What is an x-ray grid and why do you need to use one? To make it simple a grid is a device used to improve the contrast and image quality of the radiographic image. As the x-ray beam enters the body and strikes bone and tissue it produces what is called scatter radiation. Scatter radiation is x-ray and does expose film but generally is undesired x-ray and shows up as noise and causes a blurring of the x-ray image. The best way to prevent this unintended result of scatter radiation is to remove or block it. This is where the grid comes in to place. The x-ray grid has the job of blocking scatter radiation. It accomplishes this by absorbing the scatter radiation before it reaches the x-ray film or digital capture device. To block or absorb scatter radiation small thin lead strips are placed a long side each other inside a radio-translucent material. These lead strips absorb and block a majority of the scatter. Once the scatter is removed it can no longer reach the x-ray cassette therefore allowing only primary radiation to reach the x-ray cassette and expose the x-ray film which gives a cleaner image with much more contrast. Some situations where the use of a x-ray grid would be recommended is when using more than 70 kVp since higher radiation causes more scatter, when the anatomy is more than 12 cm thick and in a chest radiograph of patients who measure more than 24 cm around. When selecting a x-ray grid there are four things to consider before purchasing one, LPI, Focal Distance, Grid Ratio & Grid Type. Listed below is a brief description of each.
LPI - Line pairs per inch also referred to as Grid Frequency. Line pairs per inch refers to the number of lead strips placed next to each other every inch or centimeter. Typical grid LPI’s are 103, 178, or 200. When using a stationary grid with CR, it is recommended to use no fewer than 178 LPI to reduce the appearance of grid lines in the image. Grid lines appear in CR because the parallel grid lines match up with the scan pattern of the laser in the CR reader. By using a higher LPI you reduce the chance of the laser scan rate matching the grid pattern.
Focal Distance – The lead strips inside the grid are angled to point at the x-ray tube which only allow the primary rays of the beam to penetrate to the film or digital capture device. The focal distance is a point where the angle of these strips if extended into space would meet. Typical focal distances are 40”, 72” and ranges of 34”-44”, & 40”-72”. For maximum penetration of the primary beam through the grid you should only use a grid within the listed focal range. By doing this you avoid grid cutoff (blocking of the primary beam) and penetration issues.
Grid Ratio – The effectiveness of a grid to absorb scatter radiation is determined by the grid ratio. The grid ratio is the ratio of the height of the lead strip to the distance between the strips. This is expressed as a ratio such as 6:1, 8:1, 10:1 and 12:1. The higher the ratio the more scatter radiation is removed which improves image contrast. By using higher grid ratios it will require a higher dose and more precise centering of the x-ray beam in order to penetrate the grid properly. Several factors should be considered when selecting the right grid ratio. Although grids with higher ratios eliminate more scatter, they tend to require higher dose which increases patient exposure and they require more precise positioning to prevent grid cutoff. The advantages of a 6:1 grid are that it requires a lower dose and is easy to position. Its use must be limited to situations in which the amount of scatter radiation is small (thin body parts, low kVp) or in which maximum image contrast is not necessary. Many times a grid between these two extremes is more desirable. Such grids generally represent a compromise between image quality, patient exposure and ease of positioning.
Grid Type – There are two types of x-ray grids, standard and decubitus. In standard grids the lead strips run parallel to the long dimension of the grid. This is standard and used for most examinations. In decubitus grids the lead strips run parallel to the short dimension of the grid. Generally decubitus grids are used when the cassette has to be turned 90 degrees in order to image patients with large body frames, for cross table work and for portable studies.