Ch 18 Grids Flashcards
What does a grid do
Cleans up scatter/fog
Improves contrast in the image
High KVP = what scale contrast
Low contrast, long scale
Low kvp = what contrast
High contrast, short scale
When do we use a grid
KVP is above 60
Body part is more than 10 cm
What are the strips in a grid made of
Thin lead foil
What is the interspace material in a grid made of
Aluminum
What are oscillating grids
Grids that move back & forth to keep grid lines out of images - magnet pulls back
Who invented the grid
Gustav Buckey
Do grids also absorb useful photons
Yes
The ratio of the height of the lead strips to the distance between the strips
Grid Ratio
Grid ratio formula
Height divided by distance
Higher ratio grids allow more or less scatter through
Less
Disadvantages of high ratio grids
Require more accurate positioning
More prone to grid errors
The higher the grid frequency the thinner or thicker the strips
Thinner
Number of grid lines per inch or centimeter
Grid frequency
Lead strips, running in only One Direction
Linear grids
Lead lines run at right angles to one another
Criss cross or cross hatched grids
Parallel grid
Lead and interspace material run parallel to one another
Focused grids
Lead lines and interspace material are angled to match the divergence of the beam
Which grids are best used with long SID
Parallel grids
Distance between the grid and the convergence point
grid radius
Reciprocating grid
Motor drives the grid back and forth
What type of grids are used in portable
Stationary grids
What can a mounted grid also be called
Potter Bucky Diaphragm
GCF no grid
1 x Mas
5:1 GCF
2 x Mas
6:1 GCF
3 x MAs
8:1 GCF
4 x MAs
12:1 GCF
5 x MAs
16:1 GCF
6 x MAs
Formula for GCF
Mas1/Mas2 = GCF1/GCF2
Focused grid is used upside down on the IR
Upside Down grid error
What happens with an upside down grid error
Good exposure in the center but you lose detail in the peripheries
Occurrs when the xray beam is angled across the lead strips
Off level grid error
IR or tube is misaligned - image appears elongated - usually only occurs with stationary grids - only can occur with a parallel grid
Off level grid error
Occurs when the CR of the xray beam is not aligned from side to side with the center of a focused grid
Off center grid error
Lateral decentering - creates image similar in appearance to off level grid error
Off center grid error
Types of grid error
Off center
Off level
Upside down
Off focus
Occurs when using an SID outside of the recommended range
Off focus grid error
What does off focus grid error cause
Cutoff - lighter along the peripheries
Causes a zebra effect
Moire Effect
Observing grid lines on the image - mainly with CR
Moire Effect
Another method for limiting scatter to reaching the IR - based on OID
Air Gap Technique
Air gap technique will also cause this - unwanted
Magnification
