AN EVALUATION OF THE BASIC CHARACTERISTICS OF A PLASTIC SCINTILLATING FIBRE DETECTOR IN CT RADIATION FIELDS.

The ionisation chamber for computed tomography (CT) is an instrument that is most commonly used to measure the computed tomography dose index. However, it has been reported that the 10 cm effective detection length of the ionisation chamber is insufficient due to the extent of the dose distribution outside the chamber. The purpose of this study was to estimate the basic characteristics of a plastic scintillating fibre (PSF) detector with a long detection length of 50 cm in CT radiation fields. The authors investigated position dependence using diagnostic X-ray equipment and dependencies for energy, dose rate and slice thickness using an X-ray CT system. The PSF detector outputs piled up at a count rate of 10 000 counts ms-1 in dose rate dependence study. With calibration, this detector may be useful as a CT dosemeter with a long detection length except for the measurement at high dose rate.

Dose reduction in general radiography for adult patients by use of a dual-side-reading photostimulable phosphor plate in a computed radiography system.

Our purpose in this study was to evaluate the potential of dose reduction in general radiography for adult patients by use of a dual-side-reading (DSR) photostimulable phosphor plate in a computed radiography system. The image quality and low-contrast detectability in terms of the contrast-detail diagram of the DSR system with use of the X-ray beam quality of the RQA 5 defined by the International Electrotechnical Commission 61267 were compared with those of a conventional single-side-reading (SSR) system. The radiographic noise of the DSR system was lower compared to that of the SSR system under the same exposure conditions. Although there were no statistical differences in low-contrast detectabilities between the SSR system and the DSR system under the same exposure levels, the DSR system showed superior detectability compared to the SSR system. We conclude that the DSR system for general radiography has the potential to reduce the patient dose.

Detectability of simulated pulmonary nodules on chest radiographs: comparison between irradiation side sampling indirect flat-panel detector and computed radiography.

OBJECTIVE: To compare the detectability of simulated pulmonary nodules on chest radiographs between an irradiation side sampling indirect flat-panel detector (ISS-FPD) and computed radiography (CR). MATERIALS AND METHODS: This study was an observer performance study. Simulated pulmonary nodules of 8 mm in diameter were superimposed on an anthropomorphic chest phantom. Chest radiographs were acquired under 2 exposure levels (4 and 3.2 mAs) with the ISS-FPD and the CR. Six thoracic radiologists evaluated all 40 images (10 patterns × 2 different exposure doses × 2 different systems) for the presence or absence of a lesion over each of 12 defined areas on a 3-megapixel monochrome liquid-crystal display. Receiver operating characteristic (ROC) curves were obtained for observation in predefined 480 areas. A jackknife method was used for statistical analysis. Differences with a P value of <0.05 were considered significant. RESULTS: The analysis of the observer detection of simulated pulmonary nodules showed larger areas under the ROC curve (AUC) by the ISS-FPD than by the CR. There was a statistically significant difference between the two systems at 3.2 mAs (P=0.0330). CONCLUSION: The ISS-FPD was superior to the CR for the detection of simulated pulmonary nodules at 3.2 mAs.

Basic imaging properties of an indirect flat-panel detector system employing irradiation side sampling (ISS) technology for chest radiography: comparison with a computed radiographic system.

The image quality and potential usefulness for patient skin-dose reduction of a newly developed flat-panel detector (FPD) system employing irradiation side sampling (ISS) were investigated and compared to a conventional computed radiography (CR) system. We used the X-ray beam quality of RQA 9 as noted in the standard evaluation method by the International Electrotechnical Commission 62220-1 to evaluate the image quality of the detector for chest radiography. The presampled modulation transfer function (MTF) of the ISS-FPD system was slightly higher than that of the CR system in the horizontal direction at more than 2.2 cycles/mm. However, the presampled MTF of the ISS-FPD system was slightly lower than that of the CR system in the vertical direction. The Wiener spectrum of the ISS-FPD system showed a 50-65 % lesser noise level than that of the CR system under the same exposure condition. The detective quantum efficiency of the ISS-FPD system was at least twice as great as that of the CR system. We conclude that the ISS-FPD system has the potential to reduce the patient skin dose compared to a conventional CR system for chest radiography.

[Necessity of delivering the calibration coefficient of a dosimeter for low energy X-rays used in mammography X-ray units].

The calibration coefficient currently supplied by the National Institute of Advanced Industrial Science and Technology (AIST) is measured with an X-ray unit equipped with a W target/Al filter. We compared the calibration coefficient, which is measured with the X-ray unit equipped with a W target/Al filter and the X-ray unit for mammography. For the calibration coefficient, which is measured at the calibration fields of the X-ray units for mammography, the difference in the calibration coefficient measured with the X-ray unit equipped with a W target/Al filter that assumed a difference in the quality index-value of less than 1.5% was 24.2% at the maximum. In addition, many differences of more than unsureness 5% of the calibration coefficient were accepted. Thus, the correct dosimetry is not being performed for mammography. To improve the precision of dosimetry for mammography, at least, it is desirable to employ the calibration coefficient using the Mo target supplied by AIST. Furthermore, the indication of quality of radiation should be made severe, too. To perform equal dosimetry with a diagnosis domain, it is necessary to supply the calibration coefficient of the other target/filters.

[Physical and imaging properties of heavy metal filter combinations in chest radiography]

We have investigated the performance characteristic of the heavy metal filters with higher atomic numbers by comparing their patient exposures, tube loadings, radiographic contrasts, and noise Wiener spectra with those of a combination of copper and aluminum filter which has been used widely in chest radiography. Seven heavy metal filters were used for this study. As for a tungsten filter, two filters different in thickness were used. One is 0.05 mm thick, and the other 0.10 mm. The other metal filters were respectively combined with a tungsten filter with a thickness of 0.05 mm. Among the all filters investigated, tube loading of tungsten filter with 0.05 mm thick is minimum. Tungsten with 0.1 mm thick and tungsten with 0.05 mm+ barium show the larger advantages in patient exposure than those of the other filters. It was found that the magnitude of patient exposure varied slightly with the difference of image receptor used. The use of heavy metal filters showed the small advantages in the patient exposure reduction ( approximately 20%) compared with that of conventional copper filter. The use of heavy metal filters showed increasing tube loading ( approximately 5 times) compared with that of conventional copper filter. Noise Wiener spectra of heavy metal filters showed comparable with those of conventional copper filter. In conclusion, the use of heavy metal filter combinations offer no significant advantages over optimal conventional filters.