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weighting factor of 0.12 , and its exposure to high doses during chest CT examinations , reducing the breast dose is important ( Lestari et al ., 2023 ; Saba et al ., 2019 ).
There are several techniques to reduce breast dose during thoracic CT examinations : ( 1 ) Iterative Reconstruction Algorithm ( IRA ) ( Foley et al ., 2013 ; Lambert et al ., 2016 ); ( 2 ) Tube Current Modulation ( TCM ) techniques ( angular or organ-based ) ( Feldle et al ., 2023 ; Foley et al ., 2013 ; Lambert et al ., 2016 ; Saba et al ., 2019 ); and ( 3 ) Using bismuth shield ( Mehnati et al ., 2019 ; Mehnati et al ., 2019 ; Saba et al ., 2019 ). The use of a bismuth shield is an effective method for reducing breast absorbed dose during CT examinations . By absorbing low-energy photons , the bismuth shield reduces the dose to the superficial organs by 26 – 57 %. Bismuth breast shields work independently of the CT scanner and can therefore be used with scanners of different models and manufacturers . The advantages of bismuth breast shields include low cost , effectiveness , and ease of manipulation ( Ko et al ., 2021 ). Despite the benefits of dose reduction , the bismuth shield has a detrimental effect on image quality , such as changes in the accuracy of the CT number and an increase in image noise and artifacts ( Foley et al ., 2013 ; Lambert et al ., 2016 ; Liao et al ., 2019 ; Saba et al ., 2019 ). The other researchers ( Alonso et al ., 2016 ; Colletti et al ., 2013 ; Einstein et al ., 2012 ; Huggett et al ., 2013 ) have conducted further investigations on the use of bismuth shields . They demonstrated that the use of bismuth shield reduced the breast surface dose but it also increased image noise , created streak artifacts and decreased image quality ( signal and contrast ). The usefulness of bismuth shield has been challenged by the American Association of Physicists in Medicine ( AAPM ) ( AAPM Report , 2024 ; Akhlaghi et al ., 2014 ) due to the decrease in the diagnostic quality of the images , unpredictable and unreliable results when it is combined with the automatic radiation control system , and loss of part of the input beam intensity .
In recent decades , researchers ( Foley et al ., 2013 ; Lestari et al ., 2023 ; Mathieu et al ., 2013 ; Mehnati et al ., 2018 ; Parker et al ., 2008 ) have sought to introduce new shield materials to reduce surface dose in CT examinations . Parker et al . ( 2008 ), Feloy et al . ( 2013 ), Mathieu et al . ( 2013 ), Mehnati et al . ( 2018 ), Lestari et al . ( 2023 ) have introduced and used various material as radiation shield for CT examination . Specifically , they have fabricated and used the following materials to reduce the surface dose and investigated on the image quality parameters as well : tungsten-antimony alloy composite ( Parker et al ., 2008 ), barium vinyl ( Foley et al ., 2013 ), copper foil filtration or lead foil filtration ( Mathieu et al ., 2013 ), and polyurethane and silicon with 5 % of bismuth ( Mehnati et al ., 2018 ), silicone rubber ( SR ) -lead ( Pb ) ( Lestari et al ., 2023 ). Feloy et al . ( 2013 ) compared the dose reduction with bismuth and barium vinyl in-plan shield . They indicated that the dose reduction with bismuth is greater than barium vinyl but the important point is that the increase in image noise was more pronounced with the bismuth shield compared to the barium vinyl shield in-plane shield . Mehnati et al . ( 2018 ) demonstrated that using polyurethane and silicone shields with 5 % bismuth reduced the breast surface dose by 57.9 % and 37.6 %, respectively .
Based on the fact that CT scans are known as the primary sources of radiation exposure for patients among other X-ray diagnostic modalities , a new shield design for thoracic CT examinations is necessary to significantly reduce the breast dose while maintaining the diagnostic quality of the images . The aim of this study is to : introduce a new shield material composed of barium sulfate and copper for reducing the radiation dose to the breast surface , investigate the effect of new shield on image quality and compare its results with other research findings .
2 Materials and methods
2.1 Shield construction
In this study , copper was used as a material with a low atomic number ( Z Cu = 29 ) and barium sulfate ( BaSo 4 ) was used as a material with a high atomic number ( Z BaSo4 = 56 ). The composition of copper and BaSo 4 removes low and medium energy rays that are not crucial for imaging , and thereby reducing patient dose . However , the use of bismuth ( Z Bi = 83 ) removes both low-energy and a significant amount of highenergy X-rays , leading to image artifacts .
Four types of radiation shields were designed for the current study . These included three different weight percentages of BaSo 4 and copper ( 100 % BaSo 4 ; 50 % BaSo 4 and 50 % copper ; 10 % BaSo 4 and 90 % copper ) and as well as one weight percentage of bismuth ( 100 % bismuth ), which were chosen to create the breast shields .
A plexiglass mold in the shape of breast was used to constructed the radiation shields . The shields were made of Room-Temperature-Vulcanizing silicone ( RTV silicon ) ( Saba et al ., 2019 ) due to proper shape stability and acceptable flexibility as a matrix and bismuth , copper and BaSo 4 as X-ray absorbing materials . RTV silicone is a type of silicone rubber in which the silicone rubber serves as a base and is mixed with a curating agent ( Saba et al ., 2019 ). The radiation shield of the breast was made in a rectangular shape , with length of 40 cm , width of 14 cm , and thickness of 5.2 mm ( Hopper , 2002 ), taking into account the anatomical shape of the breasts , so that the upper middle incision was designed to fit the sternum gap . Its edges were curved to avoid any effect on the diagnostic image . In order to minimize the image noise and CT number shifts near the shield , a 2 cm thick foam pad ( Mehnati et al ., 2018 ; Mehnati et al ., 2019 ; Saba et al ., 2019 ) was used between the breasts and the shield ( Fig . 1a ).
2.2 CT scanning
All CT scanning were performed using a 16-slice scanner ( Siemens , Germany ). The machine automatically selected conventional adult chest scan parameters based on patient anatomy and attenuation ( automatic exposure control ( AEC )), including a tube voltage of 130 kVp , tube current of 100 mA , slice thickness of 5 mm , pitch of 1.25 , and pixel size of 1.6 mm 1.2 mm . To avoid increasing the scanning parameters in thorax CT , shields were placed on the breast after scout taking the images .
2.3 Phantoms and dosimetry
Evaluation of the effectiveness of the constructed radiation shields in reducing the breast surface dose , was performed