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104 Cards in this Set
- Front
- Back
Scatter radiation is produced during a _________ interaction
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Compton (no use in demonstrating structures of interest)
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Do we worry about coherent scatter in diagnostic radiography?
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No because diagnostic energy range is 50-100 keV and the energy required for coherent is 10 keV
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Scatter radiation will reduce contrast by adding...
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More shades of gray
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Why does increasing scatter increase the density?
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Each photon that hits the IR creates density; a single high energy photon can cause multiple interactions, causing more density
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As scatter increases, radiographic density _____________ (increases/decreases) and there is a __________ (increase /decrease) in contrast
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Increases; decrease (more grays)
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As scatter decreases, radiographic density _____________ (increases/decreases) and there is a __________ (increase /decrease) in contrast
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Decreases; increase (fewer grays)
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If we increase kV and leave all other technical factors constant, what will happen?
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Overall increase in density b/c more photons are exiting the patient & striking the film
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What kind of interactions increase with an increase in kVp?
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Compton interactions (outer shell electron)
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As tissue volume increases, the amount of scatter radiation __________ (increases/decreases)
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Increases
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What are the 2 reasons to reduce your field size to only the area of interest?
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1) Controls amount of scatter produced
2) Reduce patient dose |
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Which interaction is preferred for diagnostic imaging, Compton or Photoelectric?
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Photoelectric
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As the volume of the irradiated tissue increases, which type of interaction increases?
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Compton Scatter
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As the atomic number of the irradiated material increases, which type of interaction increases?
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Compton
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An increase in _______ (mAs/kVp) would need to occur when the field size is collimated small.
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mAs
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Aperture Diaphragm
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- simplest type of beam-restricting device
- flat piece of lead w/ an opening (aperture) to allow radiation to pass through - opening can be any shape; size of opening isn't adjustable - inexpensive |
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In an aperture diaphragm, the field size to be radiated is controlled by the _____
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SID (increasing SID will increase field size to be radiated)
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Penumbra
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- "fuzzy area" out toward the edges of the primary beam
- the less we have, the better our image detail will be ("use the smallest focal spot") |
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Aperture Diaphragm effect upon Penumbra
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Little impact
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Cylinder effect upon Penumbra
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Great effect
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Cones/Cylinders
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- circular aperture diaphragm w/ metal extensions (cones are flared @ ends)
- don't have much of an effect on DR as they do on film - cylinders are constant in diameter, adjustable in length |
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Which, a cone or a cylinder, is better at reducing penumbra?
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Cylinder
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Adjustable Cylinders
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- attach beneath & on the outside of tube housing
- as cylinder length increases the amount of radiation reaching patient/film decreases - increasing cylinder length will decrease the field size - off-focus radiation will be reduced |
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Equation for Determining Cone Field Size
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Projected Image Size = (SID x lower diameter of cone)/distance from focal spot to bottom of cone
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What's the most frequently used method for beam restriction?
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Collimation
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What do the bottom shutters of the collimator control?
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Reduce penumbra along periphery of beam
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What do the upper shutters of the collimator control?
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Attenuate most of the off-focus radiation from reaching film
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Collimator Test
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- a test done to check accurate alignment of the light source & the x-ray beam
- US Federal Standard = 2% SID error (ex. 40" SID x 2% = .8" allowable error) |
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Automatic Collimators
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- positive beam limitation (PBL) device
- adjusts dimension of x-ray beam to IR placed in bucky - will allow you to collimate smaller, but not bigger than the field size of the IR |
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What does the mirror in a collimator act as?
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A filter that absorbs low energy x-ray photons to 1 mm/Al eq
(Added Filtration) |
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What is the total filtration goal standard?
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2.5 mm of aluminum equivalent (mm/Al eq)
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Ancillary Devices
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- lead blockers are pliable lead sheets that prevent radiation from striking the film cassette
- lead masks cut to specific shape & size |
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What's the purpose of a grid?
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To improve the contrast of the image by absorbing the patient-generated scatter radiation (Compton) before it strikes the film
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When are grids used?
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1) Body parts larger than 10 cm
2) Procedures using 60 kVp or higher |
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What are the vertical strips in a grid designed to do?
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Absorb scatter radiation (lead) RADIOPAQUE
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What are the interspace materials in a grid designed to do?
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Allow remnant photons to strike the film (aluminum or plastic fiber) RADIOLUCENT
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How will the grid strips be running in a rectangular grid?
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With the long axis of the rectangle (used for portables)
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3 Expectations about Outcome of Image when Using Grids
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1) Higher contrast
2) Reduction in density (fewer photons hit IR) 3) Grid artifacts |
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What is the grid ratio?
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The relationship of the HEIGHT of the lead strips to the DISTANCE BETWEEN the strips
GR = h / D |
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What does the thickness of the lead strips in a grid represent in the grid ratio?
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Nothing (irrelevant in terms of calculating grid ratio)
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A(n) __________ (proportional/inverse) relationship exists between the distance between the lead strips & grid ratio WHEN the height of the grid strips is constant
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Inverse
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High Grid Ratio
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- stops scatter radiation better
- higher contrast images - requires higher adjustment in technique - attenuate more of the remnant beam (have to increase mAs) |
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What is the grid frequency?
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The number of vertical strip per unit of length/width
Vertical strips will be thinner as the frequency per unit of distance increases |
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Grid Patterns
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- linear (parallel strips of lead)
- focused grids [diagnostic] - cross-hatched or criss-cross (2 linear patterns 90 degrees to each pattern) [mammo] |
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What causes grid cutoff with a cross-hatched grid?
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Tube angulation
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Do linear grids allow tube angulation?
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Yes; must be parallel w/ lead strips
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Focused Grids
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- linear-positioned lead strips
- angled similar to angulation of primary beam divergence - REQUIRES PROPER SID (to reduce scatter & allow remnant photons to pass) |
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Latitude
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- freedom from usual restraints, limitations, or regulations
- high latitude = larger allowable changes - grid latitude: amount you can be off & still get acceptable image |
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In focused grids, as grid ratio increases, the latitude for SID __________ (increases/decreases)
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Decreases (small error in SID can cause large grid cutoff)
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Where are permanent grids located?
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- within Potter-Bucky diaphragm, above film location
- grid lines parallel w/ table length, or vertical direction of wall bucky |
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As grid ratio or frequency increases, the radiographic density ___________ (increases/decreases)
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Decreases (more scatter removed along w/ more remnant; less photons striking film)
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As grid ratio or frequency increases, the radiographic contrast ___________ (increases/decreases)
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Increases (beam less polyenergetic, more homogenous)
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Which grid type is used most frequently?
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Focused grids
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Grid Conversion Factors (GCF)
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- "constants" used to determine change in technical factors needed for specific grid ratios
- mAs adjustments must be made when changing grid ratio |
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GCF Formula
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GCF = mAs with grid / mAs without grid
mAs with grid = GCF x mAs without grid mAs without grid = mAs with grid/GCF |
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GCF - No grid
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mAs increase 1X, GCF = 1
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GCF - 5:1 Grid
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mAs increase 2X, GCF = 2
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GCF - 6:1 Grid
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mAs inceases 3X, GCF = 3
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GCF - 8:1 Grid
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mAs increase 4X, GCF = 4
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GCF - 12:1 Grid
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mAs increase 5X, GCF = 5
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GCF - 16:1 Grid
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mAs increase 6X, GCF = 6
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Off-Level Grid Error
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- occurs when tube is angled across the long axis of the grid strips or grid is unleveled in comparison to tube
- typically happens in portable grids - appearance: decrease in density - To Fix: level grid under patient, angle tube parallel w/ length of grid |
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Off-Center Grid Errors
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- Tube isn't centered (CR) w/ the center of the focused grid (tube not centered to center of grid cassette)
- occurs in fixed or portable grids - decrease in density |
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Off-Focus Grid Errors
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- SID outside acceptable range of focused grid
- occurs w/ fixed or portable grids - decrease in density at lateral margins of image |
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Upside-Down Grid Error
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- center of grid will allow the x-ray beam to pass unrestricted flipped upside down or not
- on edges, grid strips are angled opposite to angle of divergence - severe decrease in density @ lateral margins |
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Does collimation change the diverging angle of the primary beam?
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No
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Air Gap Technique
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- alternate method to use a grid if one isn't available
- use large OID (will reduce amount of scatter reaching the film with the air gap) - need to increase SID to reduce magnification - kVp = 70 at least |
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Reverse-Cassette Technique
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- flip cassette so remnant radiation strikes back side of cassette first
- foil acts as filter, stopping Compton radiation - increase in mAs needed to maintain density |
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Grids do not CREATE scatter, they __________ scatter
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ABSORB (patient creates scatter)
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What kind of latitude does a high grid ratio grid have?
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Narrow concerning SID (precise) & alignment (centered & leveled)
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What size GCF do high grid ratio grids have?
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Larger (takes more mAs to get good density)
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Who are the forefathers of technique?
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Ed C. Jerman (1920s), Arthur Fuchs (1940s)
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What's the goal of any exposure system?
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Consistency
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Exposure System Selection
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- often incorporated into pre-programmed control consoles
- suggested starting points - variables change according to anatomical part thickness |
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Calipers
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- device that measures part thickness
- important QC measure when not using AEC |
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What are the 2 main methods of part measurement?
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- Central ray entrance & exit
- Thickest part |
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Fixed kVp Systems
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- developed by Fuchs during WW2
- reduction in patient dose, provides more info within image, increased consistency in image receptor exposure, shorter exposure times - lengthens exposure latitude (gives variability in quantity of mAs to try to hit "sweet spot") - Disadvantage: higher amount of scatter results in reduced image contrast |
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Variable kVp Systems
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- developed by Jerman (1925)
- first exposure "method" - used when kVp range was limited by equipment generators - allow for small changes in exposure for small changes in part thickness, higher contrast - Disadvantage: more patient dose, higher repeat rate, less latitude |
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Anatomically Programmed Radiography (APR) Systems
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- commonly used computerized exposure control
- provides choices of anatomical parts w/ suggested exposure factors |
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APR Systems & AEC
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- selection of exposure time (mAs) eliminated
- must still choose mA, kVp, distance |
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Establishing Fixed kVp Exposure Systems
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- kVp held constant (mAs varies, eliminates effects of multiple variables)
- kVp has effect on contrast, type of interaction, scatter, average energy of beam - provides consistency, user friendly |
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Optimal kVp
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- maximum kVp that result in images w/ appropriate contrast
- produces lower contrast & minimum patient dose |
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Establishing a Variable kVp Exposure System
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- kVp is varied while mAs is constant
- thicker body part requires higher kVp value; mAs is specified for each body part - Disadvantage: varying kVp from image to image lessens control control over the variables it controls |
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Historical Development of Radiographic Practice
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- +2 for each additional cm of tissue
- (2 kVp x part cm) + 30 = new kVp - not accurate (often too low kVp) - mAs values determined through trial & error |
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Variable kVp System Flaws
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- 15% rule not thoroughly understood (used +/- 10 kVp as compensation for doubling or halving mAs)
- at low kVp, dramatic effect on image; not as profound at high kVp |
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Establishing Stepped Variable kVp System
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- straight variable kVp charts often resulted in too much or too little contrast
- remedied w/ multiple kVp scales |
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3 Criteria of kVp Scale
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1) All film contrast is acceptable to radiologist
2) Small part kVp recommendations provide adequate penetration 3) Large part kVp recommendations limit scatter & fog |
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mAs Scale
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Step 1: acceptable kVp range = 70-84 kVp (for all part thickness)
Step 2: must compensate w/ mAs via 15% rule Step 3: fine tuning (measure w/ calipers) |
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Automatic Exposure Controls (AEC)
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- devices designed to reproduce images w/ consistent density & contrast regardless of differences in anatomical sizes or textures of same anatomical part
- Goal: reproducibility & consistency btw. patients & technologists |
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AED (Automatic Exposure Devices)
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- phototiming: older method of AEC
- ionization chambers: most current method used |
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Phototimers
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- use a photomultiplier tube to detect the amount of remnant radiation exiting from the patient
- phosphor screen emits light when struck by radiation, light gathered by tube & converted to electric current, charges the capacitor, reaches a preset level, terminates exposure (quick process) - cannot use a lead-backed cassette w/ these |
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Ionization Chambers
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- placed between the patient & the IR (measures remnant radiation leaving patient)
- filled w/ air; measure amount of ionization that occurs in air within chamber - terminates exposure when predetermined amount of air has been ionized - typical 3 chamber configuration seen in clinic |
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What is a critical skill to have when using ionization chambers?
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Critical positioning skills are needed to place the area of interest over the correct ionization chamber
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The use of AEC requires the art of ________________
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Positioning
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What does the AEC do when all 3 chambers are selected?
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Averages the signals from the cells; when appropriate density is reached the exposure is terminated
Cell receiving most radiation will contribute the greatest electrical signal |
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Density Controls
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- regulate image density (-2, -1, 0/N, +1, +2)
- DENSITY CONTROLS SHOULD NOT BE USED TO COMPENSATE FOR PATIENT PART THICKNESS OR KVP CHANGES |
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What will poor patient positioning increase while using AEC?
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Repeat image rate
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Collimation w/ AEC
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- do NOT collimate close to areas of ionization chamber
- too wide of collimation will produce scatter radiation that may undercut the patient |
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Minimum Response Time
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Time necessary for AEC to respond to the ionization & send a signal to terminate exposure (0.001 second)
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What's the purpose of a back-up timer?
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Prevents extreme over exposure of the patient to radiation
Should be set to 150% of anticipated manual exposure time |
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Anatomically Programmed Radiography - Cookbook Radiography
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Specific "recipes" are used for different sized patients
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Using Film/Screen Combos w/ AEC
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- most AEC's are calibrated to a particular film/screen speed
- changing the speed will result in undesired high/low density unless setting are changed to accommodate the speed change |
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Extremity cassettes used in conjunction w/ AEC calibrated for par screens will result in __________________ (overexposed/underexposed) films
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Underexposed
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Is 100 speed considered a higher or lower speed?q
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Lower (less than 400)
Means that sensitivity to radiation is lower, need more radiation to get proper image |
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AEC Don'ts
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DON'T use AEC on extremities such as hands/feet/ankles
DON'T increase mA to increase the density of image DON'T increase the kV to increase the density of the image |