Since the development of the antiscatter grid in 1913, a major problem has been the resulting lines that can appear in the x-ray image. This critical defect created a risk of misdiagnosis by physicians. In 1920, Dr. Hollis Potter devised a solution for this problem by finding a movement patter that would eliminate the artifacts caused by the grid.
In 1968, Computed Radiography (CR) was introduced in the field of radiography. At the time, Moire patters, a wavy image from the interference between antiscatter grid and the Image Plate (IP), appeared in x-ray images, degrading the quality of the image. Bukcy trays were applied to fix this problem. When Digital Radiography (DR) was introduced the same method was applied. However, it causes an increase of dose for the patients by the moving antiscatter grid. To improve this, trials were conducted using stationary grids.
Moire interference is phenomena of optics where two slits of light will create parallel lines of light and dark. This phenomenon occurs between the imaging plate and antiscatter grid in CR and between detector and antiscatter grid in DR. Besides the basic physics creating Moire patterns, aliasing in local area sampling when the image is processed by the detector increases the problem.
The physical cause of Moires would be removed if the cell frequency of the detector and the line frequency of the antiscatter grid matched. The conventional method for eliminating the Moire is to set the ratio of the line frequency of the antiscatter grid and the cell frequency of the detector at the proper level.
During the testing performed by JPI in eliminating Moire patters in Direct DR detectors, it was confirmed that the moire is removed when the cell frequency is identical with the line frequency of the antiscatter grid. But, in addition, it was observed that when the line frequency of the antiscatter grid reached certain frequencies the line artifacts and Moire from the antiscatter grid disappeared. These results were attributed to the detection ability of the phosphor and detector cells.
The method of Moire elimination through an antiscatter grid should be different for each specification of detector and x-ray equipment. Therefore, it is highly recommended that any x-ray company communicates with the antiscatter grid manufacturer to discuss the solution of Moire elimination as well as optimal grid specification.