Tomonobu Haba

Abstract Purpose The energy threshold is an important parameter for precise material identification employing photon‐counting techniques. However, in such applications, the appropriate energy threshold has not been clarified. Therefore, we aimed to determine the appropriate energy threshold range for precise material identification, focusing on effective atomic number (Z) values as an index. Methods The atomic number was estimated using a previously proposed algorithm and Monte Carlo simulations. This algorithm included three steps: calculating the attenuation factor from the incident photon counts on a photon‐counting detector, correcting the beam‐hardening effects, and estimating the atomic number from the attenuation factor index using the calibration curve. Monte Carlo simulations were performed to add Poisson noise to an ideal x‐ray spectrum. The total number of incident x‐rays was set in the range of 10³–10⁶. The x‐ray spectra were generated at tube voltages of 50–120 kV. Polymethyl methacrylate (Z = 6.5) and aluminum (Z = 13) were used for the analysis. The energy threshold was varied at intervals of 1 keV to estimate the atomic number. We evaluated the appropriate energy threshold range for accurately estimating the atomic number using the obtained atomic number data and statistical uncertainty under various conditions. Results The appropriate energy threshold range was found to be 31–38 keV for a tube voltage range of 50–120 kV. At this energy threshold, the atomic number can be estimated within an accuracy of ± 0.7 at 10⁵ counts for the atomic number range of 6.5 (PMMA) to 13 (Al). Conclusions We found the appropriate energy threshold range. The findings of this study are expected to be useful for appropriately setting the energy threshold during precise material identification using photon‐counting detectors for clinical applications.

This study aimed to assess fetal radiation exposure in pregnant women undergoing computed tomography (CT) and rotational angiography (RA) examinations for the diagnosis of pelvic trauma. In addition, this study aimed to compare the dose distributions between the two examinations. Surface and average fetal doses were estimated during CT and RA examinations using a pregnant phantom model and real-time dosemeters. The pregnant model phantom was constructed using an anthropomorphic phantom, and a custom-made abdominal phantom was used to simulate pregnancy. The total average fetal dose received by pregnant women from both CT scans (plain, arterial and equilibrium phases) and a single RA examination was ~60 mGy. Because unnecessary repetition of radiographic examinations, such as CT or conventional 2D angiography can increase the radiation risk, the irradiation range should be limited, if necessary, to reduce overall radiation exposure.

Abstract During fetal computed tomography (CT) imaging, because of differences in the pregnancy period and scanning conditions, different doses of radiation are absorbed by the fetus. We propose a correction coefficient for determining the fetal size-specific dose estimate (SSDE) from the CT dose index (CTDI) displayed on the console at tube voltages of 80–135 kVp. The CTDIs corresponding to pregnant women and fetuses were evaluated using a Monte Carlo (MC) simulation, and the ratio of these CTDIs was defined as the Fetus-factor. When the effective diameter of a fetus was approximately 10 cm, the Fetus-factor was 1.0. The estimated pregnant SSDE was multiplied by the Fetus-factor to estimate the fetal SSDE, which was compared with the fetal dose obtained by the MC simulation of the image of the fetal CT examination. The fetal dose could be estimated with an error of 31.5% in fetal examinations conducted using helical CT.

This study aimed to compare the dose and noise level of four tube voltages in abdominal computerized tomography (CT) examinations in different abdominal circumference sizes of pregnant women. Fetal radiation doses were measured with two anthropomorphic pregnant phantoms and real-time dosimeters of photoluminescence sensors using four tube voltages for abdominal CT. The noise level was measured at the abdomen of two anthropomorphic pregnant phantoms. In the large pregnant phantom, the mean fetal doses performed using 120 and 135 kV were statistically significantly lower than the lower tube voltages (P < 0.05). In the small pregnant phantom, the mean fetal dose performed by 100, 120, and 135 kV was significantly lower than the lowest tube voltage tested (P < 0.05). The ratios of the peripheral mean dose to the centric mean dose showed that the ratios of 80 kV were the highest and those for 135 kV were the lowest in both pregnant phantoms. The ratios of the peripheral mean dose to the centric mean dose decreased as the tube voltage increased. Compared with low tube voltages, high tube voltages such as 120 and 135 kV could reduce radiation doses to the fetus without compromising the image uniformity in abdominal CT examinations during pregnancy. On low tube voltage protocols, the dose near the maternal skin surface may be increased in large pregnant women because of reduced penetration of the x rays.

<title>Abstract</title> Organ-effective modulation (OEM) is a computed tomography scanning technique that reduces the exposure dose to organs at risk. Ultrasonography is commonly used for prenatal imaging, but its reliability is reported to be limited. Radiography and computed tomography (CT) are reliable but pose risk of radiation exposure to the pregnant woman and her fetus. Although there are many reports on the exposure dose associated with fetal CT scans, no reports exist on OEM use in fetal CT scans. We measured the basic characteristics of organ-effective modulation (X-ray output modulation angle, maximum X-ray output modulation rate, total X-ray output modulation rate, and noise modulation) and used them in a Monte Carlo simulation to evaluate the effect of this technique on fetal CT scans in terms of image quality and exposure dose to the pregnant woman and fetus. Using ImPACT MC software, Monte Carlo simulations of OEM_ON and OEM_OFF were run on 8 cases involving fetal CT scans. We confirmed that the organ-effective modulation X-ray output modulation angle was 160°; the X-ray output modulation rate increased with increasing tube current; and no modulation occurred at tube currents of 80 mA or below. Our findings suggest that OEM has only a minimal effect in reducing organ exposure in pregnant women; therefore, it should be used on the anterior side (OEM_ON,front) to reduce the exposure dose to the fetus.

Presently, the scanning start angle of the X-ray tube of X-ray computed tomography (CT) scanners cannot be controlled. As a result, there is room for reducing patient dose because the peaks of the dose distributions may overlap during multiphasic CT imaging. This study investigated methods of dose reduction by performing a Monte Carlo simulation of the X-ray tube scanning start angle and locally absorbed dose in multiphasic CT imaging. In the Monte Carlo simulation, the largest decrease in the absorbed dose was seen, when the scanning start angle between the phases was±180°. Even though with present X-ray CT scanners, the scanning start angle cannot be controlled, it is possible to decrease the absorbed dose by taking the orbital synchronized scanning and scanning range into consideration. In future we hope that, we will be able to easily reduce the dose by controlling the scanning start angle.

Size-specific dose estimate (SSDE) was proposed by the American Association of Physicists in Medicine (AAPM) Task Group 204 to consider the effect of patient size in the x-ray CT dose estimation. Size correction factors to calculate SSDE were derived based on the conventional weighted CT dose index (CTDIw) equation. This study aims to investigate the influence of Bakalyar's and the authors' own CTDIw equations on the size correction factors described by the AAPM Task Group 204, using Monte Carlo simulations. The simulations were performed by modeling four types of x-ray CT scanner designs, to compute the dose values in water for cylindrical phantoms with 8-40 cm diameters. CTDI100 method and the AAPM Task Group 111's proposed method were employed as the CT dosimetry models. Size correction factors were obtained for the computed dose values of various phantom diameters for the conventional, Bakalyar's, and the authors' weighting factors. Maximum difference between the size correction factors for the Bakalyar's weighting factor and those of the AAPM Task Group 204 was 27% for a phantom diameter of 11.2 cm. On the other hand, the size correction factors calculated for the authors' weighting factor were in good agreement with those from the AAPM Task Group 204 report with a maximum difference of 17%. The results indicate that the SSDE values obtained with the authors' weighting factor can be evaluated by using the size correction factors reported by the AAPM Task Group 204, which is currently accepted as a standard.

X-ray image evaluation is commonly performed by determining the detective quantum efficiency (DQE). DQE is calculated with a presampled modulation transfer function (MTF), incident photon fluence, and digital noise power spectrum (NPS). Accurate evaluation of MTF, incident photon fluence, and NPS is important for precise DQE determination. In this study, we focused on the accuracy of the incident photon fluence in mammography. The incident photon fluence is calculated using the squared signal-to-noise ratio (SNRin2) value as specified in the International Electrotechnical Commission (IEC) 62220-1-2 report. However, the reported SNRin2 values were determined using a computer program, and the reported values may differ from those calculated from an X-ray spectrum that is measured with actual mammography equipment. Therefore, we evaluated the error range of reported SNRin2 values in mammography to assess the accuracy of the incident photon fluence. First, X-ray spectra from various mammography systems were measured with a CdTe spectrometer. Six mammographic X-ray units were used in this study. Second, the SNRin2 values were calculated from the measured X-ray spectra. The calculated values were compared to the reported values. The results show that the percentage differences between the calculated and reported SNRin2 values were within - 4.1% of each other. The results obtained in this study indicate that the SNRin2 values provided in the IEC report are a robust and convenient tool for calculating the incident photon fluence for DQE evaluation in mammography.

Swallowing computed tomography (SCT) is a relatively new technique for the morphological and kinematic analyses of swallowing. However, no optimal scan protocols are available till date. We conducted the present SCT study to estimate the patient dose at various patient reclining positions. A RANDO phantom with a thermoluminescent dosemeter was placed on a hard Table board in a semi-reclining position at the centre and off-centre. According to predetermined scan protocols, irradiation was performed to acquire scanograms at reclining angles of 55° and 65°. The effective dose was the lowest at the centre 45° (3.8 mSv) reclining angle. Comparison between the off-centre (4.6 mSv at 55°, 6.8 mSv at 65°) and centre (4.5 mSv, 5.8 mSv) values suggested that the off-centre position is undesirable with regard to the patient dose. Accordingly, we believe that SCT methods must be revised on the basis of these factors.

Purpose: The weighted computed tomography dose index (CTDIw) uses measured CTDI values at the center and periphery of a cylindrical phantom. The CTDIw value is calculated using conventional, Bakalyar's, and Choi's weighting factors. However, these weighting factors were produced from only 16- and 32-cm-diameter cylindrical phantoms. This study aims to devise new weighting factors to provide more accurate average dose in the central cross-sectional plane of cylindrical phantoms over a wide range of object diameters, by using Monte Carlo simulations. Methods: Simulations were performed by modeling a Toshiba Aquilion ONE CT scanner, in order to compute the cross-sectional dose profiles of polymethyl methacrylate (PMMA) cylindrical phantoms of each diameter (8-40 cm at 4-cm steps), for various tube voltages and longitudinal beam widths. Two phantom models were simulated, corresponding to the CTDI100 method and the method recommended by American Association of Physicists in Medicine (AAPM) task group 111. The dose-computation PMMA cylinders of 1 mm diameter were located between the phantom surfaces and the centers at intervals of 1 mm, from which cross-sectional dose profiles were calculated. By using linear least-squares fits to the obtained cross-sectional dose profiles data, we determined new weighting factors to estimate more accurate average doses in the PMMA cylindrical phantoms by using the CTDIw equation: CTDIw = W-center . CTDIcenter + W-periphery . CTDIperiphery. In order to demonstrate the validity of the devised new weighting factors, the percentage difference between average dose and CTDIw value was evaluated for the weighting factors (conventional, Bakalyar's, Choi's, and devised new weighting factors) in each calculated cross-sectional dose profile. Results: With the use of linear least-squares techniques, new weighting factors (W-center = 3/8 and W-periphery = 5/8 where W-center and W-periphery are weighting factors for CTDIcenter and CTDIperiphery) were determined. The maximum percentage differences between average dose and CTDIw value were 16, -12, -8, and -6% for the conventional, Bakalyar's, Choi's, and devised new weighting factors, respectively. Conclusions: We devised new weighting factors (W-center = 3/8 and W-periphery = 5/8) to provide more accurate average dose estimation in PMMA cylindrical phantoms over a wide range of diameter. The CTDIw equation with devised new weighting factors could estimate average dose in PMMA cylindrical phantoms with a maximum difference of -6%. The results of this study can estimate the average dose in PMMA cylindrical phantoms more accurately than the conventional weighting factors (W-center = 1/3 and W-periphery = 2/3). (C) 2017 American Association of Physicists in Medicine

Adequate dose management during computed tomography is important. In the present study, the dosimetric application software ImPACT was added to a functional calculator of the size-specific dose estimate and was part of the scan settings for the auto exposure control (AEC) technique. This study aimed to assess the practicality and accuracy of the modified ImPACT software for dose estimation. We compared the conversion factors identified by the software with the values reported by the American Association of Physicists in Medicine Task Group 204, and we noted similar results. Moreover, doses were calculated with the AEC technique and a fixed-tube current of 200 mA for the chest-pelvis region. The modified ImPACT software could estimate each organ dose, which was based on the modulated tube current. The ability to perform beneficial modifications indicates the flexibility of the ImPACT software. The ImPACT software can be further modified for estimation of other doses.

We developed a k-factor-creator software (kFC) that provides the k-factor for CT examination in an arbitrary scan area. It provides the k-factor from the effective dose and dose-length product by Imaging Performance Assessment of CT scanners and CT-EXPO. To assess the reliability, we compared the kFC-evaluated k-factors with those of the International Commission on Radiological Protection (ICRP) publication 102. To confirm the utility, the effective dose determined by coronary computed tomographic angiography (CCTA) was evaluated by a phantom study and k-factor studies. In the CCTA, the effective doses were 5.28 mSv in the phantom study, 2.57 mSv (51%) in the k-factor of ICRP, and 5.26 mSv (1%) in the k-factor of the kFC. Effective doses can be determined from the kFC-evaluated k-factors in suitable scan areas. Therefore, we speculate that the flexible k-factor is useful in clinical practice, because CT examinations are performed in various scan regions.

The American Association of Physicists in Medicine (AAPM) task group 204 has recommended the use of size-dependent conversion factors to calculate size-specific dose estimate (SSDE) values from volume computed tomography dose index (CTDIvol) values. However, these conversion factors do not consider the effects of 320-detector-row volume computed tomography (CT) examinations or the new CT dosimetry metrics proposed by AAPM task group 111. This study aims to investigate the influence of these examinations and metrics on the conversion factors reported by AAPM task group 204, using Monte Carlo simulations. Simulations were performed modelling a Toshiba Aquilion ONE CT scanner, in order to compute dose values in water for cylindrical phantoms with 8-40-cm diameters at 2-cm intervals for each scanning parameter (tube voltage, bow-tie filter, longitudinal beam width). Then, the conversion factors were obtained by applying exponential regression analysis between the dose values for a given phantom diameter and the phantom diameter combined with various scanning parameters. The conversion factors for each scanning method (helical, axial, or volume scanning) and CT dosimetry method (i.e., the CTDI100 method or the AAPM task group 111 method) were in agreement with those reported by AAPM task group 204, within a percentage error of 14.2 % for phantom diameters aeyen11.2 cm. The results obtained in this study indicate that the conversion factors previously presented by AAPM task group 204 can be used to provide appropriate SSDE values for 320-detector-row volume CT examinations and the CT dosimetry metrics proposed by the AAPM task group 111.

Purpose: Patient dose estimation in X-ray computed tomography (CT) is generally performed by Monte Carlo simulation of photon interactions within anthropomorphic or cylindrical phantoms. An accurate Monte Carlo simulation requires an understanding of the effects of the bow-tie filter equipped in a CT scanner, i.e. the change of X-ray energy and air kerma along the fan-beam arc of the CT scanner. To measure the effective energy and air kerma distributions, we devised a pin-photodiode array utilizing eight channels of X-ray sensors arranged at regular intervals along the fan-beam arc of the CT scanner. Methods: Each X-ray sensor consisted of two plate type of pin silicon photodiodes in tandem - front and rear photodiodes - and of a lead collimator, which only allowed X-rays to impinge vertically to the silicon surface of the photodiodes. The effective energy of the X-rays was calculated from the ratio of the output voltages of the photodiodes and the dose was calculated from the output voltage of the front photodiode using the energy and dose calibration curves respectively. Results: The pin-photodiode array allowed the calculation of X-ray effective energies and relative doses, at eight points simultaneously along the fan-beam arc of a CT scanner during a single rotation of the scanner. Conclusions: The fan-beam energy and air kerma distributions of CT scanners can be effectively measured using this pin-photodiode array. (C) 2016 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.

The primary study objective was to assess radiation doses using a modified form of the Imaging Performance Assessment of Computed Tomography (CT) scanner (ImPACT) patient dosimetry for cardiac applications on an Aquilion ONE ViSION Edition scanner, including the Ca score, target computed tomography angiography (CTA), prospective CTA, continuous CTA/cardiac function analysis (CFA), and CTA/CFA modulation. Accordingly, we clarified the CT dose index (CTDI) to determine the relationship between heart rate (HR) and X-ray exposure. As a secondary objective, we compared radiation doses using modified ImPACT, a whole-body dosimetry phantom study, and the k-factor method to verify the validity of the dose results obtained with modified ImPACT. The effective dose determined for the reference person (4.66 mSv at 60 beats per minute (bpm) and 33.43 mSv at 90 bpm) were approximately 10% less than those determined for the phantom study (5.28 mSv and 36.68 mSv). The effective doses according to the k-factor (0.014 mSv center dot mGy(-1)center dot cm(-1); 2.57 mSv and 17.10 mSv) were significantly lower than those obtained with the other two methods. In the present study, we have shown that ImPACT, when modified for cardiac applications, can assess both absorbed and effective doses. The results of our dose comparison indicate that modified ImPACT dose assessment is a promising and practical method for evaluating coronary CTA.

The relationship between heart rate (HR) and computed tomography dose index (CTDI) was evaluated using an electrocardiogram (ECG) gate scan for scan applications such as prospective triggering, Ca scoring, target computed tomography angiography (CTA), prospective CTA and retrospective gating, continuous CTA/CFA (cardiac functional analysis) and CTA/CFA modulation. Even in the case of a volume scan, doses for the multiple scan average dose were similar to those for CTDI. Moreover, it was found that the ECG gate scan yields significantly different doses. When selecting the optimum scan, the doses were dependent on many factors such as HR, scan rotation time, active time, prespecified cardiac phase and modulation rate. Therefore, it is necessary to take these results into consideration when selecting the scanning parameters.

The longitudinal dose profile in a computed tomography dose index (CTDI) phantom had been studied by many researchers. The cross-sectional dose profile in the CTDI phantom, however, has not been studied. It is also important to understand the cross-sectional dose profile in the CTDI phantom for dose estimation in X-ray CT. In this study, the cross-sectional dose profile in the CTDI phantom was calculated by use of a Monte Carlo (MC) simulation method. A helical or a 320-detector-row cone-beam X-ray CT scanner was simulated. The cross-sectional dose profile in the CTDI phantom from surface to surface through the center point was calculated by MC simulation. The shape of the calculation region was a cylinder of 1-mm-diameter. The length of the cylinder was 23, 100, or 300 mm to represent various CT ionization chamber lengths. Detailed analyses of the energy depositions demonstrated that the cross-sectional dose profile was different in measurement methods and phantom sizes. In this study, we also focused on the validation of the weighting factor used in weighted CTDI (CTDIw). As it stands now, the weighting factor used in CTDI w is (1/3, 2/3) for the (central, peripheral) axes. Our results showed that an equal weighting factor, which is (1/2, 1/2) for the (central, peripheral) axes, is more suitable to estimate the average cross-sectional dose when X-ray CT dose estimation is performed. © Japanese Society of Radiological Technology and Japan Society of Medical Physics 2013.

Detective quantum efficiency (DQE) is widely used as a comprehensive metric for X-ray image evaluation in digital X-ray units. The incident photon fluence per air kerma (SNRin2) is necessary for calculating the DQE. The International Electrotechnical Commission (IEC) reports the SNR in2 under conditions of standard radiation quality, but this SNRin2 might not be accurate as calculated from the X-ray spectra emitted by an actual X-ray tube. In this study, we evaluated the error range of the SNRin2 presented by the IEC62220-1 report. We measured the X-ray spectra emitted by an X-ray tube under conditions of standard radiation quality of RQA5. The spectral photon fluence at each energy bin was multiplied by the photon energy and the mass energy absorption coefficient of air then the air kerma spectrum was derived. The air kerma spectrum was integrated over the whole photon energy range to yield the total air kerma. The total photon number was then divided by the total air kerma. This value is the SNRin2. These calculations were performed for various measurement parameters and X-ray units. The percent difference between the calculated value and the standard value of RQA5 was up to 2.9 %. The error range was not negligibly small. Therefore, it is better to use the new SNRin2 of 30694 (1/(mm2 μGy)) than the current SNRin2 of 30174 (1/(mm2 μGy)). © 2013 Japanese Society of Radiological Technology and Japan Society of Medical Physics.