TY - JOUR
AU - Kusumoto,, M.
AB - Abstract The aims of this study were to estimate tube current values for each X-ray projection angle used in adult chest computed tomography (CT) and abdomen–pelvis CT examinations with tube current modulation (TCM) and to validate organ doses determined using Monte Carlo (MC) simulations through comparisons with the doses measured using in-phantom dosimetry. For dose simulations, dose distribution images were obtained by inputting the geometry of a CT scanner, scan parameters including estimated TCM curves and CT images of an adult anthropomorphic phantom into MC simulation software. Organ doses were then determined from the dose distribution images. For dose measurements, organ doses were evaluated using radio-photoluminescence glass dosemeters located at various organ positions within the phantom. Relative differences between the simulated and measured organ doses were −2.5 to 11.0% and −1.5 to 10.5% for organs in chest and abdomen–pelvis CT scan ranges, respectively. Thus, the simulated and measured doses agreed well. Introduction Multi-detector row computed tomography (MDCT) scanners have contributed to substantial increases in the diagnostic applications and frequency of computed tomography (CT) examinations. CT scans deliver relatively high doses compared with conventional radiographic examinations, raising concerns about the possible detriments to patient health. Thus, dose optimisation is necessary in CT examinations to reduce the potential radiation risks. For the past 10 or more years, technologies for reducing patient doses while maintaining image quality have been under development. One of these technologies is tube current modulation (TCM), which is designed to automatically adjust the tube current in the angular direction (in the xy-plane), the longitudinal direction (along the z-axis), or both directions depending on the patient's size and shape as well as the X-ray attenuation characteristics(1–3). TCM in both the angular and longitudinal directions is considered to be the most comprehensive CT dose reduction approach and is the most commonly used method in TCM algorithms(4). Therefore, evaluation of the radiation doses received by individual radiosensitive organs during CT examinations using TCM is important for potential radiation risk assessment. One organ dose evaluation method is based on Monte Carlo (MC) simulations. To perform MC simulations, it is necessary to model several components of the MDCT scanners such as their geometries, X-ray spectra and bow-tie filter shapes. To conduct dose simulations for CT examinations using TCM, the tube current values for each X-ray projection angle are required. However, it is exceedingly difficult to access the raw projection data and to extract the tube current values. Thus, the aims of this study were to estimate the tube current values for each X-ray projection angle in CT examinations using TCM and to validate organ doses determined through MC simulations by comparing them with the doses measured using in-phantom dosimetry. Materials and Methods MC simulations All of the simulations and measurements were performed with an Aquilion 64 (Toshiba Medical Systems, Japan). Organ doses were determined based on dose distributions obtained using the ImpactMC (CT Imaging GmbH, Germany) simulation software(5, 6). This software has a graphic user interface, and the software used in this study has a graphics processor unit (GPU)-based MC algorithm. All of the MC simulations were performed using a 3.4-GHz Intel Core i7 4770 personal computer equipped with one GeForce GTX TITAN card (NVIDIA Corporation, USA). Performing dose simulations with ImpactMC requires the modelling geometry, X-ray spectrum and bow-tie filter shape of the CT scanner. The geometric characteristics such as the fan angle and distance between the focal spot and the isocentre of the scanner were provided by the manufacturer. The X-ray spectrum was estimated from a semi-empirical model developed by Tucker et al. (7) and the measurement of first half value layer of aluminium. The bow-tie filter's material was hypothesised to be aluminium, and its shape was estimated from measurements of photon attenuation by aluminium plates and the dose profile in the fan angle direction. These measurements were performed with a 10-cm long pencil ionisation chamber (model 10 × 5-3CT; Radcal Corporation, USA) connected to an electrometer (model 9015; Radcal Corporation, USA), which was fixed at the isocentre of the scanner. Aluminium plates with different thicknesses were placed between the chamber and the stationary X-ray source. The dose profiles in the fan angle direction were measured by keeping the X-ray tube fixed and moving the chamber from the isocentre of the scanner to the edge of the fan beam, as described by Turner et al. (8). In our previous study,(9) this scanner model was validated by simulating the volumetric CT dose indices (CTDIvol) for both 32 and 16 cm CTDI phantoms with ImpactMC and comparing the simulated CTDIvol values with the corresponding measured values, which revealed that the simulation results agreed with the measurements to within 5.2%. To perform the dose simulations with ImpactMC in this study, it was necessary to establish a voxelized model of an adult anthropomorphic phantom, which was generated based on CT images. The adult phantom weighed 54 kg and was 164 cm tall (Kyoto Kagaku, Japan)(10, 11). It was composed of tissue equivalent substitutes corresponding to soft tissue, lung and bone, and was divided into 25-mm thick axial slices. Each CT value was converted into a density based on the linear relationship determined using a cone beam CT electron density phantom (Model 062A; CIRS Inc., USA). Each CT value for the adult anthropomorphic phantom was also assigned to be air, lung, soft tissue or bone according to a user-defined segmentation of the Hounsfield units scale. The simulated dose results were obtained in the form of dose distribution images in which the values in each voxel were provided as absorbed doses for air. Volume of interests (VOIs) with sizes of 10 × 10 × 1 voxels, which dimensions were 9.4 × 9.4 × 5 mm3, were set at the corresponding dose measurement positions on the dose distribution images of the adult anthropomorphic phantom, and the mean simulated doses within the VOIs were calculated. Organ doses for each tissue and organ were evaluated by multiplying each simulated dose times the ratio of the mass energy absorption coefficient of soft tissue to that of air. Angular and longitudinal TCM curve estimation Aquilion 64 is equipped with a TCM system, ‘Volume-EC’, which allows the tube current to be adjusted in both the angular and longitudinal directions. This system is based on patient attenuation data from localizer radiographs, and the appropriate tube current is applied to achieve the user-selected target noise level (standard deviation, SD)(12). The tube current can then be modulated to maintain the same SD throughout the examination. Dose simulations for CT examinations using TCM require tube current values for each X-ray projection angle. However, it is difficult to obtain actual TCM data described in raw projection data. Thus, an experiment was performed using an elliptical cylinder polymethylmethacrylate (PMMA) phantom with the long axis of 24 cm, the short axis of 16 cm and the height of 15 cm to derive the estimated formula for the tube current values in TCM scans. The phantom was placed at the isocentre of the gantry, and a localizer scan was then conducted with a tube voltage of 120 kV and a tube current of 50 mA. The tube current values in the TCM scans performed using this phantom were determined based on attenuation data from the localizer radiograph. The tube current values Amin for the anterior–posterior (AP) projections, the tube current values Amax for lateral (LAT) projections and the corresponding table positions for the TCM CT scans were obtained using the CT scanner log data. After the localizer scan, the phantom was removed from the gantry and a CT dose profiler ‘Piranha’ (RTI Electronics AB, Sweden) was set at the isocentre of the gantry. The output signal was measured by scanning the dose profiler according to the TCM scan protocol. Based on this dose profile measurements, the estimated formula for the tube current values can be expressed as follows: I=(Amax−Amin2)×sin(2θ−π/2)+(Amax+Amin2).(1) In this formula, the tube current values I are represented as a sinusoidal function of the tube angle θ. Amin and Amax in TCM scans performed using the elliptical cylinder PMMA phantom exhibited no variations with table positions. As shown in Figure 1, the tube current values derived from the measured dose profiles for target SDs of 10, 15 and 20 agreed well with those estimated using Equation (1). Figure 1. Open in new tabDownload slide Tube current values derived from dose profile measurements for target SDs of 10, 15 and 20. Dashed lines show tube current values derived from the estimated formula given in Equation (1). Figure 1. Open in new tabDownload slide Tube current values derived from dose profile measurements for target SDs of 10, 15 and 20. Dashed lines show tube current values derived from the estimated formula given in Equation (1). Next, the adult anthropomorphic phantom was set on the CT table, and a localizer scan was performed in the chest and abdomen–pelvis regions. Amin, Amax and the corresponding table positions for the chest and abdomen–pelvis CT scans were obtained from the CT scanner log data. Figure 2a and b show localizer radiographs of the chest and abdomen–pelvis regions, respectively, of the adult anthropomorphic phantom. Amin and Amax for the phantom varied with table positions and could be represented as piecewise linear functions of the table position z. θ can be expressed in terms of z as follows: θ=2π(z−z0L),(2) where z0 is the table position at the beginning of the scan and L is the table increment per gantry rotation. Thus, the tube current values for all projection angles can be expressed as a function of the table position by combining Equations (1) and (2). Figure 3a and b show the TCM curves as functions of table position for the chest and abdomen–pelvis CT scans, respectively. Figure 2. Open in new tabDownload slide Localizer radiographs for (a) chest and (b) abdomen–pelvis regions of adult anthropomorphic phantom. Figure 2. Open in new tabDownload slide Localizer radiographs for (a) chest and (b) abdomen–pelvis regions of adult anthropomorphic phantom. Figure 3. Open in new tabDownload slide Estimated TCM curve and longitudinally approximated TCM curve versus table position for (a) chest and (b) abdomen–pelvis CT scans. Figure 3. Open in new tabDownload slide Estimated TCM curve and longitudinally approximated TCM curve versus table position for (a) chest and (b) abdomen–pelvis CT scans. Longitudinal TCM curve approximation The longitudinal TCM curves in this study represent an approximation of both the angular and longitudinal TCM curves and show the average tube current values per rotation at each table position in angular and longitudinal TCM CT examinations. The average tube current values were extracted from the digital imaging and communications in medicine headers of the CT images. The longitudinal TCM curves do not include tube current values in the over-ranging region because there are no image data for that region. Figure 3a and b show the longitudinally approximated TCM curves obtained from the CT images. Organ dose simulations and measurements Organ dose simulations were performed using the scan parameters for the chest and abdomen–pelvis CT examinations. The scan range used for the chest CT contained the entire pulmonary area and that used for the abdominal–pelvic CT extended from the diaphragm to the pubic symphysis. In each simulation, 1.0 × 1011 photons were produced. The statistical uncertainty of our MC results was <1% for organs within the CT scan ranges. The simulated organ doses were then compared to the measured doses using the relative differences (RDs) derived from the following equation: RD(%)=Simulated doses−Measured dosesMeasured doses×100.(3) Organ dose measurements were performed using an adult anthropomorphic phantom and a radio-photoluminescence glass dosemeter (RGD) system. The RGD system, Dose Ace (Asahi Techno Glass Co., Japan), consisted of silver-activated phosphate glass dosemeters and an automatic readout system (FGD-1000). It included two types of dosemeters: GD-302M, which does not have energy compensation filters, and GD-352M, which includes a tin filter to adjust the photon energy dependence. GD-352M dosemeters were mainly used for the CT dose measurements in the adult phantom because the response of GD-352M dosemeters is almost flat from 45 to 60 keV (the energies commonly used in CT) and was estimated to be 20%(13). GD-302M dosemeters were used for the dose measurements at positions that were too large to insert GD-352M dosemeters into the phantoms. In the chest and abdomen–pelvis CT set-ups, 225 and 237 RGDs, respectively, were inserted at the locations of the investigated tissues and organs shown in Table 1; the tissue weighting factors were taken from International Commission on Radiological Protection (ICRP) Publication 103(14). The dose measurements were performed using the same scan parameters that were used in the simulations. Table 1. Number of RGDs at the positions of various organs within an adult anthropomorphic phantom. Tissue or organ . Number of dosemeters . Brain 8 Lens 4 Salivary glands 7 Thyroid 3 Lung 14 Oesophagus 4 Breast 8 Liver 5 Stomach 4 Colon 10 Ovaries 6 Bladder 2 Testes 3 Red bone marrow and bone surfacea Cranium 8 Mandible 4 Clavicles 2 Scapulae 4 Sternum 2 Ribs 10 Cervical vertebrae 2 Thoracic vertebrae 5 Lumbar vertebrae 3 Sacrum 2 Os coxae 8 Femora 2 Humeri 2 Skin for chest CT 56 Skin for abdomen–pelvis CT 68 Remainder Oral mucosa 3 Extrathoracic region 4 Thymus 2 Heart 8 Spleen 2 Pancreas 2 Gallbladderb 1 Adrenals 4 Kidneys 4 Small intestine 6 Uterus/cervix, prostate 2 Tissue or organ . Number of dosemeters . Brain 8 Lens 4 Salivary glands 7 Thyroid 3 Lung 14 Oesophagus 4 Breast 8 Liver 5 Stomach 4 Colon 10 Ovaries 6 Bladder 2 Testes 3 Red bone marrow and bone surfacea Cranium 8 Mandible 4 Clavicles 2 Scapulae 4 Sternum 2 Ribs 10 Cervical vertebrae 2 Thoracic vertebrae 5 Lumbar vertebrae 3 Sacrum 2 Os coxae 8 Femora 2 Humeri 2 Skin for chest CT 56 Skin for abdomen–pelvis CT 68 Remainder Oral mucosa 3 Extrathoracic region 4 Thymus 2 Heart 8 Spleen 2 Pancreas 2 Gallbladderb 1 Adrenals 4 Kidneys 4 Small intestine 6 Uterus/cervix, prostate 2 aDoses for red bone marrow and bone surface were measured using the same dosimeters set in various bone tissues. bGallbladder doses were measured using one of the dosimeters used for dose measurement for liver. Table 1. Number of RGDs at the positions of various organs within an adult anthropomorphic phantom. Tissue or organ . Number of dosemeters . Brain 8 Lens 4 Salivary glands 7 Thyroid 3 Lung 14 Oesophagus 4 Breast 8 Liver 5 Stomach 4 Colon 10 Ovaries 6 Bladder 2 Testes 3 Red bone marrow and bone surfacea Cranium 8 Mandible 4 Clavicles 2 Scapulae 4 Sternum 2 Ribs 10 Cervical vertebrae 2 Thoracic vertebrae 5 Lumbar vertebrae 3 Sacrum 2 Os coxae 8 Femora 2 Humeri 2 Skin for chest CT 56 Skin for abdomen–pelvis CT 68 Remainder Oral mucosa 3 Extrathoracic region 4 Thymus 2 Heart 8 Spleen 2 Pancreas 2 Gallbladderb 1 Adrenals 4 Kidneys 4 Small intestine 6 Uterus/cervix, prostate 2 Tissue or organ . Number of dosemeters . Brain 8 Lens 4 Salivary glands 7 Thyroid 3 Lung 14 Oesophagus 4 Breast 8 Liver 5 Stomach 4 Colon 10 Ovaries 6 Bladder 2 Testes 3 Red bone marrow and bone surfacea Cranium 8 Mandible 4 Clavicles 2 Scapulae 4 Sternum 2 Ribs 10 Cervical vertebrae 2 Thoracic vertebrae 5 Lumbar vertebrae 3 Sacrum 2 Os coxae 8 Femora 2 Humeri 2 Skin for chest CT 56 Skin for abdomen–pelvis CT 68 Remainder Oral mucosa 3 Extrathoracic region 4 Thymus 2 Heart 8 Spleen 2 Pancreas 2 Gallbladderb 1 Adrenals 4 Kidneys 4 Small intestine 6 Uterus/cervix, prostate 2 aDoses for red bone marrow and bone surface were measured using the same dosimeters set in various bone tissues. bGallbladder doses were measured using one of the dosimeters used for dose measurement for liver. The RGDs were calibrated with a traceable ionisation chamber to the Japanese national standard at the National Institute of Advanced Industrial Science and Technology. The RGDs and ionisation chamber were set on a soft-tissue-equivalent slab phantom, which was then irradiated with X-ray beams of various energies. The RGD readouts were converted into absorbed doses for soft tissue by using the conversion factors obtained from the calibration. The absorbed doses for soft tissue were used to estimate the absorbed doses for all of the tissues and organs (except the breasts) because their mass energy absorption coefficients were within 5% of that for soft tissue at a diagnostic X-ray energy of >30 keV(15, 16). The errors included in the conversion from RGD readouts to organ doses for each tissue and organ were estimated to be ~10%. The absorbed doses for the breasts were evaluated by multiplying the doses measured at the appropriate positions times the ratio of the corresponding mass energy absorption coefficients. The doses for red bone marrow and bone surfaces were evaluated from the doses measured in various bone tissues and the weight fractions of red bone marrow and mineralised bone for adults that are given in ICRP Publication 70(17), based on equations described in our previous paper(18). The skin doses were evaluated from the average doses measured on the front, back, right and left sides within the scan region of the phantom and the ratio of the irradiated area to the gross surface area of the adult phantom. The doses for the ‘remainder’ organs, except for those of the muscles and lymph nodes, were evaluated by averaging the doses for 11 organs selected from the ‘remainder’ assigned in ICRP Publication 103. The effective doses were evaluated according to ICRP Publication 103. Results Figure 4a and b depict the simulated dose distributions for adult chest and abdomen–pelvis CT scans, respectively, and Table 2 lists the simulated and measured organ and effective doses for both types of scans. The RDs between the simulated and measured organ doses for organs within the chest and abdomen–pelvis scan ranges are −2.5 to 11.0% and −1.5 to 10.5%, respectively. In each case, the RDs for the organs and tissues located near the boundaries of the scan region are higher than those for the organs located within the scan region. Figure 4. Open in new tabDownload slide CT images and simulated dose distribution images of axial, coronal and sagittal planes of adult anthropomorphic phantom for (a) chest and (b) abdomen–pelvis CT scans. Figure 4. Open in new tabDownload slide CT images and simulated dose distribution images of axial, coronal and sagittal planes of adult anthropomorphic phantom for (a) chest and (b) abdomen–pelvis CT scans. Table 2. Scan parameters for adult chest and abdomen–pelvis CT scans using TCM and RDs between simulated and measured organ and effective doses. Scan protocol Chest Abdomen–pelvis Scan mode Helical Helical Tube voltage (kV) 120 120 Target SD 15 15 Average tube current (mA) 474 527 Rotation time (s) 0.5 0.5 Beam collimation (mm) 32 32 Pitch factor 0.828 0.828 Scan length (mm) 350 420 CTDIvol (mGy) 34.1 36.5 DLP (mGy cm) 1391.5 1759.4 Scan protocol Chest Abdomen–pelvis Scan mode Helical Helical Tube voltage (kV) 120 120 Target SD 15 15 Average tube current (mA) 474 527 Rotation time (s) 0.5 0.5 Beam collimation (mm) 32 32 Pitch factor 0.828 0.828 Scan length (mm) 350 420 CTDIvol (mGy) 34.1 36.5 DLP (mGy cm) 1391.5 1759.4 Organ or tissue . Simulation (mGy) . Measurement (mGy) . RD (%) . Simulation (mGy) . Measurement (mGy) . RD (%) . Brain 0.9 0.9 −3.3 0.0 0.1 −71.0 Lens 0.7 0.7 −0.9 0.0 0.1 −50.3 Salivary glands 4.8 4.6 5.3 0.1 0.2 −26.7 Thyroid 77.3 69.6 11.0 0.6 0.6 −1.6 Lung 55.4 55.4 0.0 16.5 14.8 11.6 Oesophagus 54.0 53.1 1.6 10.2 9.2 10.9 Breast 36.0 36.9 −2.5 10.8 10.2 5.4 Liver 43.9 43.6 0.9 51.3 49.5 3.7 Stomach 44.6 45.3 −1.5 54.8 53.6 2.2 Colon 10.1 8.5 19.9 55.4 55.0 0.7 Ovaries 0.2 0.2 −6.4 46.1 46.5 −1.0 Bladder 0.2 0.2 6.0 58.0 58.9 −1.5 Testes 0.0 0.1 −58.0 70.5 63.8 10.5 Red bone marrow 15.3 16.0 −4.1 21.9 23.7 −7.5 Bone surface 32.7 34.4 −5.1 50.6 56.5 −10.3 Skin 11.7 10.7 9.2 14.9 13.9 6.8 Remainder 31.2 29.8 4.9 35.8 35.2 1.8 Effective dose (mSv) 30.7 30.2 1.6 33.6 32.7 2.6 Organ or tissue . Simulation (mGy) . Measurement (mGy) . RD (%) . Simulation (mGy) . Measurement (mGy) . RD (%) . Brain 0.9 0.9 −3.3 0.0 0.1 −71.0 Lens 0.7 0.7 −0.9 0.0 0.1 −50.3 Salivary glands 4.8 4.6 5.3 0.1 0.2 −26.7 Thyroid 77.3 69.6 11.0 0.6 0.6 −1.6 Lung 55.4 55.4 0.0 16.5 14.8 11.6 Oesophagus 54.0 53.1 1.6 10.2 9.2 10.9 Breast 36.0 36.9 −2.5 10.8 10.2 5.4 Liver 43.9 43.6 0.9 51.3 49.5 3.7 Stomach 44.6 45.3 −1.5 54.8 53.6 2.2 Colon 10.1 8.5 19.9 55.4 55.0 0.7 Ovaries 0.2 0.2 −6.4 46.1 46.5 −1.0 Bladder 0.2 0.2 6.0 58.0 58.9 −1.5 Testes 0.0 0.1 −58.0 70.5 63.8 10.5 Red bone marrow 15.3 16.0 −4.1 21.9 23.7 −7.5 Bone surface 32.7 34.4 −5.1 50.6 56.5 −10.3 Skin 11.7 10.7 9.2 14.9 13.9 6.8 Remainder 31.2 29.8 4.9 35.8 35.2 1.8 Effective dose (mSv) 30.7 30.2 1.6 33.6 32.7 2.6 Table 2. Scan parameters for adult chest and abdomen–pelvis CT scans using TCM and RDs between simulated and measured organ and effective doses. Scan protocol Chest Abdomen–pelvis Scan mode Helical Helical Tube voltage (kV) 120 120 Target SD 15 15 Average tube current (mA) 474 527 Rotation time (s) 0.5 0.5 Beam collimation (mm) 32 32 Pitch factor 0.828 0.828 Scan length (mm) 350 420 CTDIvol (mGy) 34.1 36.5 DLP (mGy cm) 1391.5 1759.4 Scan protocol Chest Abdomen–pelvis Scan mode Helical Helical Tube voltage (kV) 120 120 Target SD 15 15 Average tube current (mA) 474 527 Rotation time (s) 0.5 0.5 Beam collimation (mm) 32 32 Pitch factor 0.828 0.828 Scan length (mm) 350 420 CTDIvol (mGy) 34.1 36.5 DLP (mGy cm) 1391.5 1759.4 Organ or tissue . Simulation (mGy) . Measurement (mGy) . RD (%) . Simulation (mGy) . Measurement (mGy) . RD (%) . Brain 0.9 0.9 −3.3 0.0 0.1 −71.0 Lens 0.7 0.7 −0.9 0.0 0.1 −50.3 Salivary glands 4.8 4.6 5.3 0.1 0.2 −26.7 Thyroid 77.3 69.6 11.0 0.6 0.6 −1.6 Lung 55.4 55.4 0.0 16.5 14.8 11.6 Oesophagus 54.0 53.1 1.6 10.2 9.2 10.9 Breast 36.0 36.9 −2.5 10.8 10.2 5.4 Liver 43.9 43.6 0.9 51.3 49.5 3.7 Stomach 44.6 45.3 −1.5 54.8 53.6 2.2 Colon 10.1 8.5 19.9 55.4 55.0 0.7 Ovaries 0.2 0.2 −6.4 46.1 46.5 −1.0 Bladder 0.2 0.2 6.0 58.0 58.9 −1.5 Testes 0.0 0.1 −58.0 70.5 63.8 10.5 Red bone marrow 15.3 16.0 −4.1 21.9 23.7 −7.5 Bone surface 32.7 34.4 −5.1 50.6 56.5 −10.3 Skin 11.7 10.7 9.2 14.9 13.9 6.8 Remainder 31.2 29.8 4.9 35.8 35.2 1.8 Effective dose (mSv) 30.7 30.2 1.6 33.6 32.7 2.6 Organ or tissue . Simulation (mGy) . Measurement (mGy) . RD (%) . Simulation (mGy) . Measurement (mGy) . RD (%) . Brain 0.9 0.9 −3.3 0.0 0.1 −71.0 Lens 0.7 0.7 −0.9 0.0 0.1 −50.3 Salivary glands 4.8 4.6 5.3 0.1 0.2 −26.7 Thyroid 77.3 69.6 11.0 0.6 0.6 −1.6 Lung 55.4 55.4 0.0 16.5 14.8 11.6 Oesophagus 54.0 53.1 1.6 10.2 9.2 10.9 Breast 36.0 36.9 −2.5 10.8 10.2 5.4 Liver 43.9 43.6 0.9 51.3 49.5 3.7 Stomach 44.6 45.3 −1.5 54.8 53.6 2.2 Colon 10.1 8.5 19.9 55.4 55.0 0.7 Ovaries 0.2 0.2 −6.4 46.1 46.5 −1.0 Bladder 0.2 0.2 6.0 58.0 58.9 −1.5 Testes 0.0 0.1 −58.0 70.5 63.8 10.5 Red bone marrow 15.3 16.0 −4.1 21.9 23.7 −7.5 Bone surface 32.7 34.4 −5.1 50.6 56.5 −10.3 Skin 11.7 10.7 9.2 14.9 13.9 6.8 Remainder 31.2 29.8 4.9 35.8 35.2 1.8 Effective dose (mSv) 30.7 30.2 1.6 33.6 32.7 2.6 Table 3 shows the RDs between the organ doses determined based on the MC simulation using longitudinally approximated TCM curves and the measured organ doses listed in Table 2. The RDs between the simulated and measured doses for organs within the scan ranges are −13.5 to 18.1% and −2.4 to 6.3% for the chest and abdomen–pelvis CT scans, respectively. Table 3. RDs between simulated and measured organ and effective doses in chest and abdomen–pelvis CT scans. Simulated doses were determined based on MC simulations using longitudinally approximated TCM curves. Scan protocol . Chest . Abdomen–pelvis . Organ or tissue . Simulation (mGy) . Measurement (mGy) . RD (%) . Simulation (mGy) . Measurement (mGy) . RD (%) . Brain 1.0 0.9 9.0 0.0 0.1 −59.5 Lens 0.8 0.7 5.0 0.0 0.1 −58.9 Salivary glands 4.9 4.6 6.8 0.1 0.2 −32.8 Thyroid 82.2 69.6 18.1 0.6 0.6 −1.0 Lung 57.4 55.4 3.5 15.9 14.8 7.2 Oesophagus 55.6 53.1 4.8 10.0 9.2 8.5 Breast 38.6 36.9 4.5 10.9 10.2 6.3 Liver 42.3 43.6 −3.0 52.6 49.5 6.3 Stomach 39.1 45.3 −13.5 56.9 53.6 6.1 Colon 4.6 8.5 −45.9 55.6 55.0 1.0 Ovaries 0.2 0.2 −34.5 45.4 46.5 −2.4 Bladder 0.1 0.2 −37.4 58.2 58.9 −1.3 Testes 0.0 0.1 −67.1 67.1 63.8 5.1 Red bone marrow 15.5 16.0 −3.0 21.8 23.7 −8.3 Bone surface 33.2 34.4 −3.6 49.1 56.5 −13.0 Skin 12.2 10.7 13.4 14.9 13.9 6.7 Remainder 29.2 29.8 −1.8 36.6 35.2 4.0 Effective dose (mSv) 29.9 30.2 −1.1 33.7 32.7 3.1 Scan protocol . Chest . Abdomen–pelvis . Organ or tissue . Simulation (mGy) . Measurement (mGy) . RD (%) . Simulation (mGy) . Measurement (mGy) . RD (%) . Brain 1.0 0.9 9.0 0.0 0.1 −59.5 Lens 0.8 0.7 5.0 0.0 0.1 −58.9 Salivary glands 4.9 4.6 6.8 0.1 0.2 −32.8 Thyroid 82.2 69.6 18.1 0.6 0.6 −1.0 Lung 57.4 55.4 3.5 15.9 14.8 7.2 Oesophagus 55.6 53.1 4.8 10.0 9.2 8.5 Breast 38.6 36.9 4.5 10.9 10.2 6.3 Liver 42.3 43.6 −3.0 52.6 49.5 6.3 Stomach 39.1 45.3 −13.5 56.9 53.6 6.1 Colon 4.6 8.5 −45.9 55.6 55.0 1.0 Ovaries 0.2 0.2 −34.5 45.4 46.5 −2.4 Bladder 0.1 0.2 −37.4 58.2 58.9 −1.3 Testes 0.0 0.1 −67.1 67.1 63.8 5.1 Red bone marrow 15.5 16.0 −3.0 21.8 23.7 −8.3 Bone surface 33.2 34.4 −3.6 49.1 56.5 −13.0 Skin 12.2 10.7 13.4 14.9 13.9 6.7 Remainder 29.2 29.8 −1.8 36.6 35.2 4.0 Effective dose (mSv) 29.9 30.2 −1.1 33.7 32.7 3.1 Table 3. RDs between simulated and measured organ and effective doses in chest and abdomen–pelvis CT scans. Simulated doses were determined based on MC simulations using longitudinally approximated TCM curves. Scan protocol . Chest . Abdomen–pelvis . Organ or tissue . Simulation (mGy) . Measurement (mGy) . RD (%) . Simulation (mGy) . Measurement (mGy) . RD (%) . Brain 1.0 0.9 9.0 0.0 0.1 −59.5 Lens 0.8 0.7 5.0 0.0 0.1 −58.9 Salivary glands 4.9 4.6 6.8 0.1 0.2 −32.8 Thyroid 82.2 69.6 18.1 0.6 0.6 −1.0 Lung 57.4 55.4 3.5 15.9 14.8 7.2 Oesophagus 55.6 53.1 4.8 10.0 9.2 8.5 Breast 38.6 36.9 4.5 10.9 10.2 6.3 Liver 42.3 43.6 −3.0 52.6 49.5 6.3 Stomach 39.1 45.3 −13.5 56.9 53.6 6.1 Colon 4.6 8.5 −45.9 55.6 55.0 1.0 Ovaries 0.2 0.2 −34.5 45.4 46.5 −2.4 Bladder 0.1 0.2 −37.4 58.2 58.9 −1.3 Testes 0.0 0.1 −67.1 67.1 63.8 5.1 Red bone marrow 15.5 16.0 −3.0 21.8 23.7 −8.3 Bone surface 33.2 34.4 −3.6 49.1 56.5 −13.0 Skin 12.2 10.7 13.4 14.9 13.9 6.7 Remainder 29.2 29.8 −1.8 36.6 35.2 4.0 Effective dose (mSv) 29.9 30.2 −1.1 33.7 32.7 3.1 Scan protocol . Chest . Abdomen–pelvis . Organ or tissue . Simulation (mGy) . Measurement (mGy) . RD (%) . Simulation (mGy) . Measurement (mGy) . RD (%) . Brain 1.0 0.9 9.0 0.0 0.1 −59.5 Lens 0.8 0.7 5.0 0.0 0.1 −58.9 Salivary glands 4.9 4.6 6.8 0.1 0.2 −32.8 Thyroid 82.2 69.6 18.1 0.6 0.6 −1.0 Lung 57.4 55.4 3.5 15.9 14.8 7.2 Oesophagus 55.6 53.1 4.8 10.0 9.2 8.5 Breast 38.6 36.9 4.5 10.9 10.2 6.3 Liver 42.3 43.6 −3.0 52.6 49.5 6.3 Stomach 39.1 45.3 −13.5 56.9 53.6 6.1 Colon 4.6 8.5 −45.9 55.6 55.0 1.0 Ovaries 0.2 0.2 −34.5 45.4 46.5 −2.4 Bladder 0.1 0.2 −37.4 58.2 58.9 −1.3 Testes 0.0 0.1 −67.1 67.1 63.8 5.1 Red bone marrow 15.5 16.0 −3.0 21.8 23.7 −8.3 Bone surface 33.2 34.4 −3.6 49.1 56.5 −13.0 Skin 12.2 10.7 13.4 14.9 13.9 6.7 Remainder 29.2 29.8 −1.8 36.6 35.2 4.0 Effective dose (mSv) 29.9 30.2 −1.1 33.7 32.7 3.1 Discussion The RDs between the simulated and measured organ doses for organs within the scan ranges for adult chest and abdomen–pelvis CT scans using TCM were compared with the corresponding values for each CT scan using a constant tube current. In our previous study(9), we reported RDs of −1.6 to 3.6% and −0.8 to 12.6% for chest and abdomen–pelvis CT scans, respectively. Deak et al. (5) reported that their simulated and measured doses for TCM scans using a liver phantom agreed to within 10%. Thus, the simulated doses for CT examinations using TCM were considered to agree well with the measured doses because the RDs for TCM examinations were comparable to those for CT examinations using a constant tube current. Furthermore, the RDs between the doses simulated using the longitudinally approximated TCM curves and the measured doses were similar to or slightly higher than those between the doses simulated using both angular and longitudinal estimated TCM curves and the measured doses in this study. This result demonstrates that the doses simulated using longitudinally approximated TCM curves also agree well with those measured for CT scans performed using both angular and longitudinal TCM curves. Khatonabadi et al. (19) validated a MC model for TCM scans by using an anthropomorphic phantom and reported that the doses simulated using longitudinally approximated TCM curves were only slightly different from those simulated using both angular and longitudinal TCM curves. Thus, MC simulations performed using longitudinally approximated TCM curves will be useful for evaluating organ doses in CT examinations performed using various CT scanners because it is easier to extract tube current values from CT images. However, for organs and tissues located near the boundaries of the scan regions, the RDs between the doses simulated using the longitudinally approximated TCM curves and the measured doses were higher than those between the doses simulated using both angular and longitudinal estimated TCM curves and the measured doses in this study because the longitudinally approximated TCM curves did not include the tube current values in the over-ranging region. Therefore, both angular and longitudinal estimated TCM curves will be required to obtain more accurate organ dose estimates. This study was somewhat limited because our investigations were performed using a CT scanner from only one vendor. Our method of estimating tube current values using angular and longitudinal TCM could be manufacturer dependent. Thus, this method could be utilised for TCM techniques from some vendors (e.g. ‘Volume-EC’ from Toshiba and ‘Smart mA’ from GE) but not from other vendors (e.g. ‘Care Dose4D’ from Siemens) in which the tube current is adjusted in real time in response to variations in X-ray intensity at the detector. Conclusions In this study, we estimated tube current values for each X-ray projection angle used in adult chest and abdomen–pelvis CT examinations using TCM and validated the organ doses determined using MC simulations by comparing them with the doses measured using in-phantom dosimetry. The simulated doses agreed well with the measured doses and this result shows that the tube current values in TCM scans derived using the estimated formula will agree well with the actual tube current data. The MC simulation results performed using the longitudinally approximated TCM curves yielded organ doses similar to those determined using both the angular and longitudinal estimated TCM curves. Thus, MC simulations performed using longitudinally approximated TCM curves will be useful for evaluating organ doses in CT examinations performed using various CT scanners. However, longitudinally approximated TCM curves do not include the tube current values in the over-ranging region. Therefore, both angular and longitudinal estimated TCM curves will be required to obtain more accurate organ dose estimates. Acknowledgements This study was conducted based on a Collaborative Research Agreement between the National Cancer Center and Toshiba Medical Systems. 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TI - Organ Dose Evaluations Based on Monte Carlo Simulation for CT Examinations Using Tube Current Modulation
JO - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncw144
DA - 2017-04-28
UR - https://www.deepdyve.com/lp/oxford-university-press/organ-dose-evaluations-based-on-monte-carlo-simulation-for-ct-DDXuVzEoaA
SP - 387
VL - 174
IS - 3
DP - DeepDyve
ER -