Synchrotron radiation (SR) has the properties of both high intensity and monochromaticity which cannot be obtained with conventional X-ray generated by an X-ray tube. Although studies on mammography as an application of SR to medical diagnosis have been performed, there has been little work about the dependences of glandular dose on X-ray energy, breast compositions and breast thickness. It is difficult to measure the average glandular dose directly, so we calculated conversion factors to obtain average dose from incident air kerma using Monte Carlo simulation. In this study, the following results were obtained.: (1) A difference of 1 keV has a significant effect on the average glandular dose. (2) The average glandular dose in young age is about 50 % higher than in old age when the same absorbed dose is applied in phosphor layer. (3) The average glandular dose required in mammography is less dependent on the X-ray energy than the air kerma.
With hyper-thermal neutron incidence, both of the depth dose distribution and the beam controllability are expected to be improved for neutron capture therapy (NCT). In order to clarify the irradiation characteristics of hyper-thermal neutrons and the feasibility of hyper-thermal neutron irradiation field for NCT, a “test-type” hyper-thermal neutron generator was designed and produced. Hyper-thermal neutron can be made of thermal neutrons by up-scatter with a high temperature material. Graphite of 6 cm thickness and 21 cm diameter was selected as the high temperature scatterer. The scatterer is heated up to 1200°C in maximum using molybdenum heaters. The radiation heat is shielded by reflectors of molybdenum and stainless steal. The temperature is measured using three R-type thermo-couples and controlled by a program controller. The total thickness of the generator is designed to be as thin as possible,20 cm in maximum, in the standing point of the neutron beam intensity. The thermal stability, controllability and safety of the generator at high temperature were confirmed by the heating tests. From the simulation calculations, it was confirmed that the neutron temperature shifted to be higher as the scatterer temperature was higher.
Inverse square sensitometry in the low energy region is limited by the air attenuation. Therefore, it is difficult to obtain the accurate characteristic curve for the mammography system. We applied the new method using the technique of Bednarek with the gradual change of the distance to sensitometry in the low energy region. Then we compared it with inverse square sensitometry, which corrected air attenuation by the method already suggested. As a result, the characteristic curve got by the two different techniques coincided with each other. It was confirmed that the new method was useful as sensitometry in the low energy region.
The narrow beam dosimetry for stereotactic irradiation contains several factors of uncertainty due to geometric errors such as inadequate size of dosimeter, rapid change in dose distribution and so on. These errors cannot easily be minimized under the usual process of dosimetry. However, with the Monte Carlo simulation, it is possible to estimate the dosimetric parameters in a region where conventional dosimetry is quite difficult to put into practice and to keep these geometrical errors to a minimum. In this study, we attempted to calculate the dose distribution at several depths and transverse directions in tissue and the field factor by the Monte Carlo simulation. The dose distribution indicated that the tissue peak ratio is almost independent of the size of detection volume in the horizontal direction (detection size), and the ratio varies greatly with position in the irradiation field. The transverse dose distribution indicated rapid change on the off center ratio. Thus, it was considered necessary to determine the central axis to a geometric accuracy of 1 mm or less. We calculated the ideal absorbed dose at the central axis for the detection size of 0 cm φ and obtained the field size and the detection size with which secondary electron equilibrium is established. It was also shown that a method with a reference point at a maximum dose is better than are having a reference depth fixed by incidence photon energy to obtain the field factor. The fundamental characteristics of a narrow beam extremely much variable compared with those of a broad beam, and for this reason it is emphasized that these characteristics shoud be defined precisely at the beginning of dosimetric the process.
The focused grid has been used widely in medical radiography because of removing the scatter radiation in the large exposure field. However, it is necessary to set the focused grid on the correct position. The primary radiation is also reduced too much in the misplacement of the focused grid. In particular, it is difficult to find the inclination of the focused grid which is one kind of the misplacement. The equations of the primary radiation loss in the misplacement were associated with the grid ratio. Our computer simulation using Monte Carlo methods showed the primary radiation loss obtained by simulation was less than that of the equation, because some primary radiation passing through the lead strips. As the grid density and the tube voltage of exposure have an influence on the primary radiation passed through the lead strips, we have investigated the dependence of the primary radiation loss in the inclination of the focused grid on the grid density and the tube voltage with our simulation. Our result s showed the primary radiation loss decreased with the high grid density, and decreased with the high tube voltage. We showed the dependence of the primary radiation loss on the grid density and the tube voltage, quantitatively.
Influence of the measurement site's thickness on the measured values of bone mineral density (BMD) was examined by performing statistical analyses. The measured values of BMD were classified into 7 groups (3.5,4.0,5.0,6.0,7.0,8.0, and 9.0 cm in thickness). The measurement site's thickness was found to influence the measured values of BMD as a result of one way layout analysis of variance (ANOVA). Furthermore, by Tukey's method, pairwise statistical differences were not found in thickness between 3.5 cm and 4.0 cm,3.5 cm and 5.0 cm, and 4.0 cm and 5.0 cm, but were found in thickness between other pairs of groups at p = 0.05 level. When considering 5 groups (3.5∼5.0,6.0,7.0,8.0, and 9.0 cm in thickness), the result obtained by ANOVA was the same as that of the above 7 groups. Furthermore, by Scheffe's method, pairwise statistical differences were found in thickness between all pairs of groups at p= 0.01 level. These results suggest that the measurement site's thickness influences the measured values of BMD.