Most popular because it allows angulation of tube along length of lead lines
What is a crossed/cross hatched grid? Lines run at right angles to each other Removes more scatter because if increased lead in both directions Does not allow for angulation of tube ***no room for error***
What are the 2 types of grids? (Lead lines) Focused and Parallel (non-focused)
What is a parallel (non-focused) grid? Has lines running parallel to one another Used in fluro, must be used at a minimum SID
What is a focused grid? Lines are angled to match divergence of the beam
What is the convergent point? Imaginary lines drawn from each lead line in a linear focus grid to an imaginary point
What is the convergent line? Series of convergent points connected along the length of the grid
What is focal distance? The distance between the grid and convergent line point
What is focal range? Recommended SID range of a focus grid. 40" grid = 36"-42" 72" grid = 66"-74"
5 types of Grids Wafer grid Grid cassette Grid cap Stationary grid Reciprocating/Oscillating Grid (inside Bucky)
What is a wafer grid? Matches size of cassette and is placed on top with tape
What is a grid cassette? Cassette with grid permanently mounted to front
What is a grid cap? Contained permanently mounted grid and allows cassette to slide behind. (Slides in)
What is a stationary grid? Stationary (grid lines are often seen on image after processing)
What is a reciprocating/oscillating grid? Reciprocating = back and forth movement Oscillating = circular motion Just under table surface or surface of upright Bucky on top of cassette Allows for grid lines to be blurred out
Contrast Improvement Factor formula K = contrast with grid contrast without grid ***only need to recognize formula***
Grid conversation Factor GCF1 = mAs1 GCF2 mAs2
What is grid cutoff? Decrease in number of transmitted photons that reach the image receptor because of misalignment of grid or tube.
Which grid ratio has a higher potential for grid cut off? High or low ratio. High
4 types of grid cutoff. Centering off Angle Upside down grid SID
4 causes of grid lines. Wrong Bucky Exposure time too short Broken Bucky Stationary grid
3 rules for using a grid 1. Part is >10cm 2. kVp >60 3. To increase contrast
Typical grids are.... Linear, focused, mid-ratio 8:1 to 12:1, has a focal range that includes an SID of 40" or 72"
Created by: Aborys88