The GridThe Grid

Kyle Thornton DMI 50B

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 200 m and the height is 2.4 mm. What is the grid ratio?

Hint: Ratio = h/D Step 1 – Identify h Step 2 – Identify D Step 3 – ???? Step 4 - ????

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 and low contrast improvement factors

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

Moving GridsSingle 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

Grid mounted within Bucky Tray

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 Cutoff

A big problem with linear and crossed grids

Less of a problem with focused grids

The primary beam has been absorbed

Has a negative effect on image detail, density, and contrast

Grid Errors

Off-center– Beam is not centered to

center of grid Upside down

– Focused grid only

– Causes severe grid cutoff in periphery of film

More Grid Errors

Off-focus error– Focusing distance

not observed – Focused grid only

Using the incorrect focal distance results in grid cutoff in the periphery of the image

Yet another grid error

Off-center – off-focus– Partial grid cut-off

occurs over the entire image

Even more grid errors…

Off-level– Beam is not

perpendicular to grid• Or

– Grid is not perpendicular to the beam

• Either way, you’re repeating that film

How many grid errors can there be?

The classic grid error– Focused grid placed

upside down

Demo of Focused Grid Used Upside Down

http://www.youtube.com/watch?v=HS4DPt0Gjz0&feature=related

Grid errors from the beam and grid’s perspective

Summary of grid errors and associated results Off-level

– cutoff across image; underexposed, light image Off-center

– Grid cutoff across image; underexposed, light image Off-focus

– Grid cutoff toward edge of image Focused Grid Placed Upside-down

– Severe grid cutoff toward edge of image Off-center, off-focus

– Grid cutoff on one side of image

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

Suggested Grid Conversion Factors

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 Not effective with high kVp