The contrast improvement factor (K) is the ratio between the contrast with a grid and without a grid. Meanwhile, the higher the energy of the radiation beam, the higher the grid ratio or Bucky factor needed to effectively filter out scattered radiation 8. The higher the grid ratio, the higher the Bucky factor. The value for Bucky factor range from 3 to 5 7. If Bucky factor is two, the exposure factors also needs to be increased by two to mainatain the same image quality. Grid factor = exposure necessary with grid / exposure necessary without grid The formula for Bucky/grid factor is 8:īucky factor = incident radiation / transmitted radiation Although Bucky factor is similar to the primary transmission as described above there is one major difference: Bucky factor measures both primary and secondary (scattered) radiation transmission, thus making it more practical to determine the additional amount of patient's exposure needed when switching from non-grid to grid technique in order to maintain the same image quality. The Bucky factor (also known as grid factor) 7,8 is the ratio of incident radiation falling the grid to the transmitted radiation. Measured primary transmission is always smaller than the estimated primary transmission due to assumptions given above 8. Thus, percentage surface area of the interspaces will corresponds to the percentage of primary beam transmission, as shown in the formula below 8: Primary transmission can also be estimated by the formula below, assuming that the geometric relationship between the anode and the grid is accurate, there is no grid cutt-off, and primary radiation is not absorbed in the grid interspaces. The primary transmission can be calculated by measuring the ratio of intensity of radiation with a grid over the intensity of radiation without a grid as shown below 8: Ideally, a grid should have 100% primary beam transmission while blocking all the scatter radiation. Primary transmission is the percentage of primary radiation transmitted through the grid. Low frequency is 40 to 50 strips per cm, medium frequency is 50 to 60 strips per cm, and greater than 60 strips per cm is high frequency 7. This is typically 30-80 strips (or grid lines) per cm.
The strip line density (number of strips per cm) is 1/(D+d), where d is the thickness of the strip.
A grid ratio of 8:1 is generally used for 70-90 kVp technique and 12:1 is used for >90 kVp technique. The working ability of a grid is described by the grid ratio, which is the ratio of the height of the lead strips (h) to the distance between two strips, i.e. the interspace (D) 7. As scattered radiation is increased in "thicker" patients and at larger field sizes, grids are useful in such scenarios to improve image contrast. The strips can be oriented either linear or crossed in their longitudinal axis. They are made of parallel strips of high attenuating material such as lead with an interspace filled with low attenuating material such as carbon fiber or organic spacer 7. Grids are placed between the patient and the x-ray film to reduce the scattered radiation reaching the detector (produced mainly by the Compton effect) and thus improve image contrast.
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