Anti-scatter grid
An anti -scatter grid (also called bucky screen , colloquial X-ray grid , English Potter bucky grid or diaphragm ) is a device in X-ray technology that is attached in front of the image receiver ( screen , detector or film ) and reduces the incidence of scattered radiation on it. This increases the contrast of the X-ray image. The first anti-scatter grid was developed by Gustav Peter Bucky in 1913 . The American radiologist Hollis E. Potter (1880-1964) improved it and added a movement device in 1917.
Scattered rays are mainly caused by the scattering of the X-rays in the illuminated object. The radiation producing the image is directed directly from the X-ray tube onto the image carrier, but the scattered radiation has different directions. Scattered radiation causes a relatively even dose distribution on the detector. The relative proportion of scattered radiation increases sharply with increasing object thickness and decreases with higher energy of the X-ray radiation d. H. increasing X-ray tube voltage.
The grid is made up of narrow strips of highly absorbent material (mostly lead foil) and more permeable spacers ( mostly spacers made of aluminum or cellulose) like a slatted blind . The strips are parallel to the radiation. The desired directed radiation can penetrate the spacer strips, scattered radiation remains in the lead strips.
The grid frequency (number of lead lamellas per cm) is 20-80. Higher frequencies reduce the scattering, but increase the necessary radiation exposure of the patient. The grid ratio is the ratio of gap height to width in the grid, usually at 1:10, low in mobile grids and in pediatrics. Higher shaft ratios also result in better image quality, but increased patient stress. The bucky or Grid Factor describes the ratio of the amount of radiation with and without grid that is required for the same optical density of the X-ray film; it is typically 3-5.
Grids can be made up of differently inclined strips or parallel strips (focused on the X-ray tube). Focused grids are only suitable for the distance between the tube and the grid for which they were designed, e.g. B. 100 cm. Parallel grids, on the other hand, have shading towards the edge if the distance to the tube is small and are only suitable for larger focal distances. To reduce stray radiation in both dimensions, two grids at right angles are required, a honeycomb grid or other two-dimensional grid.
In order not to get a streaky image, the grid is moved. This movement is generated by a spring preload (pendulum grid) or by a linear motor. Before the exposure, the grid is released from the spring preload and then oscillates back and forth during the exposure (pendulum grid), or the linear motor starts the movement before the exposure; If the grid does not run fast enough or if it "jams" at the reversal points, stripes of the grid are shown on the image. An attempt is made to minimize the always existing play of the spindle in the linear motor at the reversal point by gently pulling the spring in one direction on the grid. The play of the spindle in the linear motor can also be minimized by filling the linear motor with grease (wheel bearing grease); This means that the game is almost zero and any wear and tear on the spindle is reduced to the maximum.
Important parameters for X-ray anti-scatter grids are specified in the European Union in the standard DIN EN 60627 (2001, last revised 2006). The contrast-improving effect of an anti-scatter grid can be attributed to its selectivity (= quotient of primary radiation permeability and scatter radiation permeability ). can be read. The exposure time extension factor indicates the extent by which the exposure time must be increased in order to achieve the same blackening with a grid as without a grid. It allows the radiation dose increased by the grid to be calculated .
The binding guideline of the German Medical Association prescribes that moving anti-scatter grids have at least 36 lines / cm and stationary grids at least 60 lines / cm. The optimal shaft conditions depend on the investigation. If children are also examined on the device, the grid must be easily removable. Children should only be examined with anti-scatter grids in exceptional cases if the examined body part is more than 12-15 cm thick and the shaft ratio is no more than 8, with fixed grids 15-17.
See also
Sources and individual references
- Theodor Laubenberger, Jörg Laubenberger: Technology of medical radiology: diagnostics, radiation therapy, radiation protection. For doctors, medical students and MTRA . Deutscher Ärzteverlag, 1999, ISBN 3-7691-1132-X .
- Robert A. Fosbinder, Densie Orth: Essentials of Radiologic Science . Lippincott Williams & Wilkins, 2011, ISBN 978-0-7817-7554-0 ( google.com [accessed February 26, 2013]).
- ^ E. Barth: In Memoriam Hollis E. Potter. ( Page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. (PDF; 188 kB). In: Radiology. 85, October 1965, pp. 775-776. doi: 10.1148 / 85.4.775
- ↑ DIN EN 60627 at the Radiology Standards Committee
- ↑ Guideline of the German Medical Association for Quality Assurance in X-ray Diagnostics, 2007 (PDF; 381 kB)
