Why Increasing the kVs utilised during imagine acquisition lowers patient dosage because more photons with higher energy pass through the patient to reach the detector. However, the resulting image has less contrast.
• Increasing filtration reduces the number of low-energy photons, but it also lessens the contrast of the resulting image.
• Increasing the film-focus distance (or focus-film distance) lowers the dose as well. The greater distance serves as an additional layer of filtration.
• A grid is designed to absorb scattered photons flowing obliquely over it while allowing photons passing perpendicular to it to pass through. They are made up of parallel strips of highly attenuating metal. Because they reduce the number of photons reaching the detector after they have passed through the patient, they actually increase the dose because more photons must be fired at the patient to keep the number of photons reaching the X-ray detector sufficient to form an image. Because there are less dispersed photons in the image, the final image has a greater quality with a grid.
An air gap can be established between the patient and the detector, which helps minimise the amount of scattered photons reaching the detector (similar to a grid) and so causes the secondary dosage increase described above. Using a larger air gap will also result in increased magnification of the image.