The Grid
Kyle Thornton
DMI 50B
What Is A Grid?
Invented in 1913 by Gustaf Bucky
– Consisted of a framework containing lead foil
strips standing on edge, parallel and equidistant to
each other
In 1920, Hollis Potter invented a mechanism
for suspending the grid in a framework that
moved between the patient and film
– The motion eliminated the grid lines in the image
The grid is the most effective way to remove
secondary radiation from large radiographic
fields
What Does A Grid Do?
A grid is an important radiographic tool
A grid absorbs scatter radiation before it
reaches the film
A grid improves contrast on the film
A grid has a special composition and
many different types
Used properly, the grid is a
technologist’s best friend
Grid Construction
Grid ratio
Grid frequency
Interspace material
Lead strips
Grid Ratio
Three important dimensions of a grid
– Grid thickness - T
– Interspace material thickness - D
– Grid height - h
Grid ratio is the height divided by
interspace material thickness
– Grid ratio = h/D
Why Is Grid Ratio Important
Grid ratio determines how scatter radiation is
“cleaned up”
The higher the grid ratio, the more cleanup
Grids of higher ratios require more technique
This results in a higher patient dose
Ratios range from 5:1 - 16:1
Mammo grids have very low ratios
Grid Ratio Equation
The distance between each grid strip is
150 m and the height is 1.2 mm. What
is the grid ratio?
Hint: Ratio = h/D
Are you ready?
Answer: 8:1
Grid Frequency
The number of strips or lines per inch or centimeter is
grid frequency
Higher frequencies display less lines
Higher frequencies affect patient dose
Higher frequencies are generally associated with
higher ratios
Most grid frequencies are 60-110 lines/inch
Mammo grids have very high frequencies, but low
ratios
Interspace Material
The material between the grid strips
Maintains a precise separation between
the strips
Generally constructed from aluminum or
plastic fiber
Aluminum has definite advantages over
fiber
Grid Strips
Should be very thin and have high
scatter absorption properties
Lead is best
The entire grid is encased in aluminum
for protection
Sometimes it is further encased in
plastic for more protection
Grid Performance
Contrast improvement factor
Bucky factor
Selectivity
Contrast Improvement Factor
Grids remove scatter radiation before it
reaches the film
Therefore it improves contrast
Contrast improvement factor compares
contrast improvement with a grid to that
without a grid
Contrast Improvement Factor
Equation
K = Radiographic contrast with grid
• Radiographic contrast without grid
Most grids have a contrast improvement
of 1.5 - 2.5
Contrast improvement is higher with
higher ratio grids
Lead content also determines contrast
improvement
Bucky Factor
Also called grid factor
This compares the increased technique necessary for
grid use
Bucky factor will increase with with increasing grid
ratio
It will also increase with increasing kVp
B = Incident remnant radiation
• Transmitted remnant radiation
The amount of radiation hitting the grid will always be
greater than the amount hitting the film
Grid Selectivity
Related to grid construction itself
The total lead content of the grid has an
influence on selectivity
The more lead, the more cleanup
= Primary radiation transmitted through grid
• Scatter radiation transmitted through grid
General Rules Of Grid
Characteristics
High ratio grids have high contrast
improvement factors
High frequency grids have thin strips of
interspace material
Heavy grids have high selectivity and
high contrast improvement factors
Grid Types
Linear parallel
Crossed
Focused
Moving grids
– Single stroke
– Reciprocating
– Oscillating
Linear Parallel Grid
Simplest to construct
The grid strips are parallel
Most latitude
Crossed Grid
Two linear grids at right angles to each other
Was used primarily for pneumoencephalography
Used for high contrast studies
Very high cleanup
Not used very much
Must be centered exactly
Must be directly perpendicular to grid
Focused Grid
The strips run on one axis and are tilted
Strips are parallel to the primary x-ray
path across entire film
Must use within a proscribed distance
Grid Cutoff
A big problem with linear and crossed
grids
Less of a problem with focused grids
The primary beam has been absorbed
Has a definite effect on image detail,
density, and contrast
Moving Grids
Single Stroke
Antiquated
Grid had to be cocked with a spring
mechanism
Worked in synch with exposure time
The mechanism moved once
throughout exposure
Had to be reset for each exposure
Reciprocating Grid
Moves back and forth during exposure
Motor driven
Does not have to be reset for each
exposure
Oscillating Grid
Similar to a reciprocating grid
Moves in a circular motion as opposed
to back and forth
Advantages And Disadvantages Of
Moving Grids
Advantages
– No grid lines
– Problems occur infrequently
Disadvantages
Mechanical problems may occur
Very infrequently, motion is detected on
radiograph
Grid Errors
Off-level
– Beam is not perpendicular to grid
Off-center
– Beam is not centered to center of grid
Off-focus
– Focusing distance not observed
– Focused grid only
Upside down
– Focused grid only
– Causes severe grid cutoff in periphery of film
Grid Selection
Depends upon body part to be
radiographed
Chest radiography uses high kVp
8:1 ratio can be used for most general
work
– Up to about 90 kVp
Focused grids are generally superior
Lower ratio grids offer more positioning
latitude
Grids And Patient Dose
Patient dose increases with increasing
grid ratio
High ratio grids are generally used for
high kVp studies
Patient dose decreases with higher kVp
use
Less radiation is absorbed in tissues
with higher kVp
Memorize This Table
You Will Find It On Page 226
Grid Ratio mAs increase kVp increase
Non-grid 1X 0
5:1 2X 8-10
8:1 4X 13-15
12:1 5X 20-25
16:1 6X 30-40
Alternatives To Grid Use
Air-gap technique
OID is increased
Equal to approximately 8:1 grid
Increases magnification
Distance must be increased to overcome
magnification
– Patient dose increased
Not effective with high kVp