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73 Cards in this Set
- Front
- Back
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What Effect does scatter have on image quality? |
decreases image contrast |
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Causes of scatter radiation (3) |
High KVP (increases compton interactions) patient thickness increased field size |
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How to reduce scatter (3) |
collimation use of grids air gap technique |
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name for automatic colimator |
PBL - Positive beam limitator |
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3 types of beam restrictors |
Apeture diaphram cones and cylinders variable apeture collimator |
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causes of "cone cutting" |
when using cones and cylinders, inadequate alignment of xray source, cone and IR |
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*simplest form of beam restrictor, lined with lead |
Apeture Diaphram |
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is a modification of Apeture Diaphram |
Cones and cylinders |
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Variable Apeture |
Light localizing collimator , uses shutters to adjust light field |
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Purpose of Grid |
Absorbs scatter, Improves contrast |
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which creates more compton scatter High KVP or Low KVP? |
High KVP |
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radiation that has no interaction with the patient and does not lose any energy when traveling to IR |
Transmitted Radiation |
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radiation that has an interaction with patient tissue and moves in all directions |
Scatter radiation |
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Effect of scatter on contrast |
creates lower contrast and anatomic detail, adds general density to whole image |
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Factors that affect the amount of scatter |
Patient thickness Photon energy Field size |
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What factors do NOT affect scatter |
SID or focal size |
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Best way to reduce scatter |
collimation and low KVP |
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How does patient thickness increase scatter? |
more body tissue give more atoms available for interaction |
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which size body parts are less likely to have compton interactions large or small? |
small |
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Soft tissue causes more or less compton interactions? |
more |
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when do you use a grid? |
when patient is thicker than 10cm and/or using KVP of 60 or higher |
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Controlling factor for beam energy |
KVP |
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as KVP increases |
scatter increases |
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what happens to patient does when KVP is decreased? |
increases because more photons are absorbed by the tissue |
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Which field sizes have less scatter, larger fields or smaller fields? |
smaller image field size |
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Why does a larger image field have more scatter |
because more tissue is exposed allowing more area for interactions |
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as image field gets smaller |
scatter decreases |
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purpose of collimation |
to define the size and shape of primary beam to provide a visible light field which outlines x-ray field |
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types of collimators |
LLC-light localizing colimator PBL-Positive beam limitator Iris |
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This collimator prevents having a field size larger than the IR |
PBL-positive beam limitator |
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collimation should never exceed what? |
IR size |
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2 areas in construction of grids |
lead strips and interstitial space |
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materials used to make strips |
lead foil |
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advantages to lead foil |
ductile inexpensive cleans up scatter |
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material used in the interstitial space of a grid |
aluminum or plastic fibers |
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advantages of aluminum over plastic fibers for interstitial use in grids |
it is non absorbent(hygroscopic) , less visible grid lines, easily formed |
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disadvantage of aluminum for interstitial use in grids |
increases beam absorption (dose) |
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which material is preferred in interstitial use in grids and why |
plastic fibers because it doesnt increase patient dose |
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why is plastic preferred over aluminum in interstitial use in grids |
because it does not increase beam absorption (dose) |
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most common range for GRID RATIO |
8:1 - 10:1 |
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general range for GRID RATIO |
5:1 - 16:1 |
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Define Grid ratio |
ability to clean up scatter, height of strips to space between strips |
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Define Grid Fequency |
The number of lead strips per cm or inch |
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typical range for GRID FREQUENCY |
60-200 lines per inch |
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as Grid Ratio increase, Grid frequency.... |
increases |
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technique that uses increased OID to reduce scatter radiation reaching the IR |
Air gap technique |
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Ratio of mAs required w/ a grid to the mAs required w/o a grid to produce the same optical density |
Bucky Factor |
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Bucky Grids |
moving grids designed to blur out the grid lines and absorb scatter radiation |
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two linear grids placed on top of one another so that the lead strips form a criss-cross pattern |
Crossed grids |
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a combination of transmitted and scattered radiation that passes through the patient |
Exit radiation |
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grids whose radiopaque lead strips are tilted to align at a predetermined SID, with the divergent x-ray beam |
Focused Grids |
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Grid Cut-off |
the interception of transmitted x-ray photons by the lead strips of the grid, resulting in lighter density at one or both edges of the film |
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Oscillating grid |
mechanism that moves the grid in a circular pattern above the image receptor (with magnets) |
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grids that have parallel lead strips |
parallel grids |
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Purpose of interstitial space |
to maintain precise separation between lead strips |
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how much scatter does a 5:1 ratio grid reduce? |
85% |
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how much scatter does a 16:1 ratio grid reduce? |
97% |
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a first stage variable aperture filters what? |
filters off-focus radiation or STEM |
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these grids require high radiographic technique and result in higher patient dose |
high frequency grids |
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What is the purpose for interstitial material in a grid? |
to maintain precise separation of lead strips |
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how many lines per cm does a low frequency grid have? |
60-100 |
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how many lines per cm does a high frequency grid have? |
more than 100 |
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type of grid cut-off that is possible with using portables or a table top exam |
off-level |
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how much OID is needed to make the Air-gap technique effective? |
at least 6 inches |
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a motor driven grid moving back and forth several times during an exposure |
replicating grid |
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Disadvantages of moving bucky grids |
*require bulky mechanisms, *increased OID, *increased patient dose |
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using a moving grid vs a stationary bucky increases patient dose by how much?
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15% |
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between using high KVP and high ratio grids and low KVP and low ration grids which causes less patient dose? |
High KVP and high ration grids |
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crossed grids clean up how much scatter compared to linear grids? |
cleans up twice as much scatter as linear grids |
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If a crossed grid is not perfectly aligned with the xray beam what can occur? |
grid cutt-off |
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Tilt-table technique can only be effective if |
the table and xray tube are perfectly aligned |
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exams using a KVP over 95 require what type of grid? |
high frequency grid
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uses a lower ratio grid 6:1 or 8:1 with wide focal range 40-72 inches |
portables |
