Basic principles of a grid

The basic purpose of grid is to enhance the contrast and quality of the medical image by removing the scatter radiation.

Whenever x-rays pass through matter, some of the x-ray photons will interact with the atoms of the matter. When you take an X-ray, many incoming x-ray photons are absorbed by the bone, a high attenuation material, while other tissues absorb less. This produces an image of dark and light on the x-rayfilm. The effect is like holding up paper to a light to see the writing on the other sied. However, absorption is not the end. As part of the absorption, a second photon with less energy will be produced, flying off in a different firection than the photon that was absorbed. These “scattered” photons will create a random grayness on the image, reducing the contrast between body tissues and making it hard to read the image clearly. There are five qays x-rays interact with matter: photo electronic (PE), Compton Scattering(C), Pair Production, Tomson Scattering(R), and Photodisintegration (PD). In low energy x-rays below 100keV, only Photolelectric absorption and Compton scattering are significant.

Grids are divided into four types based on their applications. There are parallel types, focused types, crisscross types, and tapered types. A single grid may be of more than one type, but this is rare. The majority of grids used in general radiography are focused grids, focused for 40″, 40-72″, or 72″ distance from the x-ray tube, but other applications require different grids.

A grid where the absorving strips are parallel to each ohter in their longitudinal axis. Most linear grids are also focused, i.e. their strips are slightly tilted, converging at a line in space (the convergent line). A non-focused linear grid will have strips that are parallel when viewed in crosssection; this is called a parallel grid. Many X-ray tables are equipped with linear, focused grids, and the strips in these grids are parallel with the long axis of the table, allowing the X-ray tube to be tilted in this direction without changing the effectiveness of the grid.

A grid in which the absorbing strips are slightly angled towards the focal spot. The grid can therefore be used only at a specified focal distance (actually inside a narrow distance interval around this specific distance). Otherwise the grid will absorb the primary radiation and parts of the film are barely exposed. Focused grids may be linear or crossed.

A grid consisting of two superimposed parallel grids having ghe same focusing distance. Such grids ard very efficient in removing scattered radiation but must be arranged at exactly the right angle to the beam. The use of such girds is therefore limited.

A grid where the surface is tapered to peak at the center of grid, functioning similar to a focused grid. All of the strips are parallel to each other, and the tapered surface is toward the focal spot type.

Line density : line number per cm. * Equation line density = 10 / (interspacer thickness + absorber thickness) lp/cm –

Ratio : proportion of interspacer thickness vs grid thickness * Equation – Ratio = grid thickness/ interspacer thickness

Grid performance is calculated by comparing the percentage of radiation that gets through the grid (Total Transmission), the percentage of scattered radiation that gets through the grid (Scatter transmission) and the percentage of radiation that has not been scattered that gets through the grid (Primary transmission). If the grid is perfectly focused, and estimate of these can be calculated from the specifications of the grid.

For example, the percentage of total radiation that would be transmitted by the grid if all x-rays were coming in perfectly parallel to the absorber when the interspacer thickness equals 390um and the absorber thickness equals 450um, the ideal primary transmission = interspacer thickness / strip thickness X 100% = 390um / 450um X 100% = 86.7%

Primary transmission can be testedby a simple comparison of exposure on a detector with and without the grid. This measurement must be made without any object in the x-ray path. If there is an object to image, scattered radiation will be generated by that object.

For example, 100 kVp, 64mAs(200mA, 0.32sec) is selected for measuring Tp. Transmission measurements will be taken with and without a grid in the x-ray beam. The Tp (total = Total Exposure with grid / Total Exposure without grid.

To measure scattered radiation, a large uniform object is placed in the x-ray beam. Lead or another x-ray blocker is then placed in front of the object so that it blocks all primary transmission to the detector. In this way, only scattered radiation can reach the detector. You can then measure the exposure with and without the grid to calculate Scatter Transmission.

The angle of the individual lamella is formaed to focus to a particular focal spot in front of the grid. Moving from that focal point will both reduce transmission and create cut-off. Only a certain amount of movement is acceptable for medical practice. These are called ” Application Limits.” The application limits are specified in the IEC60627 international standard. Focal distance and application limits should be calculated according to the equation described in the international standard and the grid maker should assure that when the x-ray tube is moved within application limit cutoff doesn’t appear on x-ray film.

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