XrayFund

5/26/99

Control of Scatter Radiation

• if your equipment is properly installed and maintained the resulting film will show evidence of four sides of collimation
• collimator adds about 1mm of Aluminum filtration equivalent

Grids

• screens alone cannot adequately improve film contrast

• selective filtration b/w the pt and film is ineffective
• invented by Dr. Gustave Bucky, 1913

Principle function—improve film contrast

• grids are made of

1. grid material, usually lead
2. interspace material, may be plastic, aluminum, or fiber

• note—fiber is hygroscopic resulting in uneven density on the resulting films and may cause the grid to wrap
• grid material is approximately 50 microns (thickness of a piece of hair)thick separated by interspace material approximately 350 microns thick
• some primary beam will b eliminated by the grid material

• grids remove b/w 80-95% of scatter emanating from the pt going towards the film
• non-grid holders—holds cassette in front of upright—use for many neck pictures, ¼ the dose which is proportional to mAs

Grid Ratio

• height of strip/interspace thickness
• done in microns

Grid Frequency

• the number of lead lines per unit measure (e.g. 85lpi—bucky (moving) system or 103lpi—appropriate for a stationary grid)
• moving grids require fewer lines per inch

• stationary grids must have the lines very close together so the lines become imperceptible on the resulting film
• the lines are still present you just can't see them at normal distances
• up close and w/magnifying glass you can see the lines

Grid label

• tube side—have to go in a certain direction
• 60 lines/inch
• 12:1 ratio
• recommended focal rage 36" to 40"
• put in backwards is like laying sod down w/green side down

Type of Grids

• linear or parallel
• focused
• crossed—2 grids at 90° of each other—no tube tilts allowed

Grid Recommendations

• 12:1 ratio
• 103 lead lines/inch

• focused from 40 to 72 inches
• aluminum interspaced
• appropriate size
• 18" by 18" if you will only perform sectional films
• 18" by 36" if you will perform scoliosis radiography

Problems

• grid cutoff
• off level
• off center
• off focus
• upside-down

• the part is intentionally or necessarily separated from the film—results in an increased object-film distance
• this allows scatter to physically miss the receiver
• scatter that does intercept the film is attenuated by the inverse square law

• this method works best w/kVp settings below 90—due to the direction of the scatter produced
• to offset the magnification created by the increased OFD a longer focal-film distance is used
• air does not appreciably filter the beam

Kodak Video

• silver gives xray film is photosensitivity

• silver bromide—light sensitive
• film is particularly sensitive to mercury, silicon, sulfur
• some extremities and mammography is done on single emulsified film
• most film is double emulsified (on each side of the film)

Film

• emulsion is primarily halide crystals in gelatin
• may be coated on one or both sides of the film
• emulsion speed (sensitivity to radiation) is a function of grain diameter or surface area of the silver crystals
• flat (tabular grains) offer a larger cross section w/more light gathering capability than conventional grains
• tabular grains are more tolerant to developer activity (more forgiving)

Intensifying screens

• screen construction
• protective coating
• closest to film
• minimizes damage to screens from handling and dirt
• reduces static electricity

• phosphor
• converts x-ray photon energy to light
• formerly, calcium tungstate (Edison) (1900-1973)
• currently, rare earth mixtures—3M in Minnesota discovered in 1973

• screen phosphors should have:

1. a high atomic number
2. high conversion efficiency
3. light matched to film used—blue or green light source to same color film
4. minimal afterglow

• rare earth materials used for intensifying screens include:

1. gadolinium
2. lanthanum
3. yttrium

• different proportions of 1 and 2 depends upon if blue or green emitting
• Kodak primarily makes green

• reflective layer
• light is produced isotropically
• takes light produced by screen moving away from the film and directs it toward the film
• increases the speed of the screen by doubling the number of photons reaching the film

• base
• provides mechanical support for phosphor layer

• luminescence
• light produced by the screen
• x-ray photon temporarily elevates an outer shell electron from its orbit
• this adds energy to the electron
• when the electron falls back into orbit the excess energy is released in the form of light
• the wavelength of light produced is specific for the phosphor material

• screens provide approximately 95% of the film's exposure—non screen exposures require 40% more beam—it is not silver bromide w/electron
• use the film that is spectrally matched to the light from the screen
• rare earth screens manufactured by:
• Kodak give off green light (Lanex fast)
• Sterling give off blue light (Quanta Super Rapid)
• Sterling give off UV light (UV Super Rapid)—Dr. Guerbert personally likes
• Konica give off green light (KS)
• AGFA give off green light (CURIX Ortho Fast)
• We want a faster screen b/c we shoot pt's standing up, a little less detailed, reduce the work load and the pt dose