Introduction: In the course of the thesis, we investigated how the change in the distance between the focal point and the image receiver influenced the dose load of patients without changing the exposure index and without changing the image quality - the signal to noise ratio. We wanted to confirm the validity of the rule, which states that the intensity of radiation falls with the square of the distance, and that the formula under which we were regulating the conditions of exposure contributes to the choice of optimum exposition conditions. Purpose: We wanted to find out how the change in the distance of the focal point-image receiver affects the patient's dose load and the exposure index, which consequently affects the quality of the image. Due to the diagnostic usefulness of the X-ray, it is also necessary to maintain the same signal-to-noise ratio in the change in the distance between the focal point and the image receiver, so the purpose of thesis was also to find the optimum beam intensity that falls on the image receiver. Methods: First, we used a descriptive method of work with a review of professional and scientific literature. The search for the literature was made in the data base COBIB.SI. Data for the practical part were obtained by measurements in the radiological laboratory of the Faculty of Health Sciences of the University of Ljubljana. The measurements were performed on the Siemens Multix / Vertix X-ray machine on the phantom of the arm and the hip. Because of the fact that the size of the image field affects image quality and the signal-to-noise ratio, we limited it manually to the size of 24 × 30 cm. The distance focal point - image receiver was changed from the basic value by 10 cm. The ultimate goal of the distance change when imaging the arm was 50 cm and 180 cm when increasing the distance. The final goal of the distance change when imaging the hip was 65 cm and 195 cm when increasing the distance. At each change in the distance, the exposure conditions were adjusted so that the intensity of the radiation that fell to the image receiver was the same. Each time we measured product between dose and irradiated surface and EI. Results: The results are composed of two parts. In the first section, we show how the distance of the focal point-image receiver affects the patient's dose, while the second part shows how the adjustment of the exposure conditions to a certain distance of the focal point-image receiver influences the exposure index. We found that by increasing or decreasing the distance of the focal point-image receiver the dose decreases or increases, if we do not adjust exposure. If the exposure is adjusted, than dose is mainly the same. Adjusting the exposition conditions according to the square law formula exposes the exposure index around the ideal (± 6%). Discussion and conclusion: From the point of view of the patient's dosage load, it is very important to adjust the exposure conditions. The phantom imaged at distance of 50 cm from the focal point with unadjusted exposition conditions received as four times the dose as it is received with adjusted exposition conditions. This fact requires special attention in mobile radiology, where the exposure conditions are adjusted to the capabilities of the apparatus and the space where imaging. The amount of radiation on the surface changes with the square of the distance. This change can be calculated by square law and allows radiology technicians to calculate new exposure conditions at distance change and to maintain the same signal-to-noise ratio.
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