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Where is the grid of x ray placed

The grid of x ray is placed in the amplitude-reducing vibration filter of the patient’s examination table (and chest radiograph stand).
The grid of x ray plate is divided into the focal plane and the back focal plane. The focal plane is the side facing the focal point of the X-ray tube, which is also called the “incident plane”. The X-ray tube is marked or noted on the focal plane, and the center line of the grid is marked. When using, the focal plane must face the X-ray tube. At the same time, technical parameters such as the focal length, grid ratio and grid density of the grid of x ray must be marked on the upper right corner of the panel to indicate its internal performance.
When using grid of x ray, you must pay attention to the following two aspects.
The effect of focus deviation and focal length mismatch
1. Tube voltage compensation
Since the main component material of the grid is lead foil, it not only blocks the scattered rays, but also inevitably blocks a part of the original emission line, which reduces the sensitivity of the film to a certain extent, so the grid is not used in photography. It is necessary to appropriately increase the exposure conditions, that is, increase the tube voltage to increase the penetration ability of X-rays to achieve the purpose of compensation. Specific compensation data should be obtained in conjunction with clinical use
In general, low-grid ratio grids are suitable for low-kV tube voltages; high-grid ratios are suitable for high-kV tube voltages.
2. The effect of focus deviation and focal length mismatch
The lead strips of the convergent grid of x ray are all arranged towards the focal point. The convergence line should coincide with the focal point of the X-ray tube during photography, so that the original emission line is parallel to the lead strip and can pass through smoothly, while the direction of the scattered line is random, very It is less consistent with the direction of the lead strip, so most of it is absorbed by the lead strip, and the image resolution obtained is better. If the focus of the X-ray tube deviates from the convergence line of the grid of x ray, or the grid of x ray is not placed on the specified focal length, the original emission line will inevitably be lost. The percentage of the loss is proportional to the deviation and the focal length difference. It is directly proportional and has a direct multiple relationship with the grid of x ray. So the grid of x ray should be placed in the required position as much as possible. But in actual clinical use, some deviations are inevitable, so there is no requirement for the deviation of grid of x ray.
a. Center deviation should be avoided as much as possible. In the case of a static grid of x ray, there should be no deviation; when using an active grid of x ray, there will inevitably be a center deviation, but the deviation should be designed as small as possible.
b. In some occasions, the distance from the focus to the film box needs to be changed according to the different parts to be illuminated. However, the grid cannot be replaced frequently, so only a permitted range of focal length changes can be specified. When the transmittance of the original emission line at the edge of the effective area of ​​the grid is reduced by 40%, the film distance at this time is the limit value. It is generally stipulated that the change in film distance should not be greater than or less than 25% of the focal length of the grid of x ray. Therefore, the focal length indicated on the grid of x ray is the nominal focal length, and the actual focal length that can be used has a certain range. The picture of grid of x ray is as follows:


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