Changes in field size, effect on exposure index and patient dose

Uvod: Sprememba velikosti slikovnega polja vpliva na dozo, ki jo prejme pacient, hkrati pa vpliva na količino sipanega sevanja, ki nastane v pacientu in doseže slikovni receptor. Pri spreminjanju velikosti slikovnega polja je potrebno spreminjati tudi ekspozicijske pogoje, če želimo, da ostaja razmerje signal-šum na sliki enako. V primeru manjšega slikovnega polja moramo produkt toka in časa povečati. Pri tem si lahko pomagamo s faktorji spremembe ekspozicijskih pogojev pri spremembe velikosti slikovnega polja. Namen Namen diplomskega dela je ugotoviti, kako sprememba velikosti slikovnega polja vpliva na ekspozicijski indeks in dozo na pacienta ter pridobiti faktorje spremembe ekspozicijskih pogojev pri spremembi velikosti slikovnega polja za sistem računalniške radiografije. Metode dela: V diplomski nalogi smo uporabili dve metodi dela. Pri prvi metodi smo pridobili teoretične podatke s preučevanjem različne domače in tuje strokovne literature. Druga metoda dela je bila raziskava s pomočjo meritev, kjer smo spreminjali velikost slikovnega polja in merili vpliv spremembe na ekspozicijski indeks ter dozo. S pomočjo formule smo izračunali teoretične faktorje povečave ekspozicijskih pogojev. Rezultati: Pri zmanjševanju slikovnega polja se zniža ekspozicijski indeks, ko pa ekspozicijske pogoje povečamo za določen faktor, ekspozicijski indeks naraste, s tem pa se veča tudi doza. Na podlagi naših meritev moramo produkt toka in časa pri spremembi velikosti slikovnega polja z 35 × 43 cm na 24 × 30 cm izračunano povečati za faktor 1,52 (52 %), da ohranimo enak ekspozicijski indeks. Pri spremembi velikosti slikovnega polja z 24 × 30 cm na 18 × 24 cm pa moramo produkt toka in časa teoretično povečati za faktor 1,41 oziroma 41 %. Razprava in zaključek: Ko želimo izmeriti dozo, ki jo prejme pacient, je za prikaz doze bolj kot ekspozicijski indeks primeren produkt med dozo in površino. Z zmanjševanjem polja se površina obsevanega telesa zmanjšuje, zato se zmanjša tudi doza. To smo ugotovili tudi v diplomski nalogi, kadar nismo spremljali produkta toka in časa. Ugotovili smo, da se je ob enakih ekspozicijskih pogojih doza zmanjšala za približno 50 %, pri zmanjšani površini za 50 %.Introduction: Changing the size of the x-ray field affects the dose received by the patient, while at the same time influences the amount of scattered radiation that occurs in the patient and reaches the image receptor. When changing the size of the x-ray field, it is also necessary to change the exposure conditions if we want the signal-to-noise ratio in the image to remain the same. In the case of a smaller image field, the mAs must be increased. In order to do so, we can use the factors of changing the exposure conditions when resizing the x-ray field. Purpose: The purpose of this bachelor thesis is to determine how the change in the size of the x-ray field influences the exposure index and the dose received by the patient, as well as to obtain factors of changing the exposure conditions when resizing the x-ray field for the computer radiography system. Methods: Two methods were used to carry out the research. With the first method, we obtained theoretical data by studying diverse domestic and foreign professional literature. The second method of work was the measurement survey where we changed the size of the x-ray field and measured the effect of the change on the exposure index and dose. Using the formula, we calculated the theoretical factors for increasing the exposure conditions. Results: By reducing the x-ray field, the exposure index is lowered, but when the exposure conditions increase by a certain factor, the exposure index increases, thereby increasing the dose. Based on our measurements, the mAs should theoretically be increased by a factor of 1.52 (52 %) when changing the size of the x-ray field from 35 × 43 cm to 24 × 30 cm in order to maintain the same exposure index. When changing the size of the x-ray field from 24 × 30 cm to 18 × 24 cm, the mAs should theoretically be increased by a factor of 1.41 or 41%. Discussion and conclusion: When we want to measure the dose received by the patient, the product between the dose and the surface is more appropriate than the exposure index. By reducing the x-ray field, the surface of the irradiated body decreases, and therefore the dose also decreases. This is also what we found out within the framework of the present thesis when we didn\u27t change the mAs. It was established that under the same conditions of exposure, the dose was reduced by about 50%, with a reduced area of 50%