TY - JOUR
AU - Kato, Ryoichi
AB - Abstract 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. INTRODUCTION Swallowing computed tomography (SCT) is a relatively new technique for the morphological and kinematic analyses of swallowing(1–4). It involves the use of wide-beam computed tomography (CT) to cover all relevant organs. As a result, the entire oral cavity, pharynx, larynx and upper oesophagus, all of which need to be included for swallowing analysis, are included in the scan area(2). Through dynamic scanning of swallowing events over time, SCT allows the user to visualise four-dimensional dynamic images of swallowing by providing multi-phase multiplanar reconstruction images and 3D images. Currently, videofluoroscopy (VF) and videoendoscopy are the most widely used techniques for swallowing analysis; however, these techniques only provide 2D images for the assessment of swallowing kinematics. Furthermore, studies on VF have shown low intra- and inter-rater reliability, particularly for the evaluation of functional components(5–7). With regard to the accuracy of SCT for kinematic analysis, Fuji et al.(1,) have been conducting trials on this technique since 2010, and the results have provided novel and important information on swallowing events(1–4). With regard to the patient dose in SCT, Kobayashi et al.(8) clarified that the effective dose is 3.9 mSv, which is 1.0 mSv higher than that required for conventional neck CT. However, no optimal SCT scan protocols are available till date. In recent years, advances in reconstruction algorithms, including iterative reconstruction (IR) algorithms, have been useful in decreasing the patient dose in clinical practice. The tube current has been decreased to 40 from 60 mA (33% decrease), while the volumetric temporal resolution has been improved by a faster tube rotational speed of 0.275 s/rot(9). Consequently, advances in CT devices have resulted in a decreased patient dose; however, little has been reported on dose reduction techniques based on the patient’s position. The sitting position on the Table is crucial for the analysis of swallowing events. SCT is conventionally performed with the patient reclined at 45° in a reclining chair. This angle is dependent on the structure of the CT device and reclining chair and can subject the patient to a positional burden. Moreover, the reclining angle may influence the tube current and scan field of view (SFOV); thus, it is directly associated with the patient dose and image quality. A phantom study is essential for estimating the relationship between the effective dose and the reclining angle. Considering all these factors, we conducted the present SCT study to estimate the patient dose at various patient reclining positions. MATERIALS AND METHODS Scanning protocol for SCT using 320-detector row CT The third-generation 320-multidetector row CT device (320-MDCT; Aquilion ONE ViSION Edition, Toshiba Medical Systems, Otawara, Japan) boasts of a big gantry frame (2030 mm [height] × 2430 mm [width] × 1070 mm [depth]) for mounting a wide-area detector comprising an array of solid-state gadolinium oxysulphide (Gd2O2S: second-generation detector) ceramic with 896 channels and 320 segments. The effective length of the detector elements on the Z-axis, at the iso-centre, is 320 × 0.5 mm2 (160 mm), the aperture size is 780 mm, the focus–detector distance is 1072 mm, and the maximum gantry tilt angle is ±22°, which is required for an SCT procedure. In addition, it incorporates adaptive iterative dose reduction 3D (AIDR 3D), which aids in decreasing the patient dose. Automatic exposure control (AEC) is an X-ray exposure termination device based on single or dual scanography. However, AEC was not available for our study because scanograms were not obtained, and a fixed tube current was used. Furthermore, the localizer is set at the iso-centre; therefore, a volume scan (wide-beam width that enables scanning of larger volumes in a single rotation without moving the patient) for positioning and dynamic volume scan for SCT is performed. The scanning parameters used in our study were as follows: X-ray tube voltage and current, 120 kV and 40 mA, respectively; tube rotation time and total scan time, 0.275 s/rot and 3.3 s (0.275 s/rot × 12 rot), respectively; beam width, 160 mm; SFOV, 240 mm (S size); gantry tilt angle, 22°; reclining angle, 45°; and image reconstruction, AIDR 3D standard. The image quality [image noise standard deviation (SD)] obtained under these parameters was considered the standard for the determination of tube current at each reclining angle used for acquiring scanograms. A reclining chair (Tomeibrace Co., Ltd., Seto and Aska Corp., Kariya, Japan) that was fixed on a base was used. The height could not be adjusted, and the chair was designed to slide forward and backward and recline to a semi-sitting position. Whole-body dosimetry phantom An anthropomorphic RANDO Man phantom (RAN110; Phantom laboratory, Salem, NY, USA) was used in this study. This male model comes without arms and legs and measures 175 cm in height and 73.5 kg in weight (Figure 1a). It is sliced at 2.5-cm intervals and incorporates air, human cadaver skeleton or simulated bone material, soft-tissue-equivalent material, and lung tissue-equivalent material. The latter two materials are manufactured from a proprietary urethane formulation with the same effective atomic number and density as human muscle tissue and lung tissue in a median respiratory state, respectively. For dose estimation studies, a number of plugs are designed to hold thermoluminescent dosemeter (TLD) chips. In the present study, these holes were used to hold TLD chips and/or place solid Mix D plugs (Figure 1b). Optimal breasts ranging from A through E in size are available for this model, although the breast shapes are not natural because of the presence of adaptor bases for blending them with the male chest(9). Although dose estimation was performed for the chest wall region of a male phantom without breasts in the present study, we considered our model to be a gender-neutral phantom [male and female without optional breasts (size A)] because TLD chips cannot be placed appropriately in the breast. Figure 1. View largeDownload slide Materials used for effective dose measurement during swallowing computed tomography (SCT) examinations at various reclining angles. (a) An anthropomorphic RANDO Man phantom without arms and legs (175 cm in height and 73.5 kg in weight). (b) A thermoluminescent dosemeter (TLD) chip manufactured from Mg2SiO4:Tb (2 mmφ × 12 mm) and a filter (4 mmφ × 15 mm). (c) A 0.5-mm Al cylinder plate (99.9%) for half-value layer (HVL) measurement. Figure 1. View largeDownload slide Materials used for effective dose measurement during swallowing computed tomography (SCT) examinations at various reclining angles. (a) An anthropomorphic RANDO Man phantom without arms and legs (175 cm in height and 73.5 kg in weight). (b) A thermoluminescent dosemeter (TLD) chip manufactured from Mg2SiO4:Tb (2 mmφ × 12 mm) and a filter (4 mmφ × 15 mm). (c) A 0.5-mm Al cylinder plate (99.9%) for half-value layer (HVL) measurement. TLD system TLD chips manufactured from Mg2SiO4:Tb were used in this study (MSO-S; Kyokko, Japan; 2 mmφ × 12 mm, glass capsule type, lower detection limit at 200 μGy). Before X-ray irradiation, the TLD chips were annealed at 400°C in an annealing oven (AO-CH; Toreck, Japan; temperature error less than ± 2% of the set value) for 1 h. The TLD chips were protected with a holder (C; Kyokko, Japan; 4 mmφ × 15 mm) and individually calibrated using a chamber with a 6-cm3 volume (10X5–6; Radcal Corporation, Monrovia, CA, USA) and a dosemeter (9015; Radcal) that was annually calibrated by a standard dosimetry laboratory (Figure 2). A diagnostic X-ray device (KXO-81 with 2.5-mm Al and 0.1-mm Cu filters at 120 kV; Toshiba Medical Systems, Otawara, Japan) with an air kerma of 10 mGy and an effective energy of 54.6 keV [half-value layer (HVL) of aluminium (99.9%), 7.88 mm Al] was used as the irradiation source. The X-ray beam quality, which was the same as that offered by 320-MDCT at 120 kV, was previously measured. After the initial exposure dose in the centre was measured using an ionisation chamber with a 3-cm3 volume (10X-3CT; Radcal Corporation, Monrovia, CA, USA), 0.5-mm Al cylinder plates (99.9%) was placed and the HVL was measured (Figure 1c). Following irradiation for calibration, the TLD chips were read after 24 h, considering the possibility of fading, using a TLD reader (Model 3000; Kyokko, Hiroshima, Japan). The obtained values were corrected for fading. Calibration factors (f) were obtained only for TLD chips with a co-efficient of sensitivity that was within 10% of the mean value. The calculation of uncertainty with regard to TLD measurements was based on the Joint Committee for Guides in Metrology(10) and the UK Institute of Physical Sciences in Medicine analysis of TLD uncertainties for patient dose measurements(11). For the treatment of uncertainties, random (Type A as TLD calibration reading, TLD background reading for calibration, calibration dose, TLD measurement reading and TLD background reading for measurement) and systematic (Type B as dosemeter calibration, TLD energy response, TLD dose linearity, TLD signal fading and temporal variation of TLD reader) errors are taken into account. Random errors for the organ doses were combined using propagation of errors. For the systematic errors identified for the measurements, we referred to some previous studies(12–15). The overall uncertainty for individual TLD measurements was estimated by combining the random and systematic uncertainties in a quadrature. In the present study, the uncertainty was estimated to be in the range of 5–10%. Figure 2. View largeDownload slide Experimental setup for the TLD chips calibration using a chamber with a 6-cm3 volume and dosemeter were as follows: source chamber distance, 100 cm; chamber floor distance, 60 cm; and X-ray field, 10 cm × 10 cm. The TLD chips with the holder were arranged at the same height as chamber. Figure 2. View largeDownload slide Experimental setup for the TLD chips calibration using a chamber with a 6-cm3 volume and dosemeter were as follows: source chamber distance, 100 cm; chamber floor distance, 60 cm; and X-ray field, 10 cm × 10 cm. The TLD chips with the holder were arranged at the same height as chamber. Determination of scanning parameters To determine the tube current for each reclining angle, we used a scanogram. Figure 3 shows the experimental setup. A RANDO phantom without TLD chips was placed on the scanner Table in a semi-reclining position at an angle of 30°. A scanogram was obtained at 120 kV and 10 mA. In this scanogram, a SD value of 40 mA at a gantry tilt angle of 7° was determined; these angles were equivalent to the patient position during SCT (patients reclining angle, 45°; gantry tilt angle, 22°). At the same SD, tube currents for gantry tilt angles of −3° and −13° were estimated. In the phantom study, these determined tube currents were used for SCT at patient reclining angles of 55° and 65°, respectively (gantry tilt angle, 22°). Figure 3. View largeDownload slide Experimental setup for our phantom study for effective dose measurement during swallowing computed tomography (SCT) examinations at various reclining angles. A RANDO phantom without TLD chips was placed on the scanner Table in a semi-reclining position at an angle of 30°. In this scanogram, a SD value of 40 mA at a gantry tilt angle of 7° was determined; these angles were equivalent to the standard SCT (patients reclining angle, 45°; gantry tilt angle, 22°). At the same SD, tube currents for gantry tilt angles of −3° and −13° were estimated. In the phantom study, these determined tube currents were used for SCT at patient reclining angles of 55° and 65°, respectively (gantry tilt angle, 22°). Figure 3. View largeDownload slide Experimental setup for our phantom study for effective dose measurement during swallowing computed tomography (SCT) examinations at various reclining angles. A RANDO phantom without TLD chips was placed on the scanner Table in a semi-reclining position at an angle of 30°. In this scanogram, a SD value of 40 mA at a gantry tilt angle of 7° was determined; these angles were equivalent to the standard SCT (patients reclining angle, 45°; gantry tilt angle, 22°). At the same SD, tube currents for gantry tilt angles of −3° and −13° were estimated. In the phantom study, these determined tube currents were used for SCT at patient reclining angles of 55° and 65°, respectively (gantry tilt angle, 22°). Patient dose estimation using 320-MDCT The placement of TLD chips was carefully considered, and these were placed at locations corresponding to the following organs: breasts, colon, lung, stomach, gonads, bladder, oesophagus, liver, thyroid, brain, salivary glands, adrenal glands, the respiratory tract, gallbladder, heart, kidney, pancreas and spleen. For the estimation of doses absorbed by the bone marrow, bone surface, and skin, we referred to the Nishizawa et al.(16). The bone marrow dose was assessed by weighting according to the distribution of red bone marrow, and the bone surface dose was assessed by weighting according to the whole bone mineral distribution (Table 1)(16). The number of TLD chips used for each organ was as follows: lungs, 59; stomach, three; colon, 10; bone marrow, 48; breast, 6; gonads, 12; bladder, 9; liver, 3; bone surface, 46; skin, 117; brain, 33 and others, 19. The TLD chips can be duplicated in some cases. Each scan condition was separately repeated, with a total of 327 TLD chips placed throughout the body (Figure 4). Table 1. Red bone marrow and mineralised bone distributions (weight and fraction) according to the study by Nishizawa. Bone  Reb bone marrow weight (g)  Reb bone marrow fraction  Mineralised bone weight (g)  Mineralised bone fraction  Skull  55.6  0.072  663.0  0.179  Mandible  3.7  0.005  Clavicle  5.6  0.007  44.0  0.012  Scapulae  16.7  0.022  113.0  0.031  Sternum  20.6  0.027  18.5  0.005  Cervical vertebrae  22.2  0.029  51.5  0.014  Thoracic vertebrae  101.0  0.132  156.5  0.042  Lumbar vertebrae  85.6  0.112  128.0  0.035  Sacral vertebrae  65.8  0.086  72.5  0.020  Ribs  104.5  0.136  368.5  0.073  Llium  170.2  0.222  317.0  0.085  Femur  87.1  0.144  676.0  0.185  Patella      24.5  0.007  Tibia      383.5  0.104  Fibula      90.0  0.024  Feet      198.0  0.053  Humerus  27.9  0.036  237.0  0.064  Radius      75.0  0.020  Ulna      94.0  0.025  Hand      89.5  0.024  Total  766.5 g  1.000  3700.0  1.000  Bone  Reb bone marrow weight (g)  Reb bone marrow fraction  Mineralised bone weight (g)  Mineralised bone fraction  Skull  55.6  0.072  663.0  0.179  Mandible  3.7  0.005  Clavicle  5.6  0.007  44.0  0.012  Scapulae  16.7  0.022  113.0  0.031  Sternum  20.6  0.027  18.5  0.005  Cervical vertebrae  22.2  0.029  51.5  0.014  Thoracic vertebrae  101.0  0.132  156.5  0.042  Lumbar vertebrae  85.6  0.112  128.0  0.035  Sacral vertebrae  65.8  0.086  72.5  0.020  Ribs  104.5  0.136  368.5  0.073  Llium  170.2  0.222  317.0  0.085  Femur  87.1  0.144  676.0  0.185  Patella      24.5  0.007  Tibia      383.5  0.104  Fibula      90.0  0.024  Feet      198.0  0.053  Humerus  27.9  0.036  237.0  0.064  Radius      75.0  0.020  Ulna      94.0  0.025  Hand      89.5  0.024  Total  766.5 g  1.000  3700.0  1.000  Figure 4. View largeDownload slide Placement of the 327-TLD chips (lungs, 59; stomach, three; colon, 10; bone marrow, 48; breast, 6; gonads, 12; bladder, 9; liver, 3; bone surface, 46; skin, 117; brain, 33 and others, 19). The TLD chips can be duplicated in some cases. Figure 4. View largeDownload slide Placement of the 327-TLD chips (lungs, 59; stomach, three; colon, 10; bone marrow, 48; breast, 6; gonads, 12; bladder, 9; liver, 3; bone surface, 46; skin, 117; brain, 33 and others, 19). The TLD chips can be duplicated in some cases. The RANDO model with TLD chips was placed on a hard Table board in a semi-reclining position at an angle of 30° at the centre and off-centre (Figure 5). In this estimation study, the positioning scan (volume scan) was excluded from dose accumulation. The predetermined tube currents for reclining angles of 55° and 65° equivalent were used. These reclining angles were determined on the basis of the following conditions. First, SCT is routinely performed at a reclining of 45°. Second, precise adjustment of the angle was difficult in the experimental system. Third, an angle of 65° or greater is difficult to achieve in the clinical setting, because of the tilt angle of the CT scanner and the structure of the reclining chair. When the RANDO phantom position was off-centre, the SFOV was selected as 400 mm (large size) at 55° and 500 mm (double large size) at 65° to obtain adequate data to reconstruct a swallowing sequence. The off-centre position was set at ~10 and 20 cm, respectively, from the centre. The tube current was the same as that at the centre position. Figure 5. View largeDownload slide Position of the RANDO phantom for patient dose estimation using a 320-multidetector row computed tomography (MDCT) device. Figure 5. View largeDownload slide Position of the RANDO phantom for patient dose estimation using a 320-multidetector row computed tomography (MDCT) device. After X-ray irradiation for each scan protocol, the amount of fluorescence M (C/kg) was measured by a TLD reader and corrected by an individual calibration factor f (Gy kg/C).   Dair=M×f·(Gy) (1)where, Dair (Gy) is the air-absorbed dose. The values for each organ were averaged to calculate the organ- and/or tissue-absorbed dose, D (Gy), assuming that TLDs were in the ideal positions for covering all organs.   D=Dair×(μen/ρ)soft tissue etc(μen/ρ)air (2)where, μen/ρ (cm2/g) is the ratio of the mass energy absorption coefficients in air and the organs. The sex-averaged equivalent dose HT (Sv) for all organs or tissues was calculated from D and the radiation weighting factor (1.0) specified in International Commission on Radiological Protection (ICRP) Publication 103(17).   HT=D×1.0 (3) To calculate the contribution effective dose, E (Sv), the equivalent dose was multiplied by the age- and sex-averaged tissue weighting factor WT specified in ICRP Publication 103(17). Other evaluated organs included the adrenal gland, gallbladder, heart, kidney, pancreas, prostate gland (in males), small intestine, spleen and uterus (in females).   E=∑WT×HT (4) RESULTS Scanning parameters Analysis of a scanogram for various reclining angles revealed tube currents of 50 and 70 mA for reclining angles of 55° and 65°, respectively, in order to obtain the same image quality as that at 45° and 40 mA (SD, 18.5 at 0.5-mm slice thickness). When the SFOV size was changed, the tube currents remained the same. Therefore, we used these tube currents for dose estimation at the centre and off-centre phantom positions as Table 2. Table 2. The scanning parameters and doses. Scan conditions  Centre 45°  Off-centre 55°  Off-centre 65°  Centre 55°  Centre 65°  Tube voltage (kV)  120  120  120  120  120  Tube current (mA)  40  50  70  50  70  Rotation time (s/rot)  0.275  0.275  0.275  0.275  0.275  Exposure time (s)  3.3  3.3  3.3  3.3  3.3  SFOV (mm)  240(S)  400(L)  500(LL)  240(S)  240(S)  CTDIvol (mGy)  26.2  30.0  42.5  32.8  45.9  DLP (mGy cm)  419.3  479.9  679.9  524.1  733.8  Scan conditions  Centre 45°  Off-centre 55°  Off-centre 65°  Centre 55°  Centre 65°  Tube voltage (kV)  120  120  120  120  120  Tube current (mA)  40  50  70  50  70  Rotation time (s/rot)  0.275  0.275  0.275  0.275  0.275  Exposure time (s)  3.3  3.3  3.3  3.3  3.3  SFOV (mm)  240(S)  400(L)  500(LL)  240(S)  240(S)  CTDIvol (mGy)  26.2  30.0  42.5  32.8  45.9  DLP (mGy cm)  419.3  479.9  679.9  524.1  733.8  Dose estimation using 320-MDCT For dose estimation at various reclining angles during SCT, we performed a phantom study using the predetermined tube currents. The absorbed dose was determined to average dose from TLDs depend on the organ on/in phantom. The TLDs were carefully distributed as evenly as possible in each organ. However, for organs where TLD placement was not possible, such as the oesophagus, the chips were placed in holes close to the organs and the dose was measured for effective dose estimation. Moreover, the dose for the red bone marrow and bone surface is difficult to evaluate in a phantom study; therefore, the method reported by Nishizawa et al.(16) was used. The accuracy of TLDs is within 10%; therefore, a large difference in measurements for the same internal organs because of differences in position, size and surrounding structures, is not considered an outlier. The absorbed dose for the oesophagus (20.03 mGy) at the centre 45° reclining angle was 11 and 35% lower than that at the off-centre 55° (22.48 mGy) and 65° (30.91 mGy) angles, respectively (Table 3). The same trend was observed for the centre 55° (25.79 mGy) and 65° (39.38 mGy) angles. The absorbed doses for the thyroid gland (28.49–39.38 mGy) were the highest among all organs. Although the doses were not influenced by the off-centre reclining angle and/or increase in tube current, the doses at the centre 55° (33.36 mGy) and 65° (33.98 mGy) reclining angles showed an increasing trend. The absorbed dose for the eye showed the same trend. The absorbed doses for organs in the trunk region, such as the lung, stomach, colon, breast, and liver, showed an increase with an increase in the reclining angle from centre 45° to off-centre 65°. A similar trend was observed by an increase in the angle from centre 45° to centre 65°, although the dose increase was smaller. The absorbed doses for the lung, as estimated from 59 TLD chips, showed large SD values because of doses from primary X-rays or scatter X-rays; the salivary gland doses showed the same trend. With regard to the effective dose (Table 2), the dose at centre 45° (3.8 mSv) was the lowest. The effective doses at off-centre 55° (4.6 mSv) and centre 55° (4.5 mSv) were similar. However, the effective dose at off-centre 65° (6.8 mSv) was higher than that at centre 65° (5.8 mSv). The centre 65° angle resulted in a 20% dose reduction compared with the off-centre 65° angle. Table 3. Organ- and tissue-absorbed doses and the effective dose for swallowing computed tomography estimated in a phantom study conducted at various reclining angles. Organ/tissue  Organ dose (mGy)  Current SCT  Hope SCT  Centre 45°  Off-centre 55°  Off-centre 65°  Centre 55°  Centre 65°  Lung (n = 59)  3.57 ± 3.5  4.82 ± 4.3  15.64 ± 10.0  3.57 ± 3.6  6.93 ± 6.4  Stomach (n = 3)  0.26 ± 0.1  0.34 ± 0.1  1.33 ± 0.3  0.32 ± 0.0  0.49 ± 0.1  Colon (n = 10)  0.06 ± 0.0  0.10 ± 0.0  0.25 ± 0.0  0.08 ± 0.0  0.14 ± 0.0  Bone marrow (n = 48)  3.77 ± 0.4  8.77 ± 0.4  7.94 ± 0.6  5.15 ± 0.5  6.99 ± 0.7  Breast (n = 6)  0.56 ± 0.0  0.79 ± 0.1  1.83 ± 0.0  0.57 ± 0.0  0.88 ± 0.0  Gonads (n = 12)  0.04 ± 0.0  0.04 ± 0.0  0.12 ± 0.0  0.06 ± 0.0  0.09 ± 0.0  Thyroid gland (n = 3)  33.38 ± 0.3  28.49 ± 0.1  30.67 ± 6.5  33.36 ± 1.7  39.38 ± 0.5  Oesophagus (n = 8)  20.03 ± 5.0  22.48 ± 3.2  30.91 ± 3.0  25.79 ± 7.7  30.38 ± 7.5  Bladder (n = 9)  0.03 ± 0.0  0.03 ± 0.0  0.10 ± 0.0  0.04 ± 0.0  0.06 ± 0.0  Liver (n = 3)  0.25 ± 0.0  0.36 ± 0.0  1.42 ± 0.0  0.31 ± 0.0  0.55 ± 0.0  Bone surface (n = 46)  2.91 ± 0.8  2.65 ± 0.8  3.45 ± 1.0  2.91 ± 1.4  4.62 ± 1.9  Skin (n = 117)  11.44 ± 12.3  12.75 ± 9.3  16.95 ± 12.6  13.39 ± 13.7  18.57 ± 18.1  Brain (n = 33)  1.87 ± 0.9  2.07 ± 2.4  2.52 ± 1.2  6.17 ± 4.2  9.21 ± 6.2  Salivary gland (n = 1)  16.10 NS  14.84 NS  8.48 NS  24.38NS  28.42 NS  Remainder (n = 19)  50.08 ± 10.4  76.26 ± 6.2  53.12 ± 11.0  56.58 ± 12.1  72.22 ± 14.5  Eye lens (n = 6)  4.50 ± 0.1  2.46 ± 0.1  2.94 ± 0.1  20.11 ± 0.9  26.30 ± 1.3  Effective dose (mSv)  3.8  4.6  6.8  4.5  5.8  Organ/tissue  Organ dose (mGy)  Current SCT  Hope SCT  Centre 45°  Off-centre 55°  Off-centre 65°  Centre 55°  Centre 65°  Lung (n = 59)  3.57 ± 3.5  4.82 ± 4.3  15.64 ± 10.0  3.57 ± 3.6  6.93 ± 6.4  Stomach (n = 3)  0.26 ± 0.1  0.34 ± 0.1  1.33 ± 0.3  0.32 ± 0.0  0.49 ± 0.1  Colon (n = 10)  0.06 ± 0.0  0.10 ± 0.0  0.25 ± 0.0  0.08 ± 0.0  0.14 ± 0.0  Bone marrow (n = 48)  3.77 ± 0.4  8.77 ± 0.4  7.94 ± 0.6  5.15 ± 0.5  6.99 ± 0.7  Breast (n = 6)  0.56 ± 0.0  0.79 ± 0.1  1.83 ± 0.0  0.57 ± 0.0  0.88 ± 0.0  Gonads (n = 12)  0.04 ± 0.0  0.04 ± 0.0  0.12 ± 0.0  0.06 ± 0.0  0.09 ± 0.0  Thyroid gland (n = 3)  33.38 ± 0.3  28.49 ± 0.1  30.67 ± 6.5  33.36 ± 1.7  39.38 ± 0.5  Oesophagus (n = 8)  20.03 ± 5.0  22.48 ± 3.2  30.91 ± 3.0  25.79 ± 7.7  30.38 ± 7.5  Bladder (n = 9)  0.03 ± 0.0  0.03 ± 0.0  0.10 ± 0.0  0.04 ± 0.0  0.06 ± 0.0  Liver (n = 3)  0.25 ± 0.0  0.36 ± 0.0  1.42 ± 0.0  0.31 ± 0.0  0.55 ± 0.0  Bone surface (n = 46)  2.91 ± 0.8  2.65 ± 0.8  3.45 ± 1.0  2.91 ± 1.4  4.62 ± 1.9  Skin (n = 117)  11.44 ± 12.3  12.75 ± 9.3  16.95 ± 12.6  13.39 ± 13.7  18.57 ± 18.1  Brain (n = 33)  1.87 ± 0.9  2.07 ± 2.4  2.52 ± 1.2  6.17 ± 4.2  9.21 ± 6.2  Salivary gland (n = 1)  16.10 NS  14.84 NS  8.48 NS  24.38NS  28.42 NS  Remainder (n = 19)  50.08 ± 10.4  76.26 ± 6.2  53.12 ± 11.0  56.58 ± 12.1  72.22 ± 14.5  Eye lens (n = 6)  4.50 ± 0.1  2.46 ± 0.1  2.94 ± 0.1  20.11 ± 0.9  26.30 ± 1.3  Effective dose (mSv)  3.8  4.6  6.8  4.5  5.8  DISCUSSION In the present study, we showed a positive correlation between the patient dose in SCT and the patient reclining angle. Our results also suggest that the off-centre position is undesirable with respect to the patient dose. In general, the image quality is poorer at the off-centre position than at the centre position. Accordingly, we believe that SCT methods must be revised on the basis of these factors. During conventional VF examinations, the reclining angle during the routine meal posture is considered ideal, and the observation of side and front images is important(18). This is also necessary for the morphological and kinematic analyses of swallowing using SCT(19). However, the patient reclining angle during SCT has limitations because of factors such as the gantry tilt angle, aperture size, and construction of the reclining chair, among others. Because of the lack of information on the role of the patient reclining angle, a fixed angle of 45° has been used in previous studies(1–3, 8, 19). However, a more flexible reclining angle that corresponds to the meal posture of the patient is desirable for accurate SCT results. When the reclining angle is larger than 45°, the patient’s image is reconstructed off-centre because of the recline of the patient chair and the tilt angle of the CT device. Therefore, a large SFOV is necessary for magnification reconstruction (display-FOV). The image quality after magnification reconstruction from the off-centre position is poor, and to position the patient in the iso-centre, a reclining chair has to be developed(1). Even if this is achieved, the dose difference caused by the reclining angle remains unclear. A study by Kobayashi et al.(8) estimated the patient dose (3.9 mSv) and operator dose (0.002 mSv) for SCT studies. However, the results of their study are limited. First, the effective dose was estimated at 60 mA because the CT scanner used in their study was not equipped with AIDR 3D. In recent years, reconstruction using AIDR 3D and/or forward projected model-based IR solution (FIRST) as full IR has become a standard for decreasing the patient dose in several CT studies. As a result, the image quality is maintained and the tube current is decreased by 33%. The reason for our patient dose value (3.8 mSv) corresponding with the value obtained by Kobayashi et al. is unclear, although the carbon backrest of the reclining chair and errors associated with the TLD chips could have played a role. Second, they confirmed the organ doses and effective dose for SCT; however, the dose required for SCT studies was not estimated using various positions. Possible explanations for this finding are as follows: in the case of obtaining the same image quality, an increase in the effective dose with an increase in the reclining angle and/or a decrease in the effective dose by a change in the patient position from off-centre to centre. Therefore, a reclining chair and CT device that enables patient positioning in the centre position needs to be developed. In conclusion, the present study estimated the dose at various patient reclining angles for the morphological and kinematic analyses of swallowing using SCT. In this study, we used a flat and hardback board and an assistance backrest as opposed to the reclining chair used in conventional SCT. This innovative technique yielded a more feasible dose estimation procedure for use in future clinical practice and studies. The estimated dose was still high and varied from 3.8 to 6.8 mSv, which is higher than that in VF (0.12–1.05 mSv)(1, 20–22). We also found a decrease in the patient dose in the centre position of both reclining angles (55° and 65°). Moreover, our results revealed that the patient reclining angle has a significant influence on the dose to organs in the chest region, such as the lungs and breasts, compared with the dose to organs in the neck region. For the operator, a dose reduction effect of a lead-free X-ray apron under the scan conditions of SCT examinations has been proven(8). Therefore, a lead-free X-ray apron should be used to protect the chest region of the patient during SCT. A limitation of our study is that the value of the absorbed dose in the neck region was uncertain value because of limitations with the arrangement of the TLD chips. Further studies using Monte Carlo simulation by ImPACT MC software(23), which can simulate the absorbed dose with the help of DICOM images, are required. Furthermore, the optimisation of scan conditions and the determination of a standard image quality are necessary for safe and effective SCT studies. CONCLUSION In conclusion, the results of this study suggest that the patient dose decreases in the centre position of both reclining angles (55° and 65°). Therefore, a reclining chair and CT device that enable patient positioning in the centre position are desirable. 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TI - SWALLOWING COMPUTED TOMOGRAPHY: DOSE ESTIMATION IN A PHANTOM STUDY CONDUCTED AT VARIOUS PATIENT RECLINING ANGLES
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncx078
DA - 2018-01-01
UR - https://www.deepdyve.com/lp/oxford-university-press/swallowing-computed-tomography-dose-estimation-in-a-phantom-study-EPHW1rUPrc
SP - 87
EP - 94
VL - 178
IS - 1
DP - DeepDyve
ER -