An exposure measurement experiment was performed to obtain frontal DSA, lateral DSA and fluoroscopic absorbed doses. In addition, the proportion of each dose to the total absorbed dose was obtained to estimate which influenced the patient-absorbed dose the most among the three in diagnostic DSA examinations for the head and neck. Three correlation coefficients were obtained to evaluate the relationship between any two doses. As a result, the following four findings were obtained:
1. The mean proportion of each absorbed dose reached a high value in the order of frontal DSA absorbed dose>lateral DSA absorbed dose>fluoroscopic absorbed dose.
2. A strongly negative correlation between the proportions of frontal DSA absorbed dose and fluoroscopic absorbed dose was observed (r=-0.864, P<0.01). Also, a slightly negative correlation between the proportions of lateral DSA absorbed dose and fluoroscopic absorbed dose was observed (r = 0.375, P<0.01). However, no correlation between the proportions of frontal DSA absorbed dose and lateral DSA absorbed dose was observed (r = -0.143, P>0.05).
3. The patient-absorbed doses, which were different from those of IVR examinations, were confirmed to be largely influenced by DSA radiographic doses.
4. The best method for the reduction of patient-absorbed doses was considered to be minimization of the frame number in DSA radiography required for diagnosis.

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The fea sibility of heavy ion computed tomography was investigated. A cylindrical polyethylene sample of 50 mm in diameter was used to establish the pencil beam scanning method and estimate the performance of the system. The sample was irradiated with carbon ions of 290 MeV/u and 400 MeV/u, and helium ions of 150 MeV/u. The residual beam energy was measured by a BGO scintillator in coincidence with two small plastic scintillators placed in front of and behind the sample to restrict the beam path and improve the spatial resolution in the CT reconstruction. The projection data were obtained by moving the sample only in the transverse direction because of its symmetrical structure. A Lucite sample with many holes of different diameters was used to demonstrate the spatial resolution of this CT system. From the reconstructed images the spatial resolution was estimated to be less than 2 mm under the conditions in this work. The electron density ratios of ethyl alcohol, water, and Lucite to polyethylene were obtained, which are in good agreement with the calculated values.

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Rossmann's formula on the power spectrum of quantum noise in a radiograph is based on an assumption that the relation between the spatial fluctuations of optical density and the relative spatial fluctuations of light energy irradiated from the intensifying screen due to absorbed x-ray quanta is linear. The reasonability of the assumption is not theoretically obvious because the relative spatial fluctuation of light energy increases as the number of absorbed x-ray quanta decreases.
Based on the shot-noise theory, the power spectrum of q uantum noise is calculated taking account of the second order of the spatial fluctuation of light energy and the spectrum is expressed as a function of the gradient, the modulation transfer function of the screen-film system, and the average number of absorbed x-ray quanta. The term caused by the nonlinearity between the spatial fluctuations of optical density and light energy per unit area for the noise power spectrum is also estimated using the mhits model for the emulsion and it is found that the effect due to the nonlinearity is little in practice.

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Most dosimetry protocols recommend the use of plane-parallel chambers for dose determination in electron beams with energies below 10 MeV. The new IAEA TRS 381 (1997) protocol includes the overall perturbation factor P_Q that consists of the inscattering correction factor P_cav(ar P_repi) and the wall correction factor P_wall(or P_wall)I. n this work, P_Qc for the commonly applied NACP, PTW/Roos and PTW/Markus planeparallel chambers was determined experimentally. For the NACP plane-parallel chamber, P_Q was obtained by comparison with a cylindrical Farmer chamber, while for the PTW/Roos and PTW/Markus chambers it was obtained by comparison with the NACP chamber. The values of P_Q for these plane-parallel chambers were measured as a function of mean electron energies ,E_z from 1.7 MeV to 11.5 MeV. It was found that for the NACP and PTW/Roos chambers, P_Q is independent of energy down to E_z= 1.7MeV, while for the PTW/Markus chamber it shows a systematic and exponential drop of about 2% with decreasing energy down to E_z=2.7 MeV. However, the decrease of P_Q for E_z=1.7 MeV was not exponential.

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