The Grid in Radiographs

·         Scattered radiation detected by a detector device be it digital or film-screen cause darkening (fog), but does not add information content to the image.

·         Grids are used to combat the effects of scatter on the image and thus improve contrast.

·         Disadvantage is that they increase patient exposure/ dose

·         Placed between patient and the detector and uses geometry to reduce the amount of scatter reaching the detector

o   Primary radiation is orientated in the same axis as the lead strips and passes through

o   Scatter is multidirectional and is absorbed by the strips (fig 8-1 C:99; fig 6-25 BB:168)

·         2 basic types of grid pattern (Orientation of the lead strips)

o   Linear

o   Crossed

o   The above may be focused (converging) or parallel

·         Grid ratio = Height (mm)/ Interspaces (mm) – this is the single most important parameter affecting performance

·         Grid frequency refers to the number bars per unit length, but does not influence the scatter cleanup – but is important in digital radiography as it may create aliasing problems

·         Parameters used to describe grid properties:

Primary transmisssion	Fraction of primary photons that pass through the grid; (I – intensity): Tp=I with grid/I without grid Calc. Tp = Interspace (mm)/    Interspace (mm) + Strip (mm)
Bucky factor High B – increases film quality; High B – increases exposure and dose	Ratio of entrance exposure to the patient without and with grid: B = Incident rad. (without grid)/ transmitted rad. (with grid)
Contrast improvement factor	Contrast with grid vs. contrast without the grid: K = contrast with/ contrast without Depends on kVp; Thickness; Field size
Lead content	Measure of amount of lead in the grid; May indicate ability to absorb scatter: mg/cm2

·         Artifacts – grid cut-off

o   Focused grid upside down

o   Lateral decentering (off center) and tilting (angulation)

o   Focus grid distance decentering (off focus)

§  Near > Far

o   Combined lateral and focus-grid distance decentering

·         Bucky grid:

o   Moves to blur out the shadows cast by lead strips

o   Reciprocate – move back and forth continuously throughout exposure

o   Important to move it very fast

o   Important to synchronize transverse motion with pulses of the generator