Organ Dose Determination in X-ray Imaging

Organ dose is the absorbed radiation energy from ionizing radiation to an organ, divided by the organ mass. Organ doses of a patient cannot be measured directly in the patient, but their determination requires dose measurements in anthropomorphic patient models i.e. phantoms or Monte Carlo simulations. Monte Carlo simulations can be performed for example by using computational phantoms or patient’s computed tomography (CT) images. Organ doses can be estimated based on measurable dose quantities, such as air kerma, kerma-area product and volume-weighted CT dose index, by using suitable conversion coefficients. Conversion coefficient is the organ dose divided by the measured or calculated examination-specific dose quantity. According to the current knowledge, the probability of radiation induced stochastic effects, which include cancer risk and risk of hereditary effects, increases linearly as a function of the radiation dose. The organ dose is a better quantity for estimating the patient specific risk than the effective dose, which is meant to be used only for populations, and it does not consider patient age or gender. Moreover, the tissue weighting factors that are used in the effective dose calculation are based on whole body irradiations, but in X-ray examinations only a part of the patient is exposed to radiation. The phantoms used in medical dosimetry are either computational or physical, and computational phantoms are further divided into mathematical and voxel phantoms. Phantoms from simplified to as realistic as possible have been developed to simulate different targets, but the organ doses determined based on them can differ largely from the real organ doses of the patient. There are also standard and reference phantoms in use, which offer a dose estimate to a so called average patient. Due to the considerable variation within patient anatomies, the real dose might differ from the dose to a standard or reference phantom. The aim of this thesis was to determine organ doses based on dose measurements and Monte Carlo simulations in four X-ray imaging modalities, including general radiography, CT, mammography and dental radiography. The effect of the patient and phantom thickness and radiation quality on the organ doses in a projection X-ray examination of the thorax was studied via Monte Carlo simulations by using both mathematical phantoms and patient CT images. The effect of the breast thickness on the mean glandular doses (MGDs) was determined based on measurements with phantoms of different thicknesses and collected diagnostic and screening data from patient examinations, and the radiation qualities used in patient and phantom exposures were studied. For fetal dose estimation, fetal dose conversion coefficients were determined based on phantom measurements in CT and dental radiography examinations. Additionally, the effect of lead shields on fetal and breast doses was determined in dental examinations.