Screening grid (synonym: blend, Bucca blend, Potter blend - Bucca - currently not used) - this device is designed to increase the contrast of the x-ray image due to the absorption of x-ray radiation scattered by the human body.
The screening grid consists of a housing, a raster mounted inside it, a raster movement mechanism, a control and alarm system (Fig. 1). A raster is a set of thin (0.02—0.3 mm) narrow lead plates, separated by layers of wood or cardboard, and now plastic or aluminum.
Fig. 1. Flat movable screening grid: 1 —call; 2 - cocking handle; 3 - cassette holder.
Fig. 2. Radiography with screening grid: 1 - X-ray film; 2 - grid raster; 3 - the object under study.
In the directional raster, the plates are focused on the focus of the X-ray tube , therefore the main part of the direct X-ray radiation passes between the plates. Most of the scattered radiation does not coincide in direction with the direct beam and is absorbed by the lead plates of the raster (Fig. 2). At the same time, the veil decreases due to the scattered radiation and the image contrast increases. The rasters in which the plates are arranged in parallel are called flat, but the most common screening grids with directional rasters for certain focal lengths (from 90 to 150 cm). If the raster is used not at the passport distance, then a sharp weakening of the intensity at the edges of the field is observed. The most important characteristic of the screening grid is its ratio (the ratio of the width of the gap between the plates to their height). With a decrease in the ratio, the intensity of the total (primary and secondary) radiation transmitted through the screening grid decreases, which can be compensated by an increase in the exposure or the generation voltage.
At voltages up to 100 kV, a screening grid with a ratio of 1: 6 is used, which slightly absorbs the primary radiation and filters 70–80% of the secondary radiation; at large voltages, a screening grid with a ratio of 1:12 and lower is used. There are fixed screening gratings (Lysholm gratings) and moving gratings. Lisholm screening grid has a flat raster with very thin (0.02—0.03 mm) lead plates and narrow (0.2—0.3 mm) gaps. This screening grid is placed on the cassette (or in the cassette) and allows you to conduct research in any position of the patient. It leaves thin strokes on the radiograph , which do not interfere with the perception of the X-ray image. Its disadvantages include the lack of directivity, strong absorption of primary radiation and the permeability of thin plates for scattered radiation. In the photo with a mobile screening grid, the image of lead plates is blurred and does not interfere with the reading of radiographs. The raster is driven by a spring or motor. To avoid banded shadows from the plates, the raster movement time should be somewhat longer than the shutter speed. However, with short exposures, shadows occur when the raster moves evenly due to the so-called stroboscopic effect. To combat it, they use uneven, slowing down, according to a certain law, raster movement or use vibration screening grids. For very short exposures, screening grids with a large number of plates per centimeter of raster length are used. The minimum shutter speed for a raster is indicated in his passport (for domestic screening grids - 0.04 sec.). Modern screening grids are equipped with several interchangeable rasters designed for specific voltages and exposures.