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46 Cards in this Set
- Front
- Back
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what is the primary function of a grid |
to improve visibility of detail by improving contrast |
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when does scattered radiation occur |
when a primary photon interacts with an atom within the patient and is misdirected |
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what is the result of misdirected information |
it is unwanted |
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how does scattered radiation impair image quality |
by reducing contrast and obscuring recorded detail |
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what is the primary beam that leaves the tube |
polyenergetic, contains photons of various energies |
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kVp is what? not what? |
is peak, not average |
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what does the primary beam consist of |
brems radiation characteristic radiation |
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brems radiation
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caused by the conversion of energy of electrons that are stopped by the target
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characteristic radiation |
is emitted by the target metal due to excitation of its atoms |
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scatter radiation is produced by what |
compton interaction and some characteristic radiation resulting from photoelectric interaction |
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what happens to compton scattered photons energy as KVP increases |
it increases |
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what kind of energy does characteristic photons have? |
extremely low, but many of the scattered compton photons have enough energy to pass through the body and approach the film from diff directions |
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how much of the density should be a result of scattered radiation |
no more than 1/4 |
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the ratio of scattered radiation relative to primary radiation increases as.. |
1 an increase in the area of radiation field and thickness of part traversed by the beam 2 increase in the tube potential. 3 increase in the density of tissue |
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what does decreasing scatter do to contrast |
increases contrast |
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who developed the grid |
Gustav Bucky; original was cross-hatched |
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what are the 2 classifications of grids |
1 stationary 2 moving |
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what does the modern grid measure |
about .05mm in width |
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why is aluminum preferred for the lead strips |
1 improved durability of the grid 2 to absorb scattered radiation caused by interaction with the lead strips |
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grid cutoff |
when primary radiation strikes the lead strips and is absorbed |
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what is the main disadvantage of the stationary grid |
1 they show up as thin white lines on the film |
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what is the rule with an 8:1 grid |
the increase in exposure is a factor of 4 |
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what are the 2 physical factors in grid efficiency |
1 grid ratio 2 grid frequency |
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grid ratio |
ratio of the height of the lead strips to the distance between the strips. R=h/D |
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grid frequency |
number of lead strips per cm or in |
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if the grid frequency is greater, what do the lead strips have to be |
thinner, in order to maintain a particular ratio |
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as grid frequency increases, what happens to grid ratio |
it increases to maintain the same efficiency |
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what is the functional factor in grid efficiency |
selectivity |
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cutoff |
absorption of the primary radiation by the grid |
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what happens if you have a larger ratio of primary radiation to scattered radiation |
the selectivity will be greater |
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what happens to exposure as selectivity increases |
it increases |
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what are the 2 types of grids |
1 stationary 2 moving |
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what do stationary grids include |
1 cross grid: 2 grids at 90 degree angles to each other 2 grid cassette: built into the cassette; durable but limited, heavy 3 wafer grids: can be taped to reg cassette 4 slip on or cap on: durable and easy to use |
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parallel grid(non focused) |
lead strips are parallel to each other |
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periphery cutoff |
great absorption of primary radiation toward the edges of the grid |
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focused grid |
most common type; made with parallel strips that slant more as you move toward the lateral edges of the grid |
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focusing distance |
vertical distance between the convergence line and the center of the grid |
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when was the moving grid deleveloped |
1920 by dr hollis potter |
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while moving parallel to its surface, how else must the grid move |
perpendicular to the long axis of the lead strips |
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precautions for using a focused grid |
1 SID; the high the grid ratio, the more exact you have to be in regards to distance and centering 2 angulation of the beam 3 centering the tube 4 tube side vs film side; reverse cut off |
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grid specifications |
1 grid ratio 2 grid frequency 3 focusing distance 4 source to grid distance limits (+ or - 25%) 5 contrast improvement factor |
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grid moving mechanisms |
1 single stroke type: "cocked" then released either by a string or an electromagnetic tripping device 2 reciprocating: moved continuously during exposure (min exposure 1/20 second) 3 oscillating or trill: oscillates or vibrates over a rapidly decreasing distance until after the end of exposure |
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causes of grid lines with moving grids (corduroy effect) |
1 synchronism: beam not generated continuously, but in intermittent showers or bursts of photons corresponding to the peaks in the voltage applied to the tube 2 exposure starting: before the grid has reached full speed or continuing after the grid travel has slowed down or stopped 3 grid moving: too slowly may produce either clearly visible grid lines or density variations on the radiograph |
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why have stationary grids replaced moving grids |
1 cheaper 2 less malfunctions 3 less exposure to patient |
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what happens if you increase the air gap (OID) |
allows more of the scattered photons to move laterally outside the film area |
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when must you increase exposure |
when using the air gap technique |
