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 OEMON and OEMOFF 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 (OEMON,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.

The Computed Tomography Dose Index (CTDI) is an indicator for dose management in computed tomography (CT), but has limited use for patient dosimetry. To evaluate the patient dose, the size-speci c dose estimate (SSDE), reported by the American Association of Physics in Medicine task groups 204, 220, and 293, must be calculated by the CTDI vol (z) displayed on the CT console and the conversion factor f(D(z)) from the effective diameter (D Eff ) or water equivalent diameter (D w ). However, no reports have veri ed the appropriateness of using the 320-mm diameter phantom for dose assessment in CT examinations involving the lower limbs. Therefore, we validated a new method for evaluating the SSDE(z) of the lower limbs, using two 160-mm diameter phantoms instead of the 320-mm diameter phantom. The CTDI vol (z) obtained from Monte Carlo (MC) simulation study was reliable because they were almost the same as obtained in a dosimetry study. The conversion factor f (D (z l.l .)) for the lower limbs was evaluated based on the CTDI vol (z) obtained by MC simulation performed using two polymethyl methacrylate cylinder phantoms of 160-mm diameter. The MC simulation was performed by the International Commission on Radiological Protection publication 135 reference adult phantom and was used to evaluate the absorbed dose of the pelvis, lower limbs, knees, and ankles. The dose showing the greatest difference was the lower limbs, which was 8.3 mGy (16%) lower than the absorbed dose. Thus, the SSDE (z l.l .) could be estimated from the CTDI 320 vol (z) displayed on the CT scanner console.

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.

The Computed Tomography Dose Index (CTDI) is an indicator for dose management in computed tomography (CT), but has limited use for patient dosimetry. To evaluate the patient dose, the size-specific dose estimate (SSDE), reported by the American Association of Physics in Medicine task groups 204, 220, and 293, must be calculated by the CTDIvol(z) displayed on the CT console and the conversion factor f(D(z)) from the effective diameter (DEff) or water equivalent diameter (Dw). However, no reports have verified the appropriateness of using the 320-mm diameter phantom for dose assessment in CT examinations involving the lower limbs. Therefore, we validated a new method for evaluating the SSDE(z) of the lower limbs, using two 160-mm diameter phantoms instead of the 320-mm diameter phantom. The CTDIvol(z) obtained from Monte Carlo (MC) simulation study was reliable because they were almost the same as obtained in a dosimetry study. The conversion factor f (D (zl.l.)) for the lower limbs was evaluated based on the CTDIvol(z) obtained by MC simulation performed using two polymethyl methacrylate cylinder phantoms of 160-mm diameter. The MC simulation was performed by the International Commission on Radiological Protection publication 135 reference adult phantom and was used to evaluate the absorbed dose of the pelvis, lower limbs, knees, and ankles. The dose showing the greatest difference was the lower limbs, which was 8.3 mGy (16%) lower than the absorbed dose. Thus, the SSDE (zl.l.) could be estimated from the \({\text{C}\text{T}\text{D}\text{I}}_{\text{v}\text{o}\text{l}}^{320}\left(\text{z}\right)\) displayed on the CT scanner console.