GRIDS
When a beam of X-ray passes through the patient, the beam is absorbed and scattered. The absorbed primary beam gives a useful shadow while the scattered radiation will tend to spoil the shadow.Scattered radiation contributes a constant background fog to the filmimage. This will increase the noise in the image. The ratio between the amounts of scattered radiation energy to the amount of primary radiation energy at a point is called as scatter to primary ratio (SPR).The SPR increases with thicker patient and larger field sizes. Forexample, in a abdomen radiography, only 20% of the photons contributes to the image formation and the other 80% energy goes as scattered radiation. Hence, scattered radiation must be removed, in order to increase the image contrast.The scattered radiation can be removed by a grid, placed in between the film and the patient. The grid consists of a series of parallel lead or tantalum strips of thickness ‘c’ (50 mm) and of height ‘h’ separated by spacers of low attenuating material of width ‘b’ (350 mm) . Aluminum or plastic fibers are used as low attenuating spacers. The grid is positioned between the patient and the detector,so that its long axis is pointed towards the X-ray beam. The primary X-rays coming out of the patient, passes through the inter space,since it is parallel in direction. The scattered X-rays, which are in non parallel direction, strike the grid bars and get absorbed. The ratio of the primary transmission to the scatter transmission of a grid is called the selectivity.
GRID RATIO
The ability of the grid to discriminate against scattered radiation is measured by the grid ratio, which is defined as the ratio of the height(h) to the width of the spacer (b) between the lead strips.
Grid ratio = h/b
As the grid ratio increases, the grid removes more scatter radiations.
The strip line density is 1/(b + c) lines per unit length. Typical grid ratio ranges from 4:1 to 16:1 and strip line densities are 25–60 lines per cm.
The performance of a grid can be understood by contrast
improvement factor. It is ratio between image contrast with grid and image contrast without grid at 100 kVp. Higher grid ratio provides higher contrast improvement factor. However, it increases patient dose, as it employs higher exposure techniques. Bucky factor (Gustave- Bucky,1913) is another parameter which relates the patient dose. It is the ratio between the patient dose with grid and patient dose with out grid. Bucky factor increases with increase of kVp and grid ratio.
TYPES OF GRID
Grids may be classified as (i) parallel grid, (ii) crossed grid, (iii) focused grid, and (iv) moving grid (Potter-Bucky). In a parallel grid, the leadstrips are parallel to each other in their longitudinal axis. Most of the X-ray tables are provided with linear grids. It is easy to design, but has the property of grid cutoff. This means that the attenuation of primary radiation is greater at the edges and it can be partial or complete cutoff. The distance of grid cutoff may be estimated from the ratio of source to image distance (SID) to grid ratio.
Crossed grid is made up of lead strips that are parallel to the long axis and short axis of the grid. Usually, it is designed with two parallel grids, that are perpendicular each other. The grid ratio of crossed grids is equal to the sum of the ratios of the two parallel grids. Crossed grids are efficient in removing scatter radiations and has higher contrast improvement factor and high grid ratio. It is useful at high kVp and tilt-table exposure techniques. The disadvantage includes difficulty in positioning, proper alignment of tube and table, and higher patient dose. Crossed grid also suffers from grid cutoff.
Focused grid is made mainly to reduce grid cutoff. The lead strip lies in a imaginary radial lines of a circle, whose center is the focal spot. The strips are parallel to the divergence of the X-ray beam. The grids are marked with focal distance and the side facing the target. If it is reversed, grid cutoff may occur, hence enough care is needed to position focused grid.
MOVING GRID
When a focused or parallel grid is used, each lead strip will appear on the radiograph as very fine line. These lines may spoil the information in the film. However, these lines may be removed by moving the grid during the radiographic exposure. This is the principle of Potter- Bucky grid (Hollis E. Potter,1920). Generally, focused grids are used as moving grids. The grid may be made to move continuously in one direction. The grid motion is timed by the exposure control of the X-ray machine. It starts moving just before the X-rays are turned on and continues to move even after the exposure is off. The traveling period of the grid should be greater then the exposure time. There are two types of moving grids, namely, (i) reciprocating grid and (ii) oscillating grid. The reciprocating grid is driven by a motor and the grid moves back and forth several times, during exposure. The distance traveled may be 2 cm. In the oscillating type, the grid is kept in a frame, which has 2–3 cm clearance on all sides. An electromagnet pulls and releases the grid before the exposure.
The grid oscillates in circular path about the frame and comes to rest after 20–30 seconds. Moving grids increases the distance between patient and film, resulting in magnification, cassette motion and image blur. However, the motion blur is undetectable and hence used widely. The use of grid will always increases the exposure, because it will absorb some of the primary radiation. In order to reduce the exposures,grids with smaller ratios are preferred. Low ratio grids such as 8:1 is used with energies up to 90 kVp. High-ratio grids such as 12:1 are preferred for high energy radiation. In mammography, grid ratio of 4:1 or 5:1 is used. These grids produce films with better contrast with increased patient dose. Grids are generally used for body parts > 12 cm thick or techniques > 70 kVp. Grid can produce artifacts when improperly aligned.
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