Anti-scatter grid
An anti-scatter grid (even Bucky diaphragm, umgangssprachl. X-ray grid, Eng. Potter - Bucky grid or diaphragm ) is a device in the X-ray technology that is mounted in front of the image receiver (monitor, detector or film) and reduces the incidence of scattered radiation on this. Thereby, the contrast of the X-ray image is enhanced. The first anti-scatter grid was developed in 1913 by Peter Gustav Bucky. The U.S. Radiologist Hollis E. Potter (1880-1964) improved and expanded it in 1917 to a moving device. The anti-scatter grid corresponds in principle the collimators for X-ray, gamma or particle radiation.
Scattered rays are produced predominantly by deflection of X-rays in the X-rayed object. The image -producing radiation is directed directly from the X-ray tube to the image support, but any stray radiation. Scatter causes a blurred density of X-ray films and should be particularly considered if the captured body part about 10 cm thick, and the X-ray has at least 60 kV energy.
The grid is structured and permeable spacers ( spacers made of aluminum, plastic film, fiber material, etc.) as a venetian blind from narrow strips of highly absorbent material (usually lead foil ). The strips are parallel to the radiation. The desired directional radiation can penetrate the Spacerstreifen, scattered radiation remains hanging in the lead strips.
The screen frequency (number of lead strips per cm) is located at 20-80. Higher frequencies reduce the scattering, but increase the necessary radiation exposure of the patient. The grid ratio is the ratio of gap width to height in the grid, usually at 1:10, low in mobile grids, and in pediatrics. Higher shaft ratios also require better image quality, but increased patient load. The Bucky grid or Factor describes the ratio of the amount of radiation with and without a grid, which is required for the same optical density of x-ray film; it is typically 3-5.
Scanning may be composed of ( focused on the X-ray tube ) differently inclined strip or parallel stripes. Focused grids are only suitable for the distance between tube and screen, for which they were designed, for example, 100 cm. Have parallel grid, however, at short distance from the tube to the edge shadowing and are suitable only for larger focal distances. To reduce stray radiation in two dimensions, two grid at right angles is required, a honeycomb grid or other two-dimensional grid.
To stop receiving streaky figure, the grid is moved. This movement is produced by a preload (pendulum height) or by a linear motor. Before exposure, we dismissed the grid the spring preload and then swings back and forth (pendulum height) during recording, or the linear motor starts the movement before recording; the grid does not fast enough or " clamped ", is at the reversal points, there is the mapping of stripes of the grid on the image. By slight spring tension in a direction the grid is trying to minimize the ever-present game of the spindle in the linear motor at the turning point. The play of the spindle in the linear motor can also be minimized by using a grease ( of wheel bearing grease ) is introduced into the linear motor; so the game goes to zero and the possible wear of the spindle is reduced to the maximum.
Important parameters for X-ray anti-scatter grid in the European Union in the standard DIN EN 60627 (2001, last revised 2006) determined. The contrast- enhancing effect of an anti-scatter grid can be connected to its selectivity ( = ratio of primary beam transmittance and scattered radiation transmittance ) can be read. The exposure time extension factor indicates the extent by which the exposure time must be increased to achieve the same darkness with grid like without grid. It allows a calculation of the radiation dose increased through the cracks.
The binding guideline of the German Medical Association stipulates that moving anti-scatter grid at least 36 lines / cm and standing height have at least 60 lines / cm. The optimum shaft ratios are dependent investigation. If the unit does children are examined, the grid must be easily removable. Children should only exceptionally be examined with anti-scatter grids, if the examined body part about 12-15 cm thick and the shaft ratio is at most 8, is in fixed grids 15-17.