Grids

Scattered Radiation

Scattered radiation has no useful information, but rather degrades the visibility of detail by devastating contrast.  High kVp, large field size, and large part thickness all increase the production of scattered radiation.  The patient is the primary source of scattered radiation.  Scattered radiation increases density, however, it also decreases contrast.  The ways that radiographers control scatter is by restricting the beam, compressing tissues (for example, abdomen with patient prone), and using a grid. 

Density

Using a grid means that we must increase our exposure factors to achieve optimal density.  Either kVp or mAs can increased; however, increasing mAs is preferred because increasing the kVp will also decrease the contrast.  Increasing kVp is typically only done for chest radiography and when imaging very dense tissues.  A grid conversion factor is used to determine change in mAs when going to and from a grid and non-grid technique.  When going from a non-grid to grid, multiply the mAs by the quality factor. 

Average grid conversion factors

Example

A portable knee exam is performed without a grid using 60 kVp and 2 mAs.  If the exposure is repeated using a 12:1 grid, what mAs should be used to maintain density?

2 mAs x the grid factor of 5 = 10 mAs

When converting from one grid ratio to another, the following formula is used:

Example

A satisfactory abdominal radiograph is produced using an 8:1 grid, 85 kVp and 35 mAs.  A second film is requested using a 12:1 grid.  What mAs is needed to produce a second satisfactory image?

   =      35 x 5       =     43.75

                                                   4

Contrast

The ability of the grid to improve contrast is specified by the contrast improvement factor.  The contrast improvement factor (k) is the ratio of the contrast of a radiograph made with a grid to the contrast of a radiograph made without a grid (usually 1.5 to 2.5).  Contrast is approximately doubled when a grid is used. 

Example: 

After an aluminum step wedge is placed on a tissue phantom 20 cm thick, a radiograph is made.  Without a grid, analysis of the radiograph shows an average gradient (a measure of contrast) of 1.1.  With a 12:1 grid, radiographic contrast is 2.8.  What is the contrast improvement factor of this grid?

K ₌ _2.8_ ₌  2.55

                                           1.1

Patient Dose

Using a grid requires an increase in technical factors to maintain optical density.  The increase in technical factors results in an increase in patient dose. 

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