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 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 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

Grid mounted within Bucky Tray

Advantages And Disadvantages Of Moving Grids 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 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

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