Anti - scatter Grid

The best-known way to effectively remove the greater part of radiation scatter is by the use of an x-ray anti-scatter grid. Radiation which does not travel in the same direction as the primary beam is absorbed by the lead strips of the grid. Since Dr. Gustav Bucky built the first grid in 1913, his original principle of lead foil strips standing on edge separated by x-ray transparent interspacers has remained one of the best-known technique to trap the scatter

Types of Filter Grids

X-ray grids are commercially available with either focused or parallel lead strips, and these two types are produced in either linear or crossed grid configuration. The focused grid has its leads angled progressively in such a way that lines drawn through each lead and continued out of the gird will intersect at a point known as the grid focus. When strips are not progressively angulated but are perpendicular to the surface of the grid, the grid is termed "parallel" (See Figure 1).

Both the focused or parallel grids may be made in either the linear or crossed grid type. The linear grid is made with the lengths of all its leads in the same direction. The crossed grid is usually two linear grids, one on top of the other, with the leads of the top grid crossing those of the lower grid (See Figure 2).

In general, the crossed grid will remove more scattered radiation than a linear grid of ratio equal to the combined ratios of its two parts, e.g., a crossed grid, each of whose parts has 5:1 ratio, will remove more scattered radiation than a linear grid of 10:1 ratio. This advantage is more striking at voltages under 100 KVP.

The advantage of the linear grid over the crossed grid is that it may be used in tilted-tube techniques without undue "cut-off" in the radiograph. This is true with grid ratios 8:1 and lower and only if the angle of tilt of the tube is in a direction parallel to the length of the leads. Tilting the tube at an angle across the leads will result in serious density reduction (cut off) on the film. With higher ratio grids, tube angling must be slight or focal distance long to avoid marked density variation.

Construction of Grids & Significance of Grid Ratio

The prime purpose of a grid being the absorption of stray radiation, lead strips (the material which is most practical in the absorption of x-rays) are its basic component. The strips -- five hundred or two thousands or more of them -- are set on edge, properly angled to a mean focal distance and separated by x-ray transparent interspacers. The whole is bonded together into a single flat structure, suitably covered for strength, durability and protection against moisture. The ratio of a grid is defined as the relation of the height of the lead strips to the distance between them. Thus with interspacers 5 times as high as they are wide, a grid is said to be 5:1 ratio, etc. Generally speaking, the higher the ratio of a grid, the more scattered radiation is absorbed (see Diagram).

As grid ratio increases, the necessity of having the focused grid exactly centered and perfectly level under the x-ray tube becomes more and more important. Also, it becomes more necessary to use the grid as its focal distance from the tube, instead of being able to use it through a range of distances. For example, the 40" focal distance, 16:1 ratio grid must be used at 40" for satisfactory results, and must be perfectly centered and leveled.

The 5:1 ratio focused grid, on the other hand, will give satisfactory results over a wide range of focal distances, and need not to be as accurately centered or leveled. Of course the 5:1 ratio linear grid will not have nearly the effectiveness of secondary removal that the 16:1 has, but in ost cases this may be willingly sacrificed to gain the latitude and ease-of-use of the low ratio grid. However, a 5:1 crossed grid will produce as good secondary removal as 16:1 grid at low kilo-voltages, while retaining the latitude of the 5:1 ratio.

Selection Considerations

In order to prevent the shadows cast onto the film by the grid from interfering with visualization of diagnostic detail, certain principles must be followed:

• For one, the lead should be as thin as possible to be consistent with adequate absorption of scattered radiation.The thinner the lead, the narrower the shadow it will produce on the film and the less visible it will be to the eye.

• Also, the thinner it is the less absorption of primary radiation will be in the grid. However, it must be noted that adequate absorption of scattered radiation is the function of the grid and lead must be thick enough to provide this function.

• Another factor is the relative fineness of the grid. This quality is represented by the number of lines per inch. In general, the greater the number of lines per inch, the less visible will the individual lines be, but this is subject to certain practical considerations which modify it in actual use.

Practical Considerations in Grid Selection:

• The selection of a grid to be used for a particular radiograph will be primarily dependent on the following considerations:
  
  Relative quantity of scattered radiation produced by subject being radiographed.
  
  Kilovoltage technique used.
  
  Capacity of x-ray generator.

The quantity of scattered radiation produced is dependent on the thickness and relative density of the body being radiographed. A non-grid exposure of the chest will consist of about one half scattered radiation, while a non-grid exposure of the abdomen may consist of more than 90% scattered radiation.

From this, it is apparent that for dense body sections the more effective removal of scattered radiation will provide the most striking improvement in the radiograph. This suggests the use of a high ratio grid or a crossed grid. The choice between these two grids depends on the ease of aligning the grid correctly relative to the x-ray tube, and whether a high or low voltage techniques are in use.

If there are questions about the proper centering or leveling, or if low kilovoltages are in use, a low ratio grid will present much greater advantage from the point of view of positioning latitude and cleanup. For high voltage techniques, if the grid can be accurately aligned (see effect of misalignment in Figures 1 & 2 below), greater advantages will result from the use of an 8:1 ratio crossed grid or high ratio linear grid.

At kilovoltages of the order of 100 KVP or more, comparable radiographic effect requires low milliampere-second values than at low kilovoltages, thus reducing the radiation dosage to the patient.

However, in order to maintain the same contrast range of the higher kilovoltage, it is necessary to use a higher ratio grid. The exposure factors are not the same for all ratios, and the increased exposure required for a high ratio grid may to some extent reduce the patient-dosage advantage gained by going to higher kilovoltage techniques. In general, in spite of the higher exposure factors involved, the use of high kilovoltage and high ratio grids will result in somewhat lower radiation dosage to the patient.

All radiographers must work within the limitations of the physical characteristics of the x-ray equipment at their disposal. While this may not be as important a consideration in the selection of a grid as some others, it is a factor to be considered. For instance, the maximum benefits to be derived from a 16:1 ratio grid will not be realized with a unit whose top limit is 90 KVP, although there will be some advantage over a lower ratio grid. In general, a 16:1 ratio grid will do the most good with equipment which can be used at kilovoltages above 100 KVP.

This applies also, to a lesser extent, to the 12:1 ratio grid. With a bedside or portable unit, where the likelihood of near-perfect alignment of the grid relative to the primary beam is poor, the use of the high ratio grids is practically impossible, and difficulties may be encountered even with the 8:1 ratio grids. For such use, where wide latitude in distance, centering, and leveling is necessary, the 5:1 ratio grid is advisable, and for maximum cleanup under these conditions the 5:1 crossed grid is ideal.

Selection Guidelines
Choosing the correct grid for your application may be a difficult task. MXE provides technical advice to assist you in selecting the proper grids and evaluating their performance.

(1) X-ray Grid Selection Based on Clean-up Requirements:

(2) Basic Guidelines:

Useful Link :http://www.mxe.com/