Analysis of dose measurements in computed tomographies

In this study, it was aimed to examine organ doses dosimetrically in imaging using Computed Tomography (CT) devices and to evaluate the effectiveness of the methods used to minimize radiation doses. CT is widely used due to its high diagnostic accuracy. However, the association of CT with high radiation doses poses potential risks for the patient. The study was carried out on the Siemens Somatom Scope (16-slice Multidetector CT) device. Alderson Rando anthropomorphic phantom is used to represent an adult person. Two methods were used for dose measurement. In Vivo Measurement: TLD-100 chips with lithium fluoride, magnesium, and titanium doped were used. TLDs are located in organs such as the brain, lens, cochlea, pituitary, temporal joint, parotid, thyroid, and breast. Estimation with Software: Organ doses were estimated using NCICT 3.0, a Monte Carlo-based software. Additional studies have also been conducted to determine Protective Equipment Effectiveness. Tissue-equivalent bolus (0,5 cm thick) for lenses and shielding with 0,50 mm Pb equivalent lead content were used for out-of-field radiosensitive organs (thyroid and breast). When the TLD measurement results obtained in cranial CT scan were evaluated, the highest absorbed dose (mean 33,0 mGy) was measured in lenses that were directly exposed to primary radiation and were close to the surface. The thyroid dose exposed to secondary radiation was measured as 1,73 mGy, and the mean dose in breast tissue was 0,18 mGy. This rapid decrease in absorbed dose in out-of-field organs is explained by the inverse-square law. 0,50 mm Pb equivalent lead shields effectively damped scattered low-to-medium energy photons. These shields resulted in a significant and significant dose reduction of 39,88% in thyroid dose and 55,56% in breast tissue (p<0,001). These results suggest that out-of-domain shielding is an effective tool in dose optimization. The use of tissue-equivalent bolus material resulted in a limited reduction in lens dose, which was not statistically significant. This suggests that the bolus cannot adequately reduce the primary photon flux. All absorbed doses estimated by the NCICT 3.0 software were observed to be systematically lower compared to TLD measurements. While the difference is around 29-33% in intra-field organs (Brain, Lens, Pituitary), the difference is much more pronounced in extra-abular organs (Thyroid: 76,9%, Breast: 72,2%). This large difference suggests that Monte Carlo-based software cannot fully capture instrument-specific spectral and filtration effects (especially scattering in the far field).