Errors in accuracy and precision are important problems in bone mineral density analysis. Changes in the CT numbers caused by X-ray beam-hardening have an important effe ct on bone mineral density measurements. X-ray beam-hard ening decreases as the effective energy of incident beam increases. We attached an additional filter (0.4mmCu) to the standard filter (3.3mmAl) to increase the effective energy of the incident X-ray beam. The effective energy increased from 43keV to 63keV. We measured the CT number of a calcium-hydroxyapatite rod (26mm Φ,100 mg/cm3) in a water phantom (body phantom) using different heights of the water phantom with the rod fixed in it, and usig different heights of the rod with the water phantom fixed. We compared the standard deviations (SDs)of the CT numbers with and without the additional filter. In conclusion, change in the CT number was suppressed from 1.7% to 1.1% with the additional Cu filter.
To determine an absorbed dose for proton and heavy ion beams, a quasi adiabatic solid calorimeter with carbon absorber was designed and constructed. The absorber is a graphite block with 2.5cm diameter and 4mm thick. It has contained three thermistors, one is used as a detector for the absorbed energy and the another two as the electrical temperature calibrators. We confirmed the system had a sufficient reliability to make the measurement of absorbed dose for dose rate larger than 0.5Gy/min.
Recently, a method for estimating an exponentially decaying spectrum based on complex Kalman filter in combinaiton with decaying exponential window, what is called “apodization” in the field of NMR, has been proposed. The method can produce a high resolution spectrum from a noisy exponential decaying signal and enhance the signal-to-noise ratio in the spectrum, simultaneously. We modify the method to estimate in vivo 31P -NMR spectra by means of dividing an FID into several segments and varying the effect of apodizaton fo each segment. We name these “segmented apodizaiton”. Furthermore, we introduce the forgetting factor, which is used in the recursive least square (RLS), into the complex Kalman filter and attempt to vary it in the filter operation corresponding to the apodization in each segment. we compare the segmentde apodization with the conventional one and examine the effect of the forgetting factor. Simulations is performed using the signal which is generate d by the paramenters (T*2, peak area, frequency) of 31P-NMR spectrum acquired from a mouse tumor cell under 2 Tesla condition. As a result, The forgetting factor is so effective to the estimation of in vivo 31P-NMR spectra. It is shown that one can estimate the spectrum more accurately using the segemented apodization by setting an adequate segmentation and adequate dumping rates.
The energy spectrum data of photon beams from a Cobalt-60 teletherapy unit is necessary not only for accurate radiation therapy but also for accurate dosimeter calibration. It is generally assumed that only 1.17 and 1.33 MeV gamma rays are present in the beams. However, this assumption is not completely valid because of energy degradation due to interaction of the primary gamma rays with the source material, source capsule, shielding treatment head and collimators. For this study the energy spectrum in a therapeutic beam from a Cobalt-60 teletherapy unit was calculated using a conscientious Monte Carlo simulation. The results were represented by the fluence rate per Bq in order to obtain relevant information for radiation therapy and dosimeter calibration. The proportion of unscattered primary photons to scattered photons was approximately 0.65: 0.35; the mean energy range of photons was 1.04 to 1.06 MeV. The Monte Carlo calculation of dose distributions was also made in order to confirm the validity of the energy spectra obtained in this study. The calculated PDD, using the energy spectrum data, agreed with the measured PDD in a wide range of depths. As a result, it is evident that the calculated photon energy spectra agree with the realistic energy spectra of therapeutic beams from a Cobalt-60 teletherapy unit.
For the optimization of the risk-benefit in mammography, accurate dosimetry requires standardization of both the methodology and dosimeter. One of the important issues is the asse ssment of the average glandular dose under mammography. To this end, accurate measurement of half value layer and that of air kerma are of primary importance. To see the variation among facilities, the half value layer and the entrance dose were measured at eight facilities. In this series of measurement, a shallow type ionization chamber designed to fit with soft X-ray used in mammography was used. This comparison indicates that, with the same radiographic condit ions, the variation of the dose among equipment was such that the ratio of the highest to the lowest was almost two. While the variation of the half value layer among equipment under the fixed tube voltage was as high as 15%.
Accuracy of cavity-gas calibration factors Ngas for ionization chambers using high-energy photon and electron beams from a linear accelerator was evaluated by comparing with in-air calibration using a 60Co beam. For in calibration with a linear accelerator, fluctuations in beam energy and output, and changes in beam flatness and symmetry become important problems. To overcome these problems, we investigated a new method with a tip-to-tip calibration procedure in a phantom and with an exchange between a calibrated PTW-Farmer chamber and a calibration Fanner chamber. For the 8 PTW-chambers (with acrylic wall), Ngas ratios and the uncertainties (2 SD) of the improved in-phantom calibration for the in-air 60Co calibration were 0.999± 0.005 for a 60Co beam,1.000 ± 0.005 for 4 MV x-rays,0.999 ± 0.005 for 10 MV x-rays, and 0.998 ± 0.006 for 15 MeV electrons, respectively. The uncertainties (2 SD) in Ngas ratios among beam energies were within ±0.3%. On the other hand, for 2 NE-chambers (with graphite wall), the deviations among the beam energies were observed because of errors of the physical data for the wall correction factors Pwall. Therefore, it was concluded that when the discrepancies in Pwall values do not occur, from the results for the PTW -chambers, the accuracy of our calibra tion method for ionization chambers with a linear accelerator is equivalent to the in-air 60Co calibration.