Biomedical subjects

U Fill

Publications and source records attributed to U Fill.

3 recordsLinked to original sources

PubMed2005

Average glandular dose conversion coefficients for segmented breast voxel models.

For 8 voxel models of a compressed breast (4-7 cm thickness and two orientations for each thickness) and 14 radiation qualities commonly used in mammography (HVL 0.28-0.50 mm Al), tissue dose conversion coefficients were calculated for a focus-to-film distance of 60 cm using Monte Carlo methods. The voxel models were segmented from a high-resolution (slice thickness of 1 mm) computed tomography data set of an ablated breast specimen fixated while being compressed. The contents of glandular tissues amounted to 2.6%, and were asymmetrically distributed with regard to the midplane of the model. The calculated tissue dose conversion coefficients were compared with the recent literature values. These earlier tissue dose conversion coefficients were also calculated using Monte Carlo methods and breast models of various thickness, but these consist of homogeneous mixtures of glandular and adipose tissues embedded in 5 mm pure adipose tissue both at the entrance and exit sides. The results show that the new glandular tissue dose conversion coefficients agree well with the literature values for those cases where the glandular tissue is predominantly concentrated in the upper part of the model. In the opposite case, they were lower by up to 40%. These findings reveal a basic problem in patient dosimetry for mammography: glandular dose is not only governed by the average breast composition, which could be derived from the breast thickness, but also by the local distribution of glandular tissue within the breast, which is not known.

Air↗

PubMed2003

The application of voxel phantoms to the internal dosimetry of radionuclides.

Extensive calculations of specific absorbed fractions (SAFs) for monoenergetic photon sources were performed using a Monte Carlo photon transport code together with seven male and female adult voxel models based on computed tomographic data of real persons. These models offer greater realism with respect to organ topology than the mathematical phantoms commonly used in the past. Due to individual anatomical differences, large variations in photon SAFs between the voxel models were found that can amount to orders of magnitude for very low photon energies. However, in many cases, the larger differences were found between MIRD-type and voxel models, since the inter-organ distances tend to be larger in the MIRD-type phantoms than in reality, due to over-simplification of organ shapes. Furthermore, organ absorbed doses per incorporated activity were evaluated for two selected radiopharmaceuticals. Although a method was found to largely eliminate the influence of organ mass on SAFs for organ self-absorption, the absorbed dose coefficients varied by several tens of per cent between the individual voxel models, thus indicating a significant influence of individual photon SAFs for organ cross-fire on organ absorbed dose. Again, 43% of the MIRD organ dose values were outside the range of doses spanned by the voxel models. Effective dose showed a variation of only up to 26% between the single voxel models for the radiopharmaceuticals considered.

Adult↗

PubMed2002

Organ dose conversion coefficients for external photon irradiation of male and female voxel models.

New organ equivalent dose conversion coefficients are presented for whole body irradiation with monoenergetic photons of energies between 10 keV and 10 MeV for idealized geometries and seven adult male and female voxel models. The geometries are broad parallel photon beams in anterior-posterior, posterior-anterior, left- and right-lateral direction and a full 360 degree rotation around the body length axis. Dose differences between the different voxel models are below approximately 30% for some organs and geometries in the energy range between 60 and 200 keV, but they can be up to 100% or more in single cases, due to differences in stature and individual anatomical details. For low photon energies, the differences may amount to hundreds of per cent. Extensive comparisons of the dose conversion coefficients with respective values calculated using mathematical body models revealed various degrees of unrealistic positioning of single organs in the latter models. Examples are the kidneys, spleen and stomach that are located too superficially in the mathematical models. Over- or underestimations of several tens of per cent may, thus, occur for the mathematical models, compared to the voxel models considered. In contrast to previous assumptions, when the mathematical models have been used to establish reference organ dose conversion coefficients, it can be concluded that they do not properly represent a large population of individuals.

Anatomy, Cross-Sectional↗