Hideki Kato

In medical linear accelerators, radioactivation is induced on the target and neighborhood parts by photoneutrons accompanying a photo-nuclear reaction and leading to higher acceleration energy. We measured the residual radiation from the radioactivated materials according to the time, and tried to identify radioactivated nuclides and their relative quantities by means of measurement results. It was presumed that the main source of residual radiations was the Target, Flattening filter and Primary collimator in the linac head. Among those materials (copper, tungsten), we calculated decrement curves of residual radiations from radioactivated nuclides generated with photo-nuclear reaction or thermal neutron capture reaction by various ratios, and we investigated the ratio that best fit the measured data. Consequently, it was presumed that ⁶⁶Cu generated with thermal neutron capture reaction contributed the most to residual radiation, followed by ⁶²Cu generated with photo-nuclear reaction contributed. It is important to understand various characteristics of these nuclides and to undertake management of the device.

The characteristics of activation after high-energy X-rays have been generated by medical linear accelerators were measured using an ionization chamber. Radiation doses increased with rising X-ray energy, based on 10 MV, 15 MV, and 18 MVX-ray measurements. When the total irradiation dose was changed, radiation dose increased with total irradiation dose. When the collimator opened, the radiation dose at a position 15 cm from the isocenter reached about the maximum, which was 2.2 times the dose at the isocenter. The radiation dose became about 0.3 times its level at a position 40 cm from the isocenter, in the outer irradiation field. The dose distribution in the treatment room became almost the same dose extending from the isocenter to 200 cm. Radiation dose decreased gradually while moving away from the target on the treatment beam axis. But it increased again as it approached the floor face. The occupational exposure dose, which was presumed from measurements of the radiation dose 50 cm from the isocenter, was about 0.9 mSv during a year, assuming 600 MU for 1 person, 8 people a day, and 245 days a year. Radiation dose changed with X-ray energy in the machine used, and it was a geometrical constituent in the treatment room. It is important to understand the characteristics of radiation generated by medical linear accelerators.

Radionuclide contamination is one of the accidents that might occur while carrying out a periodical inspection of nuclear power stations during normal reactor operation. When such an accident occurs, rescue and medical personnel, involved in transporting and treating affected workers run the risk of exposure to secondary radiation. In this study, the ambient dose equivalent rate at a certain distance from the surface of the human body contaminated with typical radioactive corrosion products was calculated. Further, the relationships among the adhesion area, contamination density, and secondary exposure dose were clarified. The secondary exposure dose and permissible working hours in a radiation emergency medicine were estimated by presenting these relationships in the form of a chart and by specifying the contamination levels.

In one of the accidents that might happen in the nuclear power station, there is a contamination accident caused by radioactive corrosion products during a periodic inspection. It is necessary to presume the skin absorbed dose from the adhesion area and the contamination density to forecast the level of the skin hazard by the adhesion of the radioactive substance. However, the data to forecast the local skin dose when the radioactive substance adhered handily is not maintained. In this paper, the absorbed dose in the skin surface neighborhood contaminated by radioactive corrosion products was calculated, and the relation between the adhesion area and the contamination density and the local absorption dose was derived. And, the approximate equation that forecast the integrated dose was derived from these data. As for the absorbed dose rate in depth 70 μm from the skin surface that became the index of the skin hazard, the contribution rate by ⁵⁹Fe was the highest within 30 days, and the contribution of ⁶⁰Co rose most after the 30th after the radioactive substance had adhered when the contamination density the adhesion area was the same. The relation between the initial contamination density and days that required it was graphed to reaching to the threshold by the integrated dose when the threshold dose in which the necrosis of the skin was caused was assumed to be 20 Gy. The absorbed dose commitment can be presumed from measurements of the contamination density by using this graph or the approximate equation.

It is important to precisely evaluate patient dose from a diagnostic X-ray in order to investigate medical exposure reduction. As a method of evaluating patient surface dose, computation with existing data based on exposure in air is generally used. With this method, backscatter factors and absorbed dose conversion factors are given by the parameter of the effective energy or the half value layer, making this procedure complicated. We developed program software (Surface Dose Evaluation Code, SDEC) that computes the surface dose automatically, using the backscatter factor and absorbed dose conversion factor calculated by using X-ray spectral data. Because the measurement of effective energy or a half value layer is unnecessary, SDEC is a useful evaluation method.

Recently, the angiography system used for interventional radiology (IVR) provides a device for measuring dose-area product (DAP), which is compulsory in European countries. The usefulness of DAP is that one can observe patient dose in real time during IVR and can obtain an integral dose by overall IVR procedure without a dosimeter directly placed on a patient. It is important to know the most irradiated region (hot spot) of the patient's skin and its maximum value in the dose management of IVR, but this information cannot be obtained only in DAP. In this paper, we describe a new method to estimate patient surface dose distributions in IVR. We devised a sheet dubbed the "Number map", which does not obstruct the IVR procedure, to confirm the hot spot, and we developed software (named PIETA "Patient Information on Exposure Total Dose Analysis in IVR") to analyze the data from the Number map. Using this system, dose distributions of patient's skin were easily obtained, and we could easily perform patient dose management in IVR.

The anti-scatter grid is generally used as a way of eliminating scattered radiation from X-ray photographs. This does not change even if the detector system changes from an analogue system to a digital system such as the flat-panel detector. We developed a new method that uses software to eliminate scatter components from digital X-ray images taken without the use of an anti-scatter grid. With this software, scatter components are eliminated from the X-ray image according to primary-to-scatter ratios, which were calculated on the basis of an imaginary phantom constructed by the pixel value of the X-ray image and on the spectrum of irradiated X-rays. In a trial calculation using a simulation image, it was confirmed that scatter components are eliminated at a high rate that is generally constant on the whole, irrespective of the kind and presence of the inhomogeneous region. When using this technique, the amount of X-ray exposure to a patient can be substantially reduced compared with that of radiography using the anti-scatter grid. Subsequently, the patient dose can be reduced.

We computed the differential dose albedo (α_D) for high-energy X-rays on a concrete slab when the incident angle, reflection angle, and azimuth angle were changed, by means of Monte Carlo simulation. We found that α_D changed with incident, reflection, and azimuth angles to the concrete slab. On the whole, the larger the incident angle, the larger α_D tended to become. If the incident angle and reflection angle were the same, the larger the azimuth angle, the smaller α_D tended to become. When the incident, reflection, and azimuth angles were the same, the smaller the X-ray energy was, the larger α_D became, in the order of 10 MV, 6 MV, and 4 MV X-rays.

Spectra of scattered X-rays in the maze of a Linac (X-ray energies of 4, 6, and 10 MV)room were estimated by means of the Monte Carlo simulation, and air kerma transmission factors of the X-rays scat-tered through a lead shield were evaluated based on those spectra. Spectra of scattered X-rays showed a maximum in the energy area below 200 keV. The higher the accelerated electron energy, also, the smaller the scattering angle that tended to spread to the higher energy area of the distribution of spectra. The air kerma transmission factor of 120° scattered X-rays of 4 MV X-rays obtained in this study was larger than the transmission factors of 124° scattered photons of ^<60>Co gamma rays through a lead shield given in ICRP. The air kerma transmission factors of 120° scattered X-rays of 6 MV X-rays were smaller than the transmission factors of 90° scattered photons of ^<60>Co gamma rays. The air kerma transmission factors of 120° scattered X-rays of 10 MV X-rays was slightly larger than transmission factors of 90° scattered photons of ^<60>Co gamma rays. Therefore, in the case of a 4 MV X-ray Linac room, the calculation method given in the "Manual of Practical Shield Calculation of Radiation Facilities (2000)" causes underestimation of leakage doses.

We developed a new physical evaluation system for medical displays using a high-resolution single-lens reflex type digital camera. This system was designed for easy and precise measurements of the modulation transfer function, the luminance response and the noise power spectrum of the liquid crystal display and the cathode-ray tube display. Each of the measurements was achieved by one-time photographing of a test pattern with the optimized data processing. Simple composition (a digital camera and a computer only) was very suitable for measurement in medical settings. Actual measurements were performed to three medical displays, and the high reproducibility and easy operability of this system were demonstrated.

The transmission factor of annihilation γA rays (511 ke V) for radiation shielding materials has been estimated by means of the Monte Carlo simulation based on three kinds of geometries of (1) a point isotropic source in infinite thick material, (2) a point isotropic source in finite thick material and (3) vertical incident to finite thick material. The present results show that the factor for geometry (1) is the largest, that for geometry (2) is next and that of the geometry (3) is the lowest. We have also found the cases in which the factor for geometry (1) is 2-3 times larger than the factors for geometries (2) and (3).<BR>The transmission factors for main γA-ray emitting nuclides, given in "Manual of Practical Shield Calculation of Radiation Facilities (2000)", are available only for the geometry (1). The present work indicates that the use of these values the calculation for actual shield geometries often causes the overestimate of effective doses. The transmission factor data for the geometries (2) and (3) are clearly necessary for more reliable shield calculations.

Data on the percentage depth-dose of diagnostic X-rays are important in evaluating patient dose from medical exposure. We developed a new method of calculating central-axis depth-dose in a homogeneous tissue phantom irradiated by diagnostic X-rays. First, primary and scattered components of percentage depthdose for mono-energetic X-rays were calculated by means of Monte Carlo simulation, and the data were stored in data tables. Then, percentage depth-doses for individual X-rays were calculated by making use of the data tables, the photon spectrum of X-rays, and exposure conditions such as field size. This method, which can calculate depth-dose under any condition of irradiation by combining with the approximating equation of the X-ray spectrum, is useful in evaluating the percentage depth-dose of diagnostic X-rays.

When computing the amount of leakage from a diagnostic X-ray room, the amount of scattered radiation released from the irradiated body in the lateral direction must be evaluated correctly. The side-scatter factor varies owing to change in the area and/or shape of the field, but the amount of variation is not always linearly proportional to the area of the field. Because the X-ray spectrum depends on the total filtration of the X-ray tube even if the irradiation geometry, X-ray equipment, and tube potential are the same, the side-scatter factor, too, is subject to change. In this paper, we propose a new method of calculation that uses the differential side-scatter factor computed by means of a Monte Carlo simulation, for obtaining the side-scatter factor of diagnostic X-rays. This method, which can calculate the side-scatter factor under any conditions of irradiation, is useful in evaluating the side-scatter factor of diagnostic X-rays.

When computing the amount of leakage from a diagnostic X-ray room, the transmission data of X-ray beams through the shielding material, which are used in the computation, must agree with the conditions of use of the X-ray equipment. Even if the tube potential is the same, the energy spectrum of generated X-rays depends on conditions such as high voltage rectification and total filtration, and transmission through the shielding material, too, is subject to change. In this paper, we propose a new method of calculation, which uses transmission data of mono-energetic photon beams computed by means of a Monte Carlo simulation, for obtaining effective dose transmission data through the shielding material of an X-ray beam with spectral distribution. We also present effective dose transmission data of primary X-ray beams and 90°scattered X-ray beams through concrete and lead shields as determined by this method. This method, which can calculate the transmission data X-ray beams with any spectral distribution, is useful in evaluating the leakage dose of diagnostic X-ray facilities.

When computing the amount of leakage from a diagnostic X-ray room, the transmission data of X-ray beams through the shielding material, which are used in the computation, must agree with the conditions of use of the X-ray equipment. Even if the tube potential is the same, the energy spectrum of generated X-rays depends on conditions such as high voltage rectification and total filtration, and transmission through the shielding material, too, is subject to change. In this paper, we propose a new method of calculation, which uses transmission data of mono-energetic photon beams computed by means of a Monte Carlo simulation, for obtaining effective dose transmission data through the shielding material of an X-ray beam with spectral distribution. We also present effective dose transmission data of primary X-ray beams and 90°scattered X-ray beams through concrete and lead shields as determined by this method. This method, which can calculate the transmission data X-ray beams with any spectral distribution, is useful in evaluating the leakage dose of diagnostic X-ray facilities.

The backscatter factor is necessary for evaluating the skin dose of patients irradiated by diagnostic x-rays. In this study, backscatter factors for x-rays of various irradiation conditions were calculated by means of the Monte Carlo method. The backscatter factors calculated for X-rays, which had the same HVL but different tube potentials, did not always coincide with each other. From this result, it was evidenced that the backscatter factor obtained from the BSF data table with a parameter of HVL were uncertain. To avoid such uncertainties, a new convolution method to obtain backscatter factors using an x-ray spectrum and differential backscatter factors was developed. This new calculation method could be installed in a computer program with an approximating equation of x-ray spectra, and accurate backscatter factors for any x-ray and/or any irradiation field could be obtained easily.

The backscatter factor is necessary for evaluating the skin dose of patients irradiated by diagnostic x-rays. In this study, backscatter factors for x-rays of various irradiation conditions were calculated by means of the Monte Carlo method. The backscatter factors calculated for X-rays, which had the same HVL but different tube potentials, did not always coincide with each other. From this result, it was evidenced that the backscatter factor obtained from the BSF data table with a parameter of HVL were uncertain. To avoid such uncertainties, a new convolution method to obtain backscatter factors using an x-ray spectrum and differential backscatter factors was developed. This new calculation method could be installed in a computer program with an approximating equation of x-ray spectra, and accurate backscatter factors for any x-ray and/or any irradiation field could be obtained easily.

In this paper, we describe an alogorithm for photon transport calculation, named the 'voxel-by-voxel process', to be used in Monte Carlo simulation codes with inhomogeneous media in which various material regions exist. In the voxel-by-voxel process, phantoms were constructed by dividing the volume of a rectangular prism into tiny voxels of homogeneous media. The photon free path length and/or the electron range in the voxelized phantom could be calculated uniformly using the unit of the voxel. By means of this method, Monte Carlo simulation could be performed with intricate inhomogeneous media, and the standardized Monte Carlo code could be programmed.

An x-ray spectrum measured by a semiconductor detector is different from the incident x-ray spectrum to the detector, because of distortions caused by energy-dependent responses of the detector and statistical and electrical fluctuations in the signal amplifying process. In this paper, we discuss a method for correcting the statistical and electrical fluctuations of the x-ray spectrum, using the unfolding method with a function based on the Gaussian distribution. Unfolding the measured x-ray spectrum by this method, K-α and K-β characteristic x-rays were clearly separated into two line spectra, and energy resolution was improved. The unfolding method, when used to supplement the stripping method that is generally applied to x-ray spectra correction, will provide enhanced correction of x-ray spectra.

The quality of portal images can be improved by the use of computed radiography(CR)with storage phosphor technology instead of the conventional screen-film system. The frequency processing of portal images, however, sometimes produces artifacts in the field margin, which may decrease the precision of verification of the irradiation field. The object of our study was to clarify the optimum parameters of frequency processing for CR portal imaging. The FCR-7000 system with 4MV X-ray was employed. Our results showed that the usual parameters of frequency processing and irradiation field size, were useful in verifying the irradiation field. However, artifacts are likely to have an effect, especially extreme parameters of frequency processing and small fields. In addition, the following wre important in selecting the parameters of frequency processing:1)the band of frequency should be selected according to field width and frequency(RN3:0.25cycle/mm), 2)the type of frequency should be used the graininess emphasis, 3)the emphasis should be minimized(RE3.0).

The Cadmium-Telluride (CdTe) semiconductor, which can be used at normal temperatures without cooling, was recently developed for the measurement of X-ray spectra. In this paper, we calculated the energy-dependent responses of CdTe semi-conductor detectors to X-ray photon beams by means of the Monte Carlo simulation. The CdTe semiconductor showed higher K-escape fractions and lower photo-peak efficiencies than the high purity Germanium (HPGe) semiconductor that has generally been used for X-ray spectra measurements. Therefore, it is assumed that the output spectra measured by the CdTe detector are more distorted than those measured by the HPGe detector. The X-ray spectra measured by CdTe semiconductor detectors must be corrected to account for the energy dependent response of CdTe.

The X-ray spectrum, measured by a high-purity germanium (HP-Ge) semiconductor detector, is the spectrum of energy absorbed in the germanium crystal. Therefore, to obtain the incident X-ray spectrum to the detector, the output spectral data from the detector must be corrected considering the energy-dependent response of the detector. The response of a HP-Ge detector to incident photon beams depends on the shape and size of the crystal and the geometry of measurement such as the photon incident angle to the detector. Accordingly, if the response data, which were used for the correction of measured spectrum, dose not correspond to the factors of measurement, the incident spectrum can not be obtained correctly. We have developed a computer code, SPECX, for correction of output spectral data measured by a HP-Ge detector. Using this code, the incident X-ray spectrum to the detector can be obtained from the output spectral data measured by any shape and size of the HP-Ge crystal and any geometry of measurement.

The purpose of this paper is to analyze the characteristics of scattered X-rays from a square-wave chart in terms of Monte Carlo simulation study and actual measurements employing a non-screen film system. It was confirmed quantitatively from the simulation study that there exist scattered X-rays from both of the lucite and lead plate, along with L-characteristic X-rays from the lead plate in the chart. The lead mask (4mm width) on the chart could not eliminate the scattered X-ray completely. The difference in square wave response observed with and without the lead mask was about 2%. The scattered rays from the chart was able to be eliminated by the air gap method with a 15cm distance between the chart and film. In an actual MTF measurement for screen/film systems, it is considered that the effect of these scattered rays is almost negligible; however, it is required to be careful to scattered rays and L-characteristic X-rays.

It is necessary to know the dose distributions not only inside but also outside the region exposed by the useful X-ray beam, to evaluate doses in a patient during the radiographic procedures.<br>In this paper, we calculated systematically three dimensional absorbed dose distributions outside the useful beam of diagnostic X-rays in a rectangular prism water phantom using the Monte Carlo method.<br>The isodose curves, which showed the scattered dose distribution, were in turn moved further away from the useful field as simultaneously increasing the X-ray tube voltage and/or the field size. But, this movement of isodose curves in the low dose rates tends to gradually decrease with the increase of the tube voltage as a result of the probability density of photon scattering angle dependence on the photon energy. When the width of the X-ray field is larger than the free path length in the water of the scattered photons, the distance of the isodose curves from the edge of the field is almost constant.<br>This systematic data of dose distributions is very useful to estimate doses at various locations when patients undergo radiologic examinations.

For diagnostic X-ray images, the quantity of scattered radiation, which influences the image quality, is generally estimated in terms of the "scatter fraction". To improve the image quality, it is necessary to devise a way to decrease the influence of scattered radiation. For that purpose, one must have an understanding of the mechanism of influence on the images, together with the quantity that is expressed by scatter fractions. In the present study, by means of a Monte Carlo simulation, we calculated the energy spectra of the primary and scattered radiation, which was absorbed on the screen systems in radiography, and analyzed the influence of the scattered radiation. As a result, not only quantity, which is expressed by the scatter fractions, but also the spectrum make a difference to the influence of scattered radiation. Also, scatter fractions depend on exposure parameters and screen systems; however, mechanisms of dependence are different respectively.

Recently, high-energy photon narrow beams, less than several cm in diameter, were going to use for the purpose of the stereotactic radiosurgery with a linear accelerator. In this paper, we analyzed the physical characteristics of high-energy X-ray narrow beams by means of a Monte Carlo simulation, and discussed their peculiar phenomena. When the beam size of high-energy X-ray became smaller, in addition to a decrement in the scattered doses, the decrease in the primary doses began from a certain beam size due to the failure of the electron equilibrium in the side direction. This caused sharp fall-off on the percentage depth doses, off-center ratios and output factors, with a shallowing the maximum dose depth. In dose calculations for high-energy X-ray narrow beams, it is necessary to estimate the primary doses, which take into account the electron transport.

Isodose charts for oblique incidence are simply obtained from normal isodose data by means of correcting methods such as the TAR method, the effective SSD method etc. Although, in these correcting methods, the depth dose data on the beam axis remained as the normal depth dose data, which were measured on the geometry of perpendicular incicence. In this paper, the primary and scattered dose on the beam axis for ^<60>Co gamma-ray oblique incidence were calculated by means of the Monte Carlo simulation, and the variation of the percentage depth dose and scatter factor were evaluated for oblique incident angles. The scattered dose distribution were altered for change in the oblique incident angle. Also, for increasing the angle, PDD were decreased and the scatter factor were increased. If the depth dose for oblique incidence were calculated using normal PDD data and normal scatter factors, the results become an underestimation of the shallow region up to several cm, and an overestimation for the deep region.

The data table of a 1cm dose equivalent per fluence to estimate the effective dose equivalents conservatively from external photon beams were given by ICRP. However these data were calculated with single energy photon beams; therefore, it must be careful when for applying the data to photon beams with spectral distributions. In this paper, we calculated the 1cm dose equivalent per unit exposure for spectral photon beams by means of a Monte Carlo simulation, and ascertained the suitability of the usual method which estimates dose equivalents by means of ICRP data corresponding to the effective energy of the spectral beam. Consequently, the 1cm dose equivalent per unit exposure calculated for primary X-ray beams and for scattered photon beams from an irradiated body were in good agreement with the ICRP data corresponding to the effective energies of each beam. However, those calculated for X-rays transmitted through a lead shield showed a considerably lower value than ICRP data corresponding to their effective energies.

In this paper, a new primary dose calculation method at points where electron equilibrium is not attained for high-energy photon beams. For this purpose, we originated a concept of a differential primary dose, and calculated this data for a 10 MV X-ray by means of the Monte Carlo simulation. Using this differential primary dose data and the primary Kerma data, a new three-dimensional calculation method of primary doses, which takes into account the electron transport, was derived. Primary doses in a heterogeneous media calculated by means of this method were in good agreement with primary doses calculated by the direct Monte Carlo simulation.