Scatter Radiation 2 Flashcards
Grid Efficiency
How well a grid removes scattered radiation from the X-ray beam
It is determined by physical factors.
Physical Factors
deal with the physical structure of the grid.
Lead content
the greater the lead content, the more efficient the grid at removing scatter.
Grid ratio
The ratio of the height of the lead strip (h) to the space between the strips (d). r = h/d
The greater the ratio, the more efficient removing scatter.
Higher ratio absorbs more primary radiation so mA needs to be increased
Higher grid ratio means lower grid latitude.
Bucky factor or grid conversion factor
mAs with grid / mAs without grid
Changing from one type of grid to another equation
New mAs = original mAs x new BF / old BF
Grid frequency
The number of lead strips per unit distance
Can be measured in either #/inch or #/cm
The greater the frequency the shorter the lead strips have to be to keep the same grid ratio
Grid type
cross hatch grids are more efficient at removing scattered radiation than linear grids.
Patient dose is higher and X-ray beam cannot be angled
Grid performance evaluation
Used to evaluate grid efficiency
Grid selectivity
Ratio between the amount of primary radiation transmitted through a grid to the amount of scattered radiation transmitted through a grid.
Contrast improvement factor
Measures how much the contrast is improved by the use of a particular grid
Grid advantages
Increased image contrast
Better visibility of detail
Grid disadvantages
Grid cutoff if not used correctly
Cost
Higher patient dose
Grid disadvantages
Grid cutoff if not used correctly
Cost
Higher patient dose
Beam restrictions
Devices to control the size of the field exposed to radiation and therefore the amount of scattered radiation
Aperture diaphragm
A sheet if lead with a certain size hole cut in the center and placed immediately beneath the X-ray tube window
Extension cone or cylinder
A long metal cylinder with an aperture opening at each end
Commonly used in dental X-ray units
Collimator
Variable aperture devices that are required for all diagnostic X-ray units
Positive beam limitation
Automatic collimating system that many collimators are equipped with
Lead vinyl masking
the use of lead vinyl for controlling scattered radiation in lateral lumbar and thoracic spine radiographs has been well documented.
Compression bands
elastic bands that attach to either side of the x-ray tabletop and have a hand crank to adjust the tension so as to gently compress the patient.
• By reducing patient thickness, scattered radiation is decreased.
Air gap technique
The object to image distance is increased to allow the scattered radiation to miss the image receptor and not affect image quality
The air does not absorb the scattered radiation
