The introduction of Computed Tomography (CT) in radiology in the 1970 s was followed by rigorous radiation protection demands due to the higher doses of this modality compared to conventional x-ray based radiography. Epidemiological studies have linked low radiation doses to breast cancer. As the International Commission for Radiation Protection (ICRP) has recently published recommendations that emphasize the radiation protection of the breast, a CT method that will contribute to reduce the radiation dose to the breast tissue is of high value in today s hospitals. Meanwhile the mentioned method is subjected to have rigorous radiographic requirements in order to be used in CT. The objective of this study is to investigate if the use of the bismuth radioprotective shielding layer will reduce the dose to the breast tissue without any addition of the beam hardening artifacts or the noise in the CT image ideally.
The Alderson female phantom (ART) was scanned using 4 different CT protocols for lung diagnostics. For each protocol, the ART phantom was scanned with different thicknesses of the bismuth radioprotective layer, in addition to one scan without shielding. The shielding layer was placed on the breasts of the ART phantom in an attempt to reduce the dose to the breast tissue .The radiation dose received by the phantom was registered by thermo luminescence dosimeters (TLD-100) at three different depth positions in the thorax of the ART phantom. The evaluation of CT image was based on the change in the Hounsfield units (HU) and the image noise as a function of the thickness of the bismuth layers on a homogeneous CT phantom made of PMMA materials.
Results strongly indicate that the dose to the breast tissue is reduced due to the use of the bismuth radioprotective layer. For the scans with a radioprotective layer the image quality was affected in some parts (the region of the breast). But in the region of diagnostic interest, the lung region, there was no remarkable decrease of the image quality: there is no indication of additional beam hardening effects or increase of the noise in the image in the lung region independently of the thickness of the bismuth layer.