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Fosbinder Chap 12
Grids, Scatter, & Reduction
| Question | Answer |
|---|---|
| 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
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