GRIDS Dr.Archana Koshy

• The radiographic grid consists of a series of Lead foil strips separated by X-ray transparent spacers .

• Invented by Dr.Gustave Bucky in 1913

• Most effective way of removing scatter radiation from large radiographic fields .

• The interspaces of grids are filled either with aluminium or some organic compound .

GRID RATIO : Ratio between the height of the Lead strips and the distance between them .

• GRID PATTERN : Orientation of the Lead strips in their longitudinal axis .

• Pattern of the grid that we see from as a view from the top

I. LINEAR GRID

II. CROSSED GRID

III. PARALLEL GRID

IV. FOCUSSED GRID

LINEAR GRID

The Lead strips are parallel to each other in their longitudinal axisMajor advantage is that they allow us to angle the X-ray tube along the length of the Grid without loss of primary radiation from grid ‘cutoff ‘

CROSSED GRID Made of two superimposed linear grids that have the same focusing distance . Grid ratio of two crossed grids is equal to the sum of the ratio of the two linear grids . Cannot be used with oblique techniques requiring angulation of the Xray tube .

FOCUSSED GRID

Grid made up of lead strips that are slightly angled so that they focus in space .May be either linear or crossed .

Line focused grids converge at a line in space called CONVERGENT LINE .

Crossed Grids converge in a point in space called CONVERGENT POINT .

FOCAL DISTANCE : Perpendicular distance between the grid and the convergent line or point .

PARALLEL GRID

Lead strips are parallel when viewed in cross section .

Focused at infinity so they do not have a convergent line .

Can only be used effectively with either very small Xray fields or long target-grid distances .

LINES PER INCH : Number of Lead strips per inch of the grid . Calculated by adding the thickness of the Lead strips and interspaces and dividing this sum into 1 .

EVALUATION OF GRID PERFORMANCE • Three main methods :

I. PRIMARY TRANSMISSION (Tp)

II. BUCKY FACTOR (B)

III. CONTRAST IMPROVEMENT FACTOR (K)

PRIMARY TRANSMISSION • Measurement of the percentage of primary

radiation transmitted through a grid . 1. Made with the grid in place to determine the

intensity of the radiation through the grid .

2. Made after removal of the grid to determine the intensity of the radiation directed at the grid .

Tp = Ip/Ip’ x 100

BUCKY FACTOR • Ratio of the incident radiation on the grid to the

transmitted radiation passing through the grid .

• Indicates the amount of exposure factors that should be increased when the change from grid to a non grid technique is made .

• Measure of the Grid’s ability to absorb scatter radiation

• Unlike primary transmission , Bucky factor indicates the absorption of both primary and secondary radiation .

B= INCIDENT RADIATION / TRANSMITTED RADIATION

CONTRAST IMPROVEMENT FACTOR • Ratio of the contrast with a grid to the contrast

without a grid .• Measure of the grid’s ability to improve contrast

which is its primary function . • It depends on : 1. kVp2. Field size3. Phantom thickness

• More closely related to the Lead content of the grid than any other factor . (g/cm²)

GRID CUT OFF • Loss of primary radiation that occurs when the

images of the Lead strips are projected wider than they would be with ordinary magnification .

• Result of a poor geometeric relationship between the primary beam and the Lead foil strips of the grid .

• Cut off is complete and no primary radiation reaches the film when the projected images of the Lead strips are thicker than the width of the interspaces.

• Amount of cut off is always greatest with high ratio grids and short grid focus distances .

• 4 SITUATIONS THAT PRODUCE GRID CUT OFF :

1. FOCUSED GRIDS USED UPSIDE DOWN

2. LATERAL DECENTERING (GRID ANGULATION )

3. FOCUS GRID DISTANCE DECENTERING

4. COMBINED LATERAL AND FOCUS-GRID DISTANCE DECENTERING .

UPSIDE DOWN FOCUSED GRID • When a focused grid is used upside down, there is

severe peripheral cut off with a dark band of exposure in the center of the film with no exposure at the periphery .

• The higher the grid ratio,narrower the exposed area .

LATERAL DECENTERING • When the X-ray tube is positioned lateral to the

convergent line but at the correct focal distance.

• Uniform loss of radiation over the entire surface of the grid, producing a uniformly light radiograph .

• 3 factors affect the magnitude of cut off from lateral decentering :

-Grid ratio-Focal distance -Amount of decentering

OFF LEVEL GRIDS When a linear grid is tilted , there is uniform loss of primary radiation across the entire surface of the grid . Effect on the film being same as that of lateral decentering .

FOCUSED GRID DISTANCE DECENTERING • The target of the X-ray tube is correctly centered

to the grid , but it is positioned above or below the convergent line .

• Cut off is greater with near than far focus –grid distance decentering

• The central portion of the film isnt affected but the periphery is light .

• The loss of primary radiation is directly proportional to the grid ratio and the distance from the center line.

COMBINED LATERAL AND FOCUS GRID DISTANCE DECENTERING • Causes an uneven exposure resulting in a film

that is light on one side and dark on the other side.

• Directly proportional to the grid ratio and decentering distance and inversely proportional to the focal distance of the grid .

MOVING GRIDS • Invented by Dr.Hollis.E.Potter in 1920 .• Also called a Bucky grid • Moved to blur out the shadows cast by the lead

strips .• Continuously moves 1 to 3 cm back and forth

throughout the exposure .

• Precautions : 1. The grid must move fast enough to blue the lead

strips 2. The transverse motion of the grid should be

synchronous with the pulses of the Xray generator .

DISADVANTAGES : • Costly • Subject to failure • May vibrate the Xray table • Places a limit on the minimum exposure time

because they move slowly . • INCREASES THE PATIENT RADIATION DOSE.

GRID SELECTION • The price of increased “cleanup “ with high ratio

grids is that patient exposure is considerably increased and that Xray tube centering becomes critical .

• 8:1 grids will give adequate results below 90 kVp

• Above 90 kvp,12:1 grids are preferred• In cases of Biplane cerebral angiography, crossed

grids are preferred as there is a great deal of scatter radiation.

AIR GAP TECHNIQUE• Scatter radiation arising from the patient from

Compton reactions disperses in all directions , so the patient acts like a large light bulb .

• The closer the patient is to the film, the greater the concentration of scatter per unit area .

• With an air gap, the concentration decreases due to more photons missing the film in the gap .

• Used in Magnification radiography and Chest radiography .

GUIDELINES TO SELECT THE OPTIMUM GAP WIDTH 1. Thicker the part,the more advantageous a larger air gap

2. The first inch of any air gap improves contrast more than any subsequent inch

3. Image sharpness deteriorates with increasing gap width unless the focal film distance is increased to compensate for the greater magnification.

4. If the gap is widened by moving the patient away from the film with a fixed focal film distance the patient is closer to the X-ray tube and his exposure increases.