Anti-Scatter Grids
An anti-scatter grid is a physical device that blocks scattered radiation. When a primary x-ray beam interacts with a body, secondary x-rays are scattered in all directions. Secondary x-rays that are traveling in a direction other than that of the primary beam cause a radiographic fog in the x-ray image. Such radiographic fog reduces the contrast of the image.
An anti-scatter grid comprises alternating sections of radiopaque material (typically lead) and radiolucent material (typically aluminum), encased in a protective, radiolucent housing. An anti-scatter grid is designed to absorb only the x-rays traveling in a direction other than that of the primary beam.
Various technical parameters of an anti-scatter grid determine its effectiveness under different conditions. An anti-scatter grid may be parallel or focused. In a parallel grid, all of the radiopaque sections are parallel to each other, and perpendicular to the surface of the grid. In a focused grid, the radiopaque sections are progressively tilted such that straight lines extended from the points at which the sections intersect with the surface of the grid would intersect at a single point. This point is defined as the focal point of the grid. Parallel grids are less expensive to manufacture than focused grids, but have the undesirable effect of absorbing more of the primary x-rays. Focused grids absorb less primary radiation, but unlike parallel grids must be used at an appropriate focal distance from the beam source, plus or minus an acceptable margin of error.
Both parallel and focused grids may be linear or crossed. A linear grid comprises a single parallel or focused anti-scatter grid. A crossed grid comprises two linear grids, one on top of the other, such that the radiopaque sections of one grid are perpendicular to those of the other. Crossed grids absorb a significantly higher percentage of the scattered radiation than linear grids, but must be positioned much more carefully relative to the source of the x-ray beam. All grids may also be fixed in position, or moving. Moving grids are attached to a mechanism that is moved as the x-rays pass through the body radiographed. This has the effect of minimizing, in the x-ray image, lines caused by the absorbence of primary x-rays by the grid.
Other technical parameters of the grid are the specific radiopaque and radiolucent materials used, the width of the sections of radiopaque material, the width of the sections of radiolucent material, the height of the grid, the ratio of the height of the grid to the width of the sections of radiopaque material (called the grid aspect ratio), the focal distance of the grid (relevant for focused grids only), and the period of time for which the grid is in motion while the digital sensor plate is being exposed to radiation (relevant for moving grids only). All of these factors determine the extent to which a grid absorbs secondary radiation, the extent to which a grid undesirably absorbs primary radiation, the proper range of focal distances for the grid, the tolerance of the grid for use outside of that range, and the dose of radiation to which the body being radiographed must be exposed in order to generate a useful x-ray image.
Anti-scatter grids are commonly used in radiography systems. Existing anti-scatter grids transmit radiation traveling in the direction of a single primary beam, so as to produce a single radiographic image.
Three Dimensional Radiographic Images
The inherent limitation of two dimensional images is a serious shortcoming of radiography as it exists today. It is desirable for a physician or researcher to know exactly where an object is located within a radiographed body. Although a two dimensional radiographic image presents an internal view of a body, it is difficult to recognize three dimensional structure within a body from a two dimensional radiograph.
There exist several rather complicated techniques for determining three dimensional information within a body. Three dimensional information can be obtained by transmission x-ray microscopy, a combination of an x-ray transmission technique with tomographical reconstruction. This combination allows the obtaining of three dimensional information about the internal microstructure of an object. An internal area is reconstructed as a set of flat cross sections which are used to analyze two and three dimensional morphological parameters. The contrast in the resulting radiographic images is a mixed combination of density and compositional information.
In some cases the compositional information can be separated form the density information with the help of a Computed Axial Tomography scan (CAT scan). A CAT scan is a medical diagnostic procedure that combines the use of x-rays with computer technology. A series of x-ray beams from many different angles are used to create cross-sectional images of a patient's body. These images are assembled by a computer into a three dimensional picture that can display organs, bones, and tissues in great detail.
However, these facilities are very complicated and expensive, and thus are not accessible for most researchers and users. What is needed is an inexpensive and readily accessible method for creating a stereo radiographic image, from which an exact location of an object within a body can be determined. This information has many important medical applications, such as surgery, physical therapy, and the like.