Chapter 18
Contrast improvement ability "K" Factor
"K" Factor Compares radiographic contrast of image with grid to radiographic contrast of image without grid Typically ranges between 1.5-3.5
Purpose of the Grid
-improves radiographic contrast in image -absorbs scattered radiation before it reaches image receptor
Grid Pattern Linear Gird
Allows primary beam to be angled along directions that lines are running In typical x-ray tube, strips run along long axis of table this allows for angling tube toward heel or feet of patient
Grid patterens
Criss-cross OR cross-hatched Linear -parallel -focused
Potter-bucky diaphragm
DR. Hollis Potter made improvements to use of grids - Realigned lead strips to run in one direction -Moved grid during exposure to make lines invisible on image
Selectivity Grids designed to absorb scatter sometimes do ....
Describes grid's ability to allow primary radiation to reach image receptor and prevent scatter Grids designed to absorb scatter - Sometimes do absorb primary radiation
Moire Effect
Digital systems -grid lines parallel to scan lines Can be prevented by high frequency grids
Grid frequency Number of lead strips per ___ or __ frequency range ?
Frequency range: 60-200 lines/in 25-80 lines/cm
Grid ratio example if grid has interspace of 0.5mm and lead strips that are 3mm high, what is its grid?
GR = 3mm/0.5mm GR = 6:1
Short axis grids
Grid lines runs across short axis of grid useful for portable chest procedures when cassette place crosswise - decreased change of grid cut-off
Grid Pattern Criss-Cross or Cross-hatched
Has horizontal lead strips Has vertical lead strips Primary beam must be centered perpendicular to grid Grid must remain flat
Selectivity High load content grids...
High lead content grids more selective
Grid performance evaluation International Commission on Radiologic Units and Measurements (ICRu) evaluated grid performance by two criteria:
International Commission on Radiologic Units and Measurements (ICRu) evaluated grid performance by two criteria: Selectivity Contrast improvement ability
Lead content of grid
Lead content -most important factor in grid's efficiency -measured in mass per unit area g/cm^2 High ratio, low frequency grids -tend to have highest lead content In general -Lead content greater in grid with high ratio and low frequency -As lead content increases, removal of scatter increases therefore contrast increases
Focused linear grids
Lead strips angled to match divergence of beam -primary beam will align with interspace martial -scatter absorbed by lead strips Convergence line Narrow positioning latitude -improper centering results in peripheral cut-off -only useful at preset SID distance -higher ratio grids require careful alignment with tube
Grid use Potter-Bucky diaphragm
Potter-Bucky diaphragm -The Bucky Mount 17" X 19" grid above cassette Moves grid during exposure
Grid movement Reciprocating Oscillating
Reciprocating - motor drives grid back and forth during exposure Oscillating -Electromagnet pulls grid to one side -Releases it during exposure
Grid conversion factor
Required increase in technique can be calculated -Grid conversion (GCF) or Bucky factor - GCF = mAs with grid mAs without grid
Digital imaging systems Very high-frequency grids ?
Very high-frequency grids 78-200 lines/in 70-80 lines/cm Recommended for use with digital systems -minimizes grid line appearance
Grids and Exposure factors Whenever grid is placed in beam to remove scatter
Whenever grid is placed in beam to remove scatter -Density/image receptor of radiograph will go down -Exposure factors must be increased to compensate for lack of density/image receptor exposure
Parallel linear grids
all lead strips parallel to one another absorb large amount of primary beam -resulting in some cut-off
Air-gap technique
alternative to grid use 10'' air gap has similar clean-up of 15:1 grid
Grids
created by radiologist, DR. Gustav Bucky (1913) - Crosshatches design allow primary radiation to reach image receptor absorb most scattered radiation primary disadvantage of grid use: grid lines or film
Creating the image Scatter
creates fog lowers contrast
Grid two types of linear grids
focused parallel
Grid Use Stationary grids
grids that can be attached to cassette for use grid cassettes
Grid dimensions h = D =
h = height of radiopaque strips D = distance between strips -thickness of interspace material
Selectivity Highly selective grids....
highly selective grids better at removing scattered radiation
Grid ratio
hight of lead strips divided by thickness of interspacing material grid ratio = h/D Higher grid ratio -more efficient in removing scatter Typical grid ratio range - 5:1 to 16:1
Scatter increases as:
kVp increases field size increases thickness of part increases Z# decreases
Indications for grid use
part thickness greater than 10 cm kVp greater than 60
Grid errors
proper alignment between x-ray tube and grid -very important Improper alignment results in cut-off Off-level Off-center Off-focus Upside-down Moire effect
Basic grid construction
radiopaque lead stripes separated by radiolucent interspace material - typically aluminum
Creating the image Transmission
responsible for dark areas
Creating the image Absorption
responsible for light areas
Grid frequency
typically, higher frequency grids have thinner lead strips