TY - JOUR
AU - Baechler,, S.
AB - Abstract The goal of this study was to develop a method to quantitatively assess the structural shielding performance of computed tomography (CT) rooms in Switzerland. The method was based on the comparison between calculated maximum allowed ambient dose rate (DR) and measured ambient DR in adjacent spaces. If the measured DR exceeds the calculated maximum allowed value, additional shielding is required; otherwise the shielding is sufficient. The maximum allowed ambient DR was calculated using two different shielding concepts either based on the tube loading of the scanner or on the accumulated dose length product of the scans. The method was validated for 36 spiral CT head protocols. The average ratio of the maximum allowed ambient DR between both concepts was 1.11 (range 0.57–2.14). Among 36 CT rooms, 7 room boundaries were insufficiently shielded. In conclusion, this method allows the authority to check on-site the compliance of the structural shielding with legal requirements. INTRODUCTION Medical X-ray units must be operated in dedicated rooms that are sufficiently shielded against leakage and scattered radiation such that the annual effective dose of an individual staying in adjacent spaces does not exceed the dose limits as established by the International Commission on Radiological Protection (ICRP) and implemented into the Swiss radiological protection legislation(1–3). Especially for high-dose applications in diagnostic radiology, such as computed tomography (CT), an appropriate room shielding design is of crucial importance. In Switzerland, until a few years ago, the calculation of the required shielding of CT rooms was based on the German standard DIN 6812:1996–04(4), which was part of the Swiss radiological protection legislation(3). In principle, the DIN concept makes use of the tube loading (TL) of the CT scanner needed for the acquisition of single CT image slices of different anatomical regions to estimate the ambient dose rate (DR) of the scattered radiation and thus to determine the required shielding of the CT room. While the application of this concept presents already some minor deficiencies for single-slice CT scanners (the effect of beam quality and scanner output is not considered), this concept becomes even more problematic when applying for multi-detector CT scanners with volume acquisition, since for identical TL several CT image slices can be reconstructed simultaneously, depending on the detector configuration(5). For that reason, a new concept has been established based on the accumulated dose length product (DLP) of the CT scans(6). This new concept allows for a more precise estimation of the amount of scattered radiation because the DLP directly reflects both the effect of beam parameters (tube voltage, beam collimation) as well as CT scanner characteristics (beam filtration, scanner output). The Federal Office of Public Health (FOPH) is the competent authority for licensing and supervision of medical X-ray units in Switzerland. As part of its supervisory activities, technical audits are performed on a regular basis in private and public hospitals and private radiological institutes operating a CT scanner in diagnostic radiology. The general objective of these technical audits is to check the compliance with the legal requirements. Thereby, special attention is paid to the assessment of the structural shielding performance of CT rooms. During the on-site audits, the FOPH verifies visually whether the existing room shielding is constructed in accordance with the radiation protection calculation. DR measurements in adjacent spaces while the CT scanner is operated would allow to quantitatively assess the structural shielding performance of CT rooms. According to the Swiss radiological protection legislation, the maximum allowed ambient dose in adjacent spaces is 0.02 mSv per week for uncontrolled areas and 0.1 mSv per week for controlled areas(3). These regulatory restrictions ensure that the annual effective doses for occupationally exposed persons and for members of the public do not exceed 5 and 1 mSv, respectively. However, in order to assess whether the measured DRs do not result in an ambient dose per week that exceeds the maximum allowed value of 0.02 and 0.1 mSv, respectively, is not straightforward. The aim of this study was therefore to develop a semi-analytical method to calculate the maximum allowed ambient DR outside CT rooms. Comparing this calculated value to the measured ambient DR in adjacent spaces during operation of the CT scanner allows to immediately assess the compliance of the CT room shielding with legal requirements. MATERIALS AND METHODS Calculation of the ambient DR outside CT rooms The required shielding of a CT room is generally based on the weekly workload of the CT scanner. In principle, the higher the weekly workload, i.e. the more scans are performed, the more leakage and scattered radiation is emitted and the more shielding is required. In Switzerland, the FOPH authorizes for the operation of each CT scanner a maximum weekly workload reflecting the institute-specific radiological practice. For large hospitals with a high patient throughput, a higher workload is authorized than for small radiological institutes performing only a few CT scans per week. It is required that the authorized weekly workload is not exceeded, otherwise the calculated shielding would not be sufficient to comply with the legal requirements. Depending on the shielding calculation concept which is either based on the TL or on the DLP, the authorized weekly workload is expressed as WTL (mA·min/week) or WDLP (Gy·cm/week), respectively(3, 6). Considering a specific CT scan of tube current I (mA), scan time t (s) and DLP (mGy·cm), the maximum allowed weekly beam-on time for that scan is derived by dividing the authorized weekly workload WTL and WDLP by the tube current and by the DLP times the scan duration, respectively. This allows to calculate the maximum allowed ambient DR (µSv/s) outside the CT room by multiplying the authorized ambient dose per week for adjacent spaces, Hw (µSv/week)(3), by the maximum allowed weekly beam-on time as follows: DRTL=Hw×IWTL×160(1) and DRDLP=Hw×DLPWDLP×t×11000,(2) where the correction factors 1/60 and 1/1000 are introduced to convert the unit of the DR to µSv/s. However, this basic approach does not reflect the real situation with sufficient accuracy. The amount of scattered radiation does not only depend on the beam-on time but also on other key parameters such as the beam collimation, scattering volume and scanner output, which represents a characteristic parameter of each CT scanner describing how much radiation is generated for specific tube settings. Effect of beam collimation The beam collimation has a major impact on the emission of scattered radiation. The larger the beam collimation, the more volume is irradiated and the more scattered radiation is emitted. This in turn results in an increased ambient DR. Supposing approximately the relationship between beam collimation and ambient DR to be linear(7), Equation (1) becomes DRTL=Hw×IWTL×160×ccmean,(3) where c (mm) represents the beam collimation of the CT scan and cmean a normalization factor of 5 mm. At that time, when the TL shielding calculation concept has been developed, most of the single-slice CT scanners in Switzerland have been operated at beam collimations of 2, 5 and 10 mm. For the calculation of the required CT room shielding, the median beam collimation of 5 mm was used. This approach was less conservative than using the maximum beam collimation of 10 mm, which would result in a reduction of the calculated ambient DR by a factor of 2. In contrast to the TL shielding calculation concept, the effect of beam collimation on the calculation of the ambient DR is already reflected by the DLP and thus Equation (2) remains valid. Effect of scattering volume In clinical CT practice, different anatomical regions are scanned resulting in different amounts of emitted scattered radiation. For the development of the semi-analytical calculation method, the scattering behaviour of the scanned anatomical regions was assumed to be similar to that of a cylindrical polymethyl methacrylate (PMMA) head or body phantom, each with a length of 15 cm and a diameter of 16 and 32 cm, respectively. This assumption is motivated by the fact that both the PMMA head and body phantom show a similar shape, volume and material density as the human head and trunk that represents the two most frequently scanned anatomical regions by CT in clinical practice. In order to estimate the effect of the scattering volume on the calculation of the ambient DR outside the CT room, both following key hypotheses are formulated. The ambient DR is supposed to be linearly related firstly to the absorbed dose per time unit in the scattering volume (i.e. the DLP per time unit) and secondly to the scattering volume itself. In a realistic clinical scenario it is expected that half of the weekly performed CT scans are applied to the head region and the other half to the trunk region. In that case, half of the authorized weekly workload WDLP is accumulated in the body phantom, WDLP,body = 1/2×WDLP, while the other half is accumulated in the head phantom, WDLP,head = 1/2×WDLP. Since the volume of the body phantom is four times larger than the volume of the head phantom, according to the second hypothesis, the DR when scanning the body phantom is also four times larger than when scanning the head phantom, i.e. Hw,body = 4×Hw,head, while Hw,head + Hw,body = Hw. These two equations can be combined to Hw,body = 4/5×Hw and Hw,head = 1/5×Hw. However, to achieve the same absorbed dose within both phantoms, the TL for the body scans must be twice as high as that for the head scans, while the other scan parameters remain unchanged, i.e. WTL,body = 1/2×WTL and WTL,head = 1/4×WTL. Replacing WTL, WDLP and Hw in Equations (2) and (3) by the phantom-specific parameters WTL,body, WTL,head, WDLP,body, WTL,head, Hw,body and Hw,head result in following equations: DRTL,head=15×Hw×I1/4×WTL×160×ccmean,(4) DRTL,body=45×Hw×I1/2×WTL×160×ccmean,(5) DRDLP,head=15×Hw×DLP1/2×WDLP×t×11000,(6) DRDLP,body=45×Hw×DLP1/2×WDLP×t×11000.(7) Effect of scanner output The scanner output depends on various beam modifying parameters such as the material composition and quality of the anode as well as the filtration. It is commonly described by the normalized weighted computed tomography dose index nCTDIw(mGy/mAs). Tabulated nCTDIw values for a wide range of CT scanners can be found in literature(8–10). According to the shielding calculation based on the TL and DLP concept(3, 6), the equivalent workload scanner output (neqCTDIw) is related to the authorized weekly workloads WDLP and WTL as follows: WDLP=CneqTDIw×WTL×cmean×350,(8) where the conversion factor of 3/50 is introduced to correct for the units. The equivalent workload scanner output defined in Equation (8) describes a hypothetical scanner output that results from the equivalence of workloads and which is independent of the specific CT scanner. To take into consideration the real scanner output in terms of nCTDIw (mGy/mAs) as presented in literature(8–10), a correction factor is introduced in Equations (4) and (5) as DRTL,head=15×Hw×I1/4×WTL×160×CnTDIw,head,CTExpoCneqTDIw,head×ccmean(9) and DRTL,body=45×Hw×I1/2×WTL×160×CnTDIw,body,CTExpoCneqTDIw,body×ccmean,(10) where nCTDIw,head,CTExpo and nCTDIw,body,CTExpo is derived from a Microsoft Excel®–based programme called CT-Expo V2.1(8) and neqCTDIw, head and neqCTDIw, body is calculated from Equation (8) using the relations WDLP, head = 1/2 × WDLP, WTL,head = 1/4 × WTL,WDLP,body = 1/2 × WDLP and WTL,body = 1/2 × WTL as CneqTDIw,head=WDLPWTL×1cmean×1003(11) and CneqTDIw,body=WDLPWTL×1cmean×503.(12) Substituting Equations (11) and (12) in Equations (9) and (10), respectively, finally results in DRTL,head=15×Hw×I×CnTDIw,head,CTExpo×cWDLP×1500(13) and DRTL,body=45×Hw×I×CnTDIw,body,CTExpo×cWDLP×1500.(14) Equations (6) and (7) must not be modified since the DLP already reflects the effect of the scanner output. Validation of the calculation method The two key hypotheses on which the calculation of the ambient DR outside CT rooms is based, namely that the ambient DR is linearly related firstly to the absorbed dose per time unit in the scattering volume (i.e. to the DLP per time unit) and secondly to the scattering volume itself, was validated both by measurements and Monte Carlo simulations. Measurements Measurements were performed on a Somatom Volume Zoom 4-slice CT scanner (Siemens Healthcare GmbH, Erlangen, Germany). A PMMA head and body phantom used as scattering volume was placed at the isocenter of the CT scanner. Ambient DR was measured using an X-ray multimeter (Barracuda, RTI Electronics, Mölndal, Sweden) together with an ionisation chamber with a volume of 1500 cc (Keithley Instruments Inc., Cleveland, Ohio, USA) at four different room positions. The measurement setup is illustrated in Figure 1. Figure 1. Open in new tabDownload slide Schematic illustration of the four measurement positions A–D for DR measurements in front view (a, left) and in side view (a, right) of the CT scanner. At each measurement position located at a distance of 1 m from the isocenter of the CT scanner the ionisation chamber was aligned in direction to the isocenter. At Position A, the axis of the ionisation chamber coincided with the scanner axis. At Positions B and C, the angle between the ionisation chamber axis and the scanner axis was 45° in horizontal and vertical direction, respectively. For Position D, the angle between the ionisation chamber axis and the scanner axis was 45° in both horizontal and vertical direction. In (b), the measurement setup with the home-made mounting support of the ionisation chamber is shown for Positions A (left) and D (right). Figure 1. Open in new tabDownload slide Schematic illustration of the four measurement positions A–D for DR measurements in front view (a, left) and in side view (a, right) of the CT scanner. At each measurement position located at a distance of 1 m from the isocenter of the CT scanner the ionisation chamber was aligned in direction to the isocenter. At Position A, the axis of the ionisation chamber coincided with the scanner axis. At Positions B and C, the angle between the ionisation chamber axis and the scanner axis was 45° in horizontal and vertical direction, respectively. For Position D, the angle between the ionisation chamber axis and the scanner axis was 45° in both horizontal and vertical direction. In (b), the measurement setup with the home-made mounting support of the ionisation chamber is shown for Positions A (left) and D (right). A spiral CT head protocol was applied for all measurements. The scan parameters were as follows: tube voltage = 120 kV, beam collimation = 20 mm, pitch = 1, rotation time = 1 s, scan length = 15 cm, scan time = 7.4 s. The CT head protocol was performed for each phantom and measurement position twice applying various tube currents (Scans 1–4, Table 1). Comparing the average ambient dose of Scan 1 with that of Scan 2 and of Scan 3 with that of Scan 4 allowed the validation of the first key hypothesis. For the validation of the second key hypothesis the average ambient dose of Scan 2 was compared with that of Scan 4 since the DLP was identical for both scans. In order to assess the amount of leakage radiation, three additional scans were performed with the smallest possible beam collimation of 1 mm and tube currents of 161, 200 and 435 mA (Scans 5–7, Table 1). For the measurements of the leakage radiation, the patient table with the phantom on top was completely retracted to avoid the emission of any scattered radiation. Ambient DR of leakage radiation was measured only at Position A (Figure 1). All results are presented as ambient dose that was derived by integrating the measured ambient DR over the total scan time. Table 1. Ambient dose at Positions A–D when applying a spiral CT head protocol of different tube currents and using a head phantom, a body phantom or no phantom at all. CT Scans 1 and 3 were performed with the same tube current of 200 mA while for CT Scans 2 and 4 the tube current has been adapted such that the resulting DLP became identical. Measurements for CT Scans 5–7 were performed only at one single Position A. Phantom . Scan . Tube current (mA) . DLP (mGy·cm) . Ambient dose (µSv) . . . . . Position A . Position B . Position C . Position D . Average . Head 1 200 562 71 92 85 83 83 Head 2 161 452 59 77 71 69 69 Body 3 200 208 89 100 97 76 91 Body 4 435 452 170 192 184 147 173 None 5 161 n.a. 1.9 n.a. n.a n.a. 1.9 None 6 200 n.a 2.3 n.a. n.a. n.a. 2.3 None 7 435 n.a. 4.4 n.a n.a. n.a. 4.4 Phantom . Scan . Tube current (mA) . DLP (mGy·cm) . Ambient dose (µSv) . . . . . Position A . Position B . Position C . Position D . Average . Head 1 200 562 71 92 85 83 83 Head 2 161 452 59 77 71 69 69 Body 3 200 208 89 100 97 76 91 Body 4 435 452 170 192 184 147 173 None 5 161 n.a. 1.9 n.a. n.a n.a. 1.9 None 6 200 n.a 2.3 n.a. n.a. n.a. 2.3 None 7 435 n.a. 4.4 n.a n.a. n.a. 4.4 Table 1. Ambient dose at Positions A–D when applying a spiral CT head protocol of different tube currents and using a head phantom, a body phantom or no phantom at all. CT Scans 1 and 3 were performed with the same tube current of 200 mA while for CT Scans 2 and 4 the tube current has been adapted such that the resulting DLP became identical. Measurements for CT Scans 5–7 were performed only at one single Position A. Phantom . Scan . Tube current (mA) . DLP (mGy·cm) . Ambient dose (µSv) . . . . . Position A . Position B . Position C . Position D . Average . Head 1 200 562 71 92 85 83 83 Head 2 161 452 59 77 71 69 69 Body 3 200 208 89 100 97 76 91 Body 4 435 452 170 192 184 147 173 None 5 161 n.a. 1.9 n.a. n.a n.a. 1.9 None 6 200 n.a 2.3 n.a. n.a. n.a. 2.3 None 7 435 n.a. 4.4 n.a n.a. n.a. 4.4 Phantom . Scan . Tube current (mA) . DLP (mGy·cm) . Ambient dose (µSv) . . . . . Position A . Position B . Position C . Position D . Average . Head 1 200 562 71 92 85 83 83 Head 2 161 452 59 77 71 69 69 Body 3 200 208 89 100 97 76 91 Body 4 435 452 170 192 184 147 173 None 5 161 n.a. 1.9 n.a. n.a n.a. 1.9 None 6 200 n.a 2.3 n.a. n.a. n.a. 2.3 None 7 435 n.a. 4.4 n.a n.a. n.a. 4.4 Monte Carlo simulations The Monte Carlo software package MCNPX® version 2.7.0 (Los Alamos National Laboratory, Los Alamos, New Mexico, USA) was used for the simulations. The geometry of the CT scanner was simulated by a cuboid of 1 m width, 1 m height and 0.5 m depth and a gantry opening of 0.6 m in diameter. The CT scanner was composed of iron for simulating dose absorption within the phantoms. For simulating the scattered radiation air was used as scanner material in order to avoid any influence of the CT scanner on the scattering behaviour. As for the measurements a cylindrical PMMA phantom of 15 cm length and 16 and 32 cm in diameter was implemented as scattering volume and positioned at the isocenter of the CT scanner along the scanner axis. The phantom was irradiated in the central plane perpendicular to the scanner axis by a fan beam of 10 mm beam width and 22.4° fan angle. 720 fan beams evenly spaced by an angle of 0.5° were used for a full rotation of the X-ray tube. The X-ray spectrum was simulated by a monoenergetic electron beam of 120 kV hitting an oblique cylinder of tungsten. The emitted Bremsstrahlung passed first through a 1 mm thick layer of stainless steel (tube filtration) followed by a 5 mm thick layer of aluminium and finally by 3.2 cm thick layer of aluminium with a gap in the form of a semi-sphere of 3 cm radius representing the bow tie filter of the CT scanner. The simulation was further refined by implementing data for the intensity attenuation of the bow tie filter for electron beams of 120 kV of the Somatom Volume Zoom CT scanner that was provided by Siemens Healthcare GmbH. The ambient dose was simulated using conversion coefficients for photon fluxes to absorbed doses(11) in nine spheres of 5 cm in diameter consisting of air. The positioning of the spheres is presented in Figure 2. For statistical reasons, simulations for the two phantoms were performed using the same number of incident particles. Figure 2. Open in new tabDownload slide Positions of the nine spheres as indicated by Numbers 1–9 in front view (left) and in side view (right) of the CT scanner for ambient DR simulations. Spheres with even numbers 2, 4, 6, 8 and odd numbers 3, 5, 7, 9 are located in the same plane perpendicular to the scanner axis, respectively. Sphere 1 is located on the scanner axis. The distance from the isocenter to each sphere is 1 m. Figure 2. Open in new tabDownload slide Positions of the nine spheres as indicated by Numbers 1–9 in front view (left) and in side view (right) of the CT scanner for ambient DR simulations. Spheres with even numbers 2, 4, 6, 8 and odd numbers 3, 5, 7, 9 are located in the same plane perpendicular to the scanner axis, respectively. Sphere 1 is located on the scanner axis. The distance from the isocenter to each sphere is 1 m. Application of the calculation method During the on-site audits in private and public hospitals and private radiological institutes, ambient DR measurements have been carried out for a total of 36 multi-detector CT scanners operated in a diagnostic radiology department. A PMMA head phantom used as scattering volume was placed at the isocenter, and for each CT scanner a standard clinical spiral CT head scan was performed. Ambient DR was measured in selected spaces adjacent to the CT room either using a DR monitor (LB 1230 UMo; Berthold Technologies, Bad Wildbad, Germany) together with a DR probe (LB 1236-H10; Berthold Technologies, Bad Wildbad, Germany), which is calibrated for X-ray energies between 30 keV and 1.3 MeV and for DRs between 0.1 µSv/h–10 mSv/h or a DR metre (RadEye B20-ER; Thermo Fisher Scientific, Waltham, MA USA), which is calibrated for X-ray energies between 17 keV and 1.3 MeV and for DRs between 0.01 µSv/h – 2 mSv/h. Details on scanner types and scan parameters are summarized in Table 2. Table 2. CT scanner characteristics (scanner type and literature-based scanner output nCTDIw,head,CTExpo(8)), maximum weekly workloads (WTL and WDLP) as well as scan parameters (tube current I, collimation c, DLP and scan time t) of the 36 CT scans used for ambient DR measurements. The collimation, DLP and scan time were displayed on the operator screen, whereas the tube current was manually calculated by dividing the displayed TL per gantry rotation by the rotation time. CT scanner characteristics . TL shielding calculation concept . DLP shielding calculation concept . Dataset . Scanner type . nCTDIw,head,CTExpo (mGy/mAs) . WTL (mA·min per week) . I (mA) . c (mm) . WDLP (Gy·cm/week) . DLP (mGy·cm) . t (s) . 1 GE Lightspeed VCT 0.226 30 000 299 20 100 411 6.5 2 GE Optima CT 520 0.178 30 000 350 20 100 698 8.9 3 GE Optima CT 660 0.167 30 000 180 20 100 368 12.6 4 GE Optima CT 660 0.167 30 000 300 20 100 1006 10.9 5 Philips Brilliance CT 16 0.130 30 000 179 12 100 986 30 6 Philips Brilliance CT 16 0.130 30 000 219 12 100 509 16 7 Philips Brilliance CT 16 0.130 30 000 263 12 100 745 15.5 8 Philips Brilliance CT 64 0.110 30 000 235 40 100 1254 10.4 9 Philips Brilliance CT 64 0.110 30 000 30 40 100 341 48.5 10 Philips Brilliance CT 64 0.110 15 000 197 40 50 992 9.6 11 Philips Brilliance iCT 0.119 30 000 138 40 100 846 12.6 12 Philips Brilliance iCT 0.119 30 000 188 12.5 100 529 13 13 Siemens Sensation Open 0.187 30 000 210 10 100 1061 19 14 Siemens Sensation 16 0.184 30 000 459 12 100 1310 11.3 15 Siemens Sensation 16 0.184 60 000 160 24 200 868 11.9 16 Siemens Definition AS 0.125 30 000 195 38.4 100 1045 18.1 17 Siemens Definition AS 0.125 30 000 91 19.2 100 472 12.3 18 Siemens Definition AS 0.125 30 000 196 4.8 100 334 16.6 19 Siemens Definition AS 0.125 30 000 189 4.8 100 272 14.2 20 Siemens Definition Flash 0.145 60 000 193 38.4 200 860 8.7 21 Siemens Definition Flash 0.145 60 000 608 24 200 1200 9.2 22 Siemens Emotion 16 0.250 30 000 160 9.6 100 618 15.1 23 Siemens Emotion 16 0.250 30 000 150 19.2 100 1238 17.9 24 Siemens Emotion 16 0.250 30 000 146 9.6 100 289 14 25 Siemens Emotion 16 0.250 30 000 140 9.6 100 319 11.3 26 Siemens Emotion 16 0.250 10 000 81 9.6 50 248 12.1 27 Siemens Volume Zoom 0.200 30 000 347 5 100 564 17 28 Toshiba Aquilion 16 0.198 15 000 230 16 50 1300 15 29 Toshiba Aquilion 16 0.198 30 000 301 8 100 1420 37.5 30 Toshiba Aquilion 32 0.198 30 000 241 16 100 1405 16.3 31 Toshiba Aquilion 64 0.198 30 000 260 16 100 2630 28 32 Toshiba Aquilion 64 0.198 30 000 250 16 100 1650 18.3 33 Toshiba Aquilion 64 0.198 30 000 301 32 100 1780 14.7 34 Toshiba Aquilion CX 0.194 30 000 200 32 100 1327 10.2 35 Toshiba Aquilion CXL 0.194 30 000 218 16 100 845 11.9 36 Toshiba Aquilion Prime 0.173 30 000 240 20 100 2480 26.3 CT scanner characteristics . TL shielding calculation concept . DLP shielding calculation concept . Dataset . Scanner type . nCTDIw,head,CTExpo (mGy/mAs) . WTL (mA·min per week) . I (mA) . c (mm) . WDLP (Gy·cm/week) . DLP (mGy·cm) . t (s) . 1 GE Lightspeed VCT 0.226 30 000 299 20 100 411 6.5 2 GE Optima CT 520 0.178 30 000 350 20 100 698 8.9 3 GE Optima CT 660 0.167 30 000 180 20 100 368 12.6 4 GE Optima CT 660 0.167 30 000 300 20 100 1006 10.9 5 Philips Brilliance CT 16 0.130 30 000 179 12 100 986 30 6 Philips Brilliance CT 16 0.130 30 000 219 12 100 509 16 7 Philips Brilliance CT 16 0.130 30 000 263 12 100 745 15.5 8 Philips Brilliance CT 64 0.110 30 000 235 40 100 1254 10.4 9 Philips Brilliance CT 64 0.110 30 000 30 40 100 341 48.5 10 Philips Brilliance CT 64 0.110 15 000 197 40 50 992 9.6 11 Philips Brilliance iCT 0.119 30 000 138 40 100 846 12.6 12 Philips Brilliance iCT 0.119 30 000 188 12.5 100 529 13 13 Siemens Sensation Open 0.187 30 000 210 10 100 1061 19 14 Siemens Sensation 16 0.184 30 000 459 12 100 1310 11.3 15 Siemens Sensation 16 0.184 60 000 160 24 200 868 11.9 16 Siemens Definition AS 0.125 30 000 195 38.4 100 1045 18.1 17 Siemens Definition AS 0.125 30 000 91 19.2 100 472 12.3 18 Siemens Definition AS 0.125 30 000 196 4.8 100 334 16.6 19 Siemens Definition AS 0.125 30 000 189 4.8 100 272 14.2 20 Siemens Definition Flash 0.145 60 000 193 38.4 200 860 8.7 21 Siemens Definition Flash 0.145 60 000 608 24 200 1200 9.2 22 Siemens Emotion 16 0.250 30 000 160 9.6 100 618 15.1 23 Siemens Emotion 16 0.250 30 000 150 19.2 100 1238 17.9 24 Siemens Emotion 16 0.250 30 000 146 9.6 100 289 14 25 Siemens Emotion 16 0.250 30 000 140 9.6 100 319 11.3 26 Siemens Emotion 16 0.250 10 000 81 9.6 50 248 12.1 27 Siemens Volume Zoom 0.200 30 000 347 5 100 564 17 28 Toshiba Aquilion 16 0.198 15 000 230 16 50 1300 15 29 Toshiba Aquilion 16 0.198 30 000 301 8 100 1420 37.5 30 Toshiba Aquilion 32 0.198 30 000 241 16 100 1405 16.3 31 Toshiba Aquilion 64 0.198 30 000 260 16 100 2630 28 32 Toshiba Aquilion 64 0.198 30 000 250 16 100 1650 18.3 33 Toshiba Aquilion 64 0.198 30 000 301 32 100 1780 14.7 34 Toshiba Aquilion CX 0.194 30 000 200 32 100 1327 10.2 35 Toshiba Aquilion CXL 0.194 30 000 218 16 100 845 11.9 36 Toshiba Aquilion Prime 0.173 30 000 240 20 100 2480 26.3 Table 2. CT scanner characteristics (scanner type and literature-based scanner output nCTDIw,head,CTExpo(8)), maximum weekly workloads (WTL and WDLP) as well as scan parameters (tube current I, collimation c, DLP and scan time t) of the 36 CT scans used for ambient DR measurements. The collimation, DLP and scan time were displayed on the operator screen, whereas the tube current was manually calculated by dividing the displayed TL per gantry rotation by the rotation time. CT scanner characteristics . TL shielding calculation concept . DLP shielding calculation concept . Dataset . Scanner type . nCTDIw,head,CTExpo (mGy/mAs) . WTL (mA·min per week) . I (mA) . c (mm) . WDLP (Gy·cm/week) . DLP (mGy·cm) . t (s) . 1 GE Lightspeed VCT 0.226 30 000 299 20 100 411 6.5 2 GE Optima CT 520 0.178 30 000 350 20 100 698 8.9 3 GE Optima CT 660 0.167 30 000 180 20 100 368 12.6 4 GE Optima CT 660 0.167 30 000 300 20 100 1006 10.9 5 Philips Brilliance CT 16 0.130 30 000 179 12 100 986 30 6 Philips Brilliance CT 16 0.130 30 000 219 12 100 509 16 7 Philips Brilliance CT 16 0.130 30 000 263 12 100 745 15.5 8 Philips Brilliance CT 64 0.110 30 000 235 40 100 1254 10.4 9 Philips Brilliance CT 64 0.110 30 000 30 40 100 341 48.5 10 Philips Brilliance CT 64 0.110 15 000 197 40 50 992 9.6 11 Philips Brilliance iCT 0.119 30 000 138 40 100 846 12.6 12 Philips Brilliance iCT 0.119 30 000 188 12.5 100 529 13 13 Siemens Sensation Open 0.187 30 000 210 10 100 1061 19 14 Siemens Sensation 16 0.184 30 000 459 12 100 1310 11.3 15 Siemens Sensation 16 0.184 60 000 160 24 200 868 11.9 16 Siemens Definition AS 0.125 30 000 195 38.4 100 1045 18.1 17 Siemens Definition AS 0.125 30 000 91 19.2 100 472 12.3 18 Siemens Definition AS 0.125 30 000 196 4.8 100 334 16.6 19 Siemens Definition AS 0.125 30 000 189 4.8 100 272 14.2 20 Siemens Definition Flash 0.145 60 000 193 38.4 200 860 8.7 21 Siemens Definition Flash 0.145 60 000 608 24 200 1200 9.2 22 Siemens Emotion 16 0.250 30 000 160 9.6 100 618 15.1 23 Siemens Emotion 16 0.250 30 000 150 19.2 100 1238 17.9 24 Siemens Emotion 16 0.250 30 000 146 9.6 100 289 14 25 Siemens Emotion 16 0.250 30 000 140 9.6 100 319 11.3 26 Siemens Emotion 16 0.250 10 000 81 9.6 50 248 12.1 27 Siemens Volume Zoom 0.200 30 000 347 5 100 564 17 28 Toshiba Aquilion 16 0.198 15 000 230 16 50 1300 15 29 Toshiba Aquilion 16 0.198 30 000 301 8 100 1420 37.5 30 Toshiba Aquilion 32 0.198 30 000 241 16 100 1405 16.3 31 Toshiba Aquilion 64 0.198 30 000 260 16 100 2630 28 32 Toshiba Aquilion 64 0.198 30 000 250 16 100 1650 18.3 33 Toshiba Aquilion 64 0.198 30 000 301 32 100 1780 14.7 34 Toshiba Aquilion CX 0.194 30 000 200 32 100 1327 10.2 35 Toshiba Aquilion CXL 0.194 30 000 218 16 100 845 11.9 36 Toshiba Aquilion Prime 0.173 30 000 240 20 100 2480 26.3 CT scanner characteristics . TL shielding calculation concept . DLP shielding calculation concept . Dataset . Scanner type . nCTDIw,head,CTExpo (mGy/mAs) . WTL (mA·min per week) . I (mA) . c (mm) . WDLP (Gy·cm/week) . DLP (mGy·cm) . t (s) . 1 GE Lightspeed VCT 0.226 30 000 299 20 100 411 6.5 2 GE Optima CT 520 0.178 30 000 350 20 100 698 8.9 3 GE Optima CT 660 0.167 30 000 180 20 100 368 12.6 4 GE Optima CT 660 0.167 30 000 300 20 100 1006 10.9 5 Philips Brilliance CT 16 0.130 30 000 179 12 100 986 30 6 Philips Brilliance CT 16 0.130 30 000 219 12 100 509 16 7 Philips Brilliance CT 16 0.130 30 000 263 12 100 745 15.5 8 Philips Brilliance CT 64 0.110 30 000 235 40 100 1254 10.4 9 Philips Brilliance CT 64 0.110 30 000 30 40 100 341 48.5 10 Philips Brilliance CT 64 0.110 15 000 197 40 50 992 9.6 11 Philips Brilliance iCT 0.119 30 000 138 40 100 846 12.6 12 Philips Brilliance iCT 0.119 30 000 188 12.5 100 529 13 13 Siemens Sensation Open 0.187 30 000 210 10 100 1061 19 14 Siemens Sensation 16 0.184 30 000 459 12 100 1310 11.3 15 Siemens Sensation 16 0.184 60 000 160 24 200 868 11.9 16 Siemens Definition AS 0.125 30 000 195 38.4 100 1045 18.1 17 Siemens Definition AS 0.125 30 000 91 19.2 100 472 12.3 18 Siemens Definition AS 0.125 30 000 196 4.8 100 334 16.6 19 Siemens Definition AS 0.125 30 000 189 4.8 100 272 14.2 20 Siemens Definition Flash 0.145 60 000 193 38.4 200 860 8.7 21 Siemens Definition Flash 0.145 60 000 608 24 200 1200 9.2 22 Siemens Emotion 16 0.250 30 000 160 9.6 100 618 15.1 23 Siemens Emotion 16 0.250 30 000 150 19.2 100 1238 17.9 24 Siemens Emotion 16 0.250 30 000 146 9.6 100 289 14 25 Siemens Emotion 16 0.250 30 000 140 9.6 100 319 11.3 26 Siemens Emotion 16 0.250 10 000 81 9.6 50 248 12.1 27 Siemens Volume Zoom 0.200 30 000 347 5 100 564 17 28 Toshiba Aquilion 16 0.198 15 000 230 16 50 1300 15 29 Toshiba Aquilion 16 0.198 30 000 301 8 100 1420 37.5 30 Toshiba Aquilion 32 0.198 30 000 241 16 100 1405 16.3 31 Toshiba Aquilion 64 0.198 30 000 260 16 100 2630 28 32 Toshiba Aquilion 64 0.198 30 000 250 16 100 1650 18.3 33 Toshiba Aquilion 64 0.198 30 000 301 32 100 1780 14.7 34 Toshiba Aquilion CX 0.194 30 000 200 32 100 1327 10.2 35 Toshiba Aquilion CXL 0.194 30 000 218 16 100 845 11.9 36 Toshiba Aquilion Prime 0.173 30 000 240 20 100 2480 26.3 For each of the 36 CT protocols presented in Table 2 and each measurement location, the DRDLP,head and DRTL,head were calculated according to Equations (6) and (13), respectively. The ratios between DRDLP, head and DRTL,head were determined to evaluate the calculation method for the TL and DLP shielding calculation concept. In order to quantitatively assess the structural shielding performance of the 36 CT rooms the ratios between the measured ambient dose rates DRmeasured and the calculated maximum allowed ambient dose rates DRDLP,head and DRTL,head were also determined. For ratios larger than 1 the structural shielding performance of the CT room did not comply with the legal requirements meaning that additional shielding was required. RESULTS Validation of the calculation method Measurements Results of the ambient DR measurements are shown in Table 1. As expected, the highest overall ambient doses were measured for Scan 4 using the highest tube current and the largest scattering volume. The variation of the ratios of the ambient dose between head and body phantom was similar for Scan 1 and 3 (from 1.25 at Position A to 0.92 at Position D, deviation of 36%) and for Scans 2 and 4 (from 2.88 at Position A to 2.13 at Position D, deviation of 35%), respectively. The ratios of the average ambient dose and of the DLP between Scans 1 and 2 were similar (1.20 vs. 1.24). The same observation holds true for the ratios between Scans 3 and 4 (0.46 vs. 0.53). These results provide strong evidence that the first key hypothesis on the linear relationship between the ambient dose and the DLP per time unit proves to be correct (Figure 3). For the identical DLP in the head and body phantom (application of Scans 2 and 4), the ratio of the average ambient dose was 2.51, while a ratio of 4 was expected to confirm the second hypothesis on the linearity between the ambient dose and the scattering volume. The contribution of the leakage radiation to the overall ambient dose was very small and ranged between 2.5 and 2.8%. Figure 3. Open in new tabDownload slide Measured ambient dose vs. DLP for Scans 1 and 2 using the head phantom (circles) and for Scans 3 and 4 using the body phantom (triangles up) according to Table 1. Two linear regression curves are plotted for the head phantom (intercept = 0.26, slope = 0.15, R2 = 0.99) and for the body phantom (intercept = 4.10, slope = 0.38, R2 = 0.99) indicating a linear relationship between ambient dose and DLP. Figure 3. Open in new tabDownload slide Measured ambient dose vs. DLP for Scans 1 and 2 using the head phantom (circles) and for Scans 3 and 4 using the body phantom (triangles up) according to Table 1. Two linear regression curves are plotted for the head phantom (intercept = 0.26, slope = 0.15, R2 = 0.99) and for the body phantom (intercept = 4.10, slope = 0.38, R2 = 0.99) indicating a linear relationship between ambient dose and DLP. Monte Carlo simulations Simulated ambient doses per incident particle are summarized in Table 3. Ambient dose for the body phantom were similar at all nine sphere positions, whereas for the head phantom ambient dose on the scanner axis (sphere position 1) was significantly lower compared to those at the other eight sphere positions. The ratio of the average ambient dose between head and body phantom was 1.50. This result is based on the same number of incident particles (same tube current) simulated for the two phantoms. For the same absorbed dose in both phantoms, the ratio of the average ambient dose between head and body phantom was calculated to be 3.09 instead of the expected ratio of 4. Table 3. Ambient doses in arbitrary units resulting from Monte Carlo simulations at the nine sphere positions for the head and body phantom.a Sphere index . Ambient doses (arbitrary units) . Ratio . . Body phantom . Head phantom . . 1 3.42 1.86 1.84 2 3.42 2.34 1.46 3 3.41 2.34 1.46 4 3.43 2.34 1.46 5 3.43 2.34 1.46 6 3.41 2.35 1.45 7 3.40 2.35 1.45 8 3.42 2.35 1.45 9 3.42 2.34 1.46 Average 3.42 2.29 1.50 Sphere index . Ambient doses (arbitrary units) . Ratio . . Body phantom . Head phantom . . 1 3.42 1.86 1.84 2 3.42 2.34 1.46 3 3.41 2.34 1.46 4 3.43 2.34 1.46 5 3.43 2.34 1.46 6 3.41 2.35 1.45 7 3.40 2.35 1.45 8 3.42 2.35 1.45 9 3.42 2.34 1.46 Average 3.42 2.29 1.50 aResults are based on the same number of incident particles (same tube current) for both phantoms. Table 3. Ambient doses in arbitrary units resulting from Monte Carlo simulations at the nine sphere positions for the head and body phantom.a Sphere index . Ambient doses (arbitrary units) . Ratio . . Body phantom . Head phantom . . 1 3.42 1.86 1.84 2 3.42 2.34 1.46 3 3.41 2.34 1.46 4 3.43 2.34 1.46 5 3.43 2.34 1.46 6 3.41 2.35 1.45 7 3.40 2.35 1.45 8 3.42 2.35 1.45 9 3.42 2.34 1.46 Average 3.42 2.29 1.50 Sphere index . Ambient doses (arbitrary units) . Ratio . . Body phantom . Head phantom . . 1 3.42 1.86 1.84 2 3.42 2.34 1.46 3 3.41 2.34 1.46 4 3.43 2.34 1.46 5 3.43 2.34 1.46 6 3.41 2.35 1.45 7 3.40 2.35 1.45 8 3.42 2.35 1.45 9 3.42 2.34 1.46 Average 3.42 2.29 1.50 aResults are based on the same number of incident particles (same tube current) for both phantoms. Application of the calculation method Results of the ratios between DRDLP,head and DRTL,head for the 36 spiral CT head protocols are presented in Figure 4. Ratios ranged between 0.57 and 2.14 with a mean value of 1.11 and a standard deviation of 0.41. No correlation was found between the ratio and the CT scanner type (data not presented). Figure 4. Open in new tabDownload slide Ratio of the calculated maximum allowed ambient DR outside the CT room between the TL shielding calculation concept (DRTL,head) and the DLP shielding calculation concept (DRDLP,head) for 36 standard clinical spiral CT head scans as described in Table 2. Figure 4. Open in new tabDownload slide Ratio of the calculated maximum allowed ambient DR outside the CT room between the TL shielding calculation concept (DRTL,head) and the DLP shielding calculation concept (DRDLP,head) for 36 standard clinical spiral CT head scans as described in Table 2. A total of 174 ambient DR measurements have been carried out in selected spaces adjacent to the 36 CT rooms. For the DLP shielding calculation concept, eight measured ambient DRs exceeded the calculated maximum allowed ambient DRs; for the TL shielding calculation concept, this was the case for seven measurements (Figure 5). Ratios between measured and calculated ambient DRs ranged between 0.001 and 45.23 for the TL shielding calculation concept and between 0.001 and 49.176 for the DLP shielding calculation concept. Regression analysis clearly showed the validity of the calculation method for both the TL and DLP shielding calculation concepts (R2 = 0.98). Figure 5. Open in new tabDownload slide Scatter plot showing the ratio between the DRmeasured and the calculated maximum allowed ambient DRs for the TL shielding calculation concept (DRTL,head) and the DLP shielding calculation concept (DRDLP,head). Ratios are presented as empty circles. For ratios lying outside of the dashed area, the room shielding was not sufficient. In addition, a linear regression curve is plotted (intercept = 0.0005, slope = 1.033, R2 = 0.98). For purposes of clarity of the figure, both axes are logarithmically scaled. Figure 5. Open in new tabDownload slide Scatter plot showing the ratio between the DRmeasured and the calculated maximum allowed ambient DRs for the TL shielding calculation concept (DRTL,head) and the DLP shielding calculation concept (DRDLP,head). Ratios are presented as empty circles. For ratios lying outside of the dashed area, the room shielding was not sufficient. In addition, a linear regression curve is plotted (intercept = 0.0005, slope = 1.033, R2 = 0.98). For purposes of clarity of the figure, both axes are logarithmically scaled. DISCUSSION In this study, a calculation method is developed to quantitatively assess the structural shielding performance of CT rooms. The concept of this method is based on the comparison between the maximum allowed and measured ambient DR in spaces adjacent to the CT room. If the measured ambient DR is lower than the calculated maximum allowed ambient DR, the structural shielding performance is sufficient, otherwise the CT room must be additionally shielded. This method provides an important tool for the radiation protection authority to check on-site the compliance of the structural shielding with legal requirements and in case of non-compliance allows to demand immediate corrective measures. Alternative methods to check the structural shielding performance as, for example, placing thermoluminescence dosemeters or DR meters at critical positions outside the CT room and monitoring ambient doses over a certain time period are much more time-consuming and expensive than the calculation method proposed in this study. The method is developed for two different concepts of calculating CT room shielding in Switzerland, an old concept based on the TL of single-slice CT scanners and a new concept based on the accumulated DLP of multi-detector CT scanners(3, 6). The advantage of the method based on the DLP shielding calculation concept is that it provides a more accurate estimation of the emitted scattered radiation since the effects of relevant beam modifying parameters such as the tube voltage, beam collimation, beam filtration and scanner output are directly reflected in the DLP. Thus, the DLP method remains also valid for modern CT scanners using tube current modulation and dual-source technology. This makes the DLP method the gold standard for the quantitative assessment of the structural shielding assessment of CT rooms in Switzerland. For the TL shielding calculation concept, the effect of beam collimation and scanner output is taken into account by introducing correction factors. The effect of tube voltage on the ambient DR is not considered since most of the CT examinations are performed at 120 kV. For those rare CT examinations with a tube voltage of 140 kV, an additional correction factor could be applied. However, compared to the overall uncertainties of the method, the impact of the tube current correction can be neglected. According to Equation (3), the effect of beam collimation on the ambient DR is assumed to be linear, which is also supported by literature(7). The beam collimation factor is defined as the beam collimation of the CT scan normalized to 5 mm. The normalization is necessary due to historical reasons. The development of the TL shielding calculation concept is based on a median beam collimation of 5 mm, which is supposed to be representative for the operation of most of the single-slice CT scanners in Switzerland at that time. Thus, applying CT scans with beam collimations larger than 5 mm will cause proportionally more scattered radiation being emitted meaning that more shielding is required, while for CT scans with beam collimations smaller than 5 mm, proportionally less scattered radiation is emitted and less shielding is required. Since the DLP already reflects the effect of beam collimation on the ambient DR, no correction factor is required for the DLP shielding concept. As expected, the calculated numerical values of neqCTDIw derived from Equation (8) differ from the numerical values of the real scanner outputs nCTDIw(8–10). More specifically, when using Equation (11) and the equivalent workloads of 60 000 mA·minute per week vs. 200 Gy·cm per week, 30 000 mA·minute per week vs. 100 Gy·cm per week or 15 000 mA·minute per week vs. 50 Gy·cm per week (Table 2), the equivalent workload scanner output for the head phantom neqCTDIw,head is calculated as a constant value of 0.222 mGy/mAs which is, for example, significantly higher than the scanner output for the Philips Brilliance CT series(8–10). This shows the importance of an appropriate scanner output correction as defined in Equations (9) and (10). The scan volume presents one of the key parameters that has a major impact on the emission of scattered radiation for both the TL and DLP shielding calculation concept. To simulate the influence of the scan volume on the scattering behaviour in clinical situations as accurately as possible, an anthropomorphic phantom should be used. However, these phantoms are not readily available and for regular supervisory activities not very practical. Moreover, the calculation of the maximum allowed ambient DR using these phantoms is a complex function of the emission angle of the scattered radiation. For these reasons, a cylindrical PMMA head phantom is used for ambient DR measurements. The head phantom is considered suitable for simulation of the scattering behaviour for CT head scans since material density, shape and volume are similar to that of the human head. Monte Carlo simulations show no differences in ambient dose between sphere Positions 2–9 for both phantoms. This is expected since the spheres are located symmetrically around the head and body phantom at the same distance from the isocenter. However, for the head phantom, the ambient dose at sphere Position 1 is significantly lower compared to the ambient dose at the other sphere positions. One possible explanation might be the directional dependence of the absorption of X-rays within the phantom. Since for the simulations only the central plane perpendicular to the scanner axis is irradiated, the absorption of X-rays is the highest along the scanner axis. A similar result is observed for the measurements (Table 1). For Scans 1 and 2 using the head phantom, the measured ambient dose is the lowest at Position A on the scanner axis, whereas using the body phantom, this difference is not observed. The percentage variation of the ratios of the measured ambient dose between head and body phantom is similar when using the same tube current and DLP, respectively (36 vs. 35%). This is explained by practical difficulties in accurately aligning and positioning the ionisation chamber at the correct distance from the isocenter. Due to the inhomogeneous scattered radiation field, even small deviations from the nominal distance of 1 m have a major influence on the measured ambient DR. Obviously, this is not the case for Monte Carlo simulations, and therefore these results are more consistent. The two key hypotheses stating that the ambient DR is linearly related both to the DLP per time unit and to the scattering volume are only partially confirmed. While the first hypothesis proved to be correct, the second hypothesis is confirmed neither by the measurements nor by the Monte Carlo simulations. It is expected that the ambient DR when scanning the body phantom would be four times higher than the ambient DR when scanning the head phantom, provided that the same DLP was absorbed in both phantoms. Measurements show a ratio between the ambient DRs for the head and body phantom of 2.5 and Monte Carlo simulations a ratio of 3.09, which is significantly lower than the expected value of 4. These differences between measurements, Monte Carlo simulations and expectation can be explained by several reasons. One reason might be that the second key hypothesis on the linear relationship between the ambient DR and the scattering volume holds only true for small volumes. For X-ray energies applied in diagnostic imaging, incident photons are either fully absorbed within the phantom (photoelectric effect) or are inelastically scattered by the electrons (Compton effect). Thus, even if the same energy is absorbed within the phantom, the ambient dose of the scattered radiation can be different, depending on which the two effects is more dominant. In soft tissue, the higher the photon energy, the more dominant the Compton effect becomes and the more scattered radiation is emitted(12). However, if the volume gets larger, the number of interactions and thus the energy loss of the photons increases. This in turn results in a decreased emission of scattered radiation. Other reasons that might explain the observed differences in the ratio of the ambient DRs between measurements, Monte Carlo simulations and expectation are the inaccurate alignment and positioning of the ionisation chamber during measurements and the lack of adequate information on scanner hardware to simulate the scanner geometry and the beam properties with sufficient accuracy. However, even though the two key hypotheses have not been confirmed exactly by the measurements and simulations, from a radiological protection point of view, they represent a conservative but valid upper limit for the calculation of the limit ambient DR. Overall, the calculated maximum allowed ambient DR for the two shielding calculation concepts shows a good agreement (Figure 4). Ideally, for each of the 36 CT head protocols the calculated maximum allowed ambient DR would be identical for both shielding concepts. Deviations from the expected ratio of 1 is caused most probably either by a discrepancy between the scanner output applied from literature(8) and the real scanner output of the CT scanners included in the study or by an incorrect registration of the scan parameters during the on-site audits. The scanner-specific leakage radiation does not explain the variability of the results. Results show that <3% of the overall ambient DR is caused by leakage radiation. It is the scattered radiation that provides the largest contribution to the ambient DR(13). The calculation method has been successfully applied to quantitatively assess the structural shielding performance of 36 CT facilities in diagnostic radiology departments in Switzerland (Figure 5). Most of the structural shielding proved to be sufficient. Only 7 out of a total of 174 ambient DR measurements exceed the calculated maximum allowed ambient DR by a factor between 1.3 and 45.2 for the TL shielding calculation concept and by a factor between 1.8 and 49.2 for the DLP shielding calculation concept. The largest deviations from the maximum allowed ambient DR are observed behind those room boundaries, where in contrast to the shielding calculation, no lead shielding is installed. Smaller deviations from the maximum allowed ambient DR are caused by an incomplete overlap of the lead shielding in double-leaf doors or door frames. Doors in general, no matter of which type (sliding doors, swing doors) represents those parts of the room boundary with the highest demands on proper shielding construction. CONCLUSION A semi-analytical calculation method was developed to quantitatively assess the structural shielding performance of CT rooms in Switzerland. The method was validated both by measurements and Monte Carlo simulations and was successfully applied during 36 on-site audits in private and public hospitals as well as radiological institutes operating a CT scanner. Thereby, areas where the room shielding was not sufficient could be easily and quickly localized. This method provides a valuable tool for the radiation protection authority to check the compliance of the structural shielding with the legal requirements during on-site audits. REFERENCES 1 ICRP . 1990 Recommendations of the International Commission on Radiological Protection. ICRP Publication 60. Ann. ICRP 21 (1–3) ( 1991 ). 2 Federal Department of Home Affairs . Radiological Protection Ordinance. SR 814.501 ( 1994 ). 3 Federal Department of Home Affairs . X-ray ordinance. SR 814.542.1 ( 1998 ). 4 DIN 6812:1996-04 . Medical X-ray equipment up to 300 kV—radiation protection rules for installation ( 1996 ). 5 Verdun , F. 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TI - Calculation of the maximum allowed ambient dose rate outside CT rooms to quantitatively assess the structural shielding performance
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncw131
DA - 2017-04-25
UR - https://www.deepdyve.com/lp/oxford-university-press/calculation-of-the-maximum-allowed-ambient-dose-rate-outside-ct-rooms-ewqX4dsDgX
SP - 226
VL - 174
IS - 2
DP - DeepDyve
ER -