How to choose the focal length of the stationary grid

The stationary grid is an accessory used to filter out the scattered scattered rays to make the captured images clearer and reduce the X-ray radiation received by doctors and patients. The usual focal lengths of stationary grid are 1.8m, 1m and 1.3m, as shown in the figure below, then how should we choose the focal length.
The focal length of the stationary grid should be selected according to the position where the patient was taken. If shooting in a standing position, the patient needs to stand in front of the film rack. At this time, the film box should be at a distance of 1.8 meters from the X-ray machine. At this time, the focal length of the stationary grid is 1.8 meters. If shooting in a lying position, the patient needs to lie on the filming bed. At this time, the film box is 1 meter away from the X-ray machine, and the focal length of the stationary grid is 1 meter. However, sometimes the hospital uses a U-arm X-ray machine. The distance between the cassette and the tube is not adjustable, and you need to shoot in the vertical and lying positions. At this time, we recommend that the customer choose a stationary grid with a focal length of 1.3 meters.

Author：Lillian