USE OF DLP FOR ESTABLISHING THE SHIELDING OF MULTI- DETECTOR COMPUTED TOMOGRAPHY ROOMS

Transcription

1 USE OF DLP FOR ESTABLISHING THE SHIELDING OF MULTI- DETECTOR COMPUTED TOMOGRAPHY ROOMS F.R. Verdun 1, A. Aroua 1, P.R. Trueb 2, F.O. Bochud 1* 1 University Institute for Radiation Physics, Switzerland 2 Federal Office of Public Health, Switzerland Abstract. The aim of the present work is to draw the attention of those who are in charge of radiation protection in the medical field to the necessity of taking into account the recent advances in CT technology when designing the shielding of CT rooms and using the DLP instead of the tube loading in ma.min when calculating the amount of scattered radiation. The approaches of the German Institute for Standardization (DIN) and the US National Council on Radiation Protection & Measurements (NRCP) were compared and a series of radiation measurements were performed in several CT rooms at the Lausanne University Hospital. The DLP was found to be more appropriate to use since it gives results that depend only on the size of the scanned object and are independent of the high voltage and the beam collimation. KEYWORDS: radiation shielding, multi-detector CT, diagnostic room, scattered radiation 1. Introduction The issue of scattered radiation around modern multi-detector CT units is of current concern and several attempts to reduce it in order to lower the exposure of the patient and the practitioner/operator, particularly during long interventional procedures such as CT fluoroscopy, or to increase the contrastto-noise ratio are reported in the literature [1-4]. But even with a reduced scattered radiation, the shielding of the CT room is an important task of radiation protection. In the design of a proper shielding of a CT room the accurate determination of the spatial distribution of scattered radiation is a necessary and various models have been proposed in the literature for this purpose [5-9]. Of particular interest are the models proposed by the US National Council on Radiation Protection and Measurements (NCRP) [10, 11] and German Institute for Standardization (DIN) [12] which are the basis for shielding calculation in many countries in the world. Many works were dedicated to the comparison of the various models [13-15] or to the comparison of the data obtained by these models and that provided by CT manufacturers [4, 16, 17]. In Switzerland the shielding of diagnostic rooms with CT scanners is designed today based on the Ordinance on Medical X-rays Units (ORX) [18], which uses the same model as that proposed by DIN and indicates the tube loading needed for a CT slice, expressed in ma.min, for a given anatomical region to be examined with several minimal loading values depending on the size of the hospital. The aim of this work is to compare the DIN model (thus the formalism used in Switzerland) and the NCRP model, to investigate the limits of using the tube loading in ma.min as the working parameter for new CT units and to explore the appropriateness of using the DLP instead. 2. Methods 2.1 The DIN Concept According to the DIN Standard, the calculation of the equivalent dose of primary radiation (Nutzstrahl H N ) is performed as follows: H N = W. H A,1. (1/x 2 ) Where * Presenting author, francois.bochud@chuv.ch 1

2 W H A,1 x is the work load in ma.min/week is the equivalent dose at 1 m per unit current loading in msv/ma.min is the distance between the focal point and the measurement point The minimal work load for CT unit given in the DIN Norm is 20'000 ma.min/week, and the equivalent dose at 1 m per unit current loading given for 120 kv and 2.5 mm Al is 13 msv/ma.min, i.e msv/mas. The calculation of the equivalent dose of scattered radiation (Streustrahlung H S ) is performed as follows: H S = W. H A,1. (1/a 2 ). f K. (1/s 2 ) Where a s f K is the distance between the focal point and the centre of the scattering phantom is the distance between the centre of the scattering phantom and the measurement point is the scatter yield (f K = for CT) Moreover, according to the DIN standard, the radiation transmitted across the tube housing is taken into account by multiplying the f K factor by a coefficient f D equal to 3 for CT. H S = W. H A,1. (1/a 2 ). f K. (1/s 2 ). f D (Eq. 1) 2.2 The NCRP Concept In the NRCP formalism the air kerma is used in the calculation of the secondary radiation (scattered and transmitted) whose contribution at 1 m from the isocenter is proportional to the DLP: K trunk = κ trunk. DLP and K head = κ head. DLP (Eq. 2) The κ factors given in publication NCRP 147, based on measurements and accounting for both scattered and transmitted radiation are: κ trunk = cm -1 and κ head = cm -1 The NRCP recommends the use of the Dose Line Integral DLI instead of the DLP since it adds a multiplicative factor that varies with collimation and which is equal to 1.2 for a 32 cm test object when the collimation is 20 mm [11]. 2.3 Measurements Dose measurements were performed on the GE 64-slice CT unit of the Lausanne University Hospital (CHUV). Two CTDI test objects ( 32 cm and 16 Plexiglas phantoms) were scanned in the helical mode. The ambient dose equivalent, H*(10), was measured at various distances along the axis of the CT scanner and on a line oriented at 45 to the side of the legs of the patient. The measurement were performed using a Smartion dosemeter calibrated in terms of H*(10) using a Cs-137 beam. The collimation widths used during the acquisitions were 20 mm for the 32x0.625 configuration and 40 mm for the 64x0.625 configuration using 120 kv and 140 kv at various tube loadings.. Following these measurements, an isodose cartography was established. 2

3 3 Results and Discussion 3.1 Comparison of the NCRP and DIN formalisms From Eq. 1 the air kerma established according to the NCRP formalism is converted into ambient equivalent dose H*(10) as follows: H*(10) = θ. K with θ = 1.5 Sv/Gy Hence: H*(10) at 1 m = 54 μsv for 100 mgy.cm and the trunk H*(10) at 1 m = 13.5μSv for 100 mgy.cm and the head (Eq. 3) From Eq. 2, and according to DIN formalism, the dose of scattered radiation, H s, for a 32 cm scanned phantom is: H s = 26 μsv for 100 mas at 120 kv and for the trunk and a beam of 1 cm This quantity can be expressed in terms of DLP, which is related to the tube loading (Q) through the normalized CT dose index ( n CTDI w in mgy/mas) and the beam length (L): DLP = Q. n CTDI w. L The dose of scattered radiation, H s, is thus expressed as: H s = DLP /( nctdi w. L). H A,1. (1/a 2 ). f K. (1/s 2 ). f D Assuming a beam 1 cm wide and a high voltage of 120 kv, it follows that: H s, trunk (msv) = (msv/mgy.cm). DLP (mgy.cm) For the head, the CTDI has to be divided by a factor 2, and the same holds for the f K factor. This leads to the following equations: H s at 1 m = 240 μsv for 100 mgy.cm and for the trunk H s at 1 m = 60 μsv for 100 mgy.cm and for the head (Eq. 4) The results of Eq. 3 and 4 are summarized in Table 1. Table 1: Scattered radiation per unit DLP established according to the NCRP and DIN formalisms Model H s / DLP [μsv/(100 mgy.cm)] trunk head NCRP DIN The ratio of the contribution of secondary radiation for trunk and head is 4; this corresponds to the ratio of the total energies imparted to the phantom. It is interesting to notice that the NCRP model differs from the DIN model by a factor of about five. This difference can however by explained by two reasons. The first one is that the DIN model considers an X-ray tube that has a total filtration of 2.5 mm Al in spite of the fact that CT unit have generally a total filtration closer to 5 mm Al than 2.5. The use of a more representative total filtration would have reduced the primary dose (and thus the secondary dose) by a factor close to 2. The second is that the introduction a factor of 3 (F D ) 3

4 concerning the transmission of radiation across the tube housing that is difficult to justify. Thus, the value proposed by the DIN standard appears to be highly overestimated when considering the actual technology of X-ray tubes used in CT units. Taking into account the limitation of the DIN standard approach we recommend to use a single value for the κ factor for the trunk, independent of the high voltage and equal to 0.5 μsv/(mgy.cm), to convert the DLP into ambient equivalent dose à 1 m, and to divide this factor by 4 in the case of the head. 3.2 Measured data Figure 1 presents the fraction of the scattered radiation at an angle of 45 for 100 mas and for a 32 cm scanned phantom. Two tube voltages (120 and 140 kv) are explored. The figure shows, as expected, that the ambient equivalent dose H*(10) varies with the inverse of the square of the distance. It indicates also that at a distance of 1 m from the isocentre and for a high voltage of 140 kv and a tube loading of 100 mas H*(10) equals 19.3 μsv and 9.7 μsv for a 40 and 20 mm beam collimation respectively. This result confirms that the scattered radiation is proportional to the value of the collimation used. Figure 1 shows also that for the same beam collimation the tube voltage change from 120 to 140 kv is reflected in an increase of the scattered radiation by about 50%. This increase is in agreement with the increase of the normalized weighted CTDI ( n CTDI w ), also by a factor of 50% after this voltage change. This variation of the fraction the scattered radiation for the same tube loading confirms the inappropriateness of using only the tube loading to estimate the dose of scattered radiation around a CT unit. Figure 1: Dose of the scattered radiation per100 mas, at an angle of 45 and for a 32 cm scanned phantom and 2 voltages (120 and 140 kv). 20 mas, 140 kv, beam 40 mm mas, 140 kv, beam 20 mm mas, 120 kv, beam 40 mm Y = M0 + M1*x +... M8*x 8 + M9*x 9 M M M R 1 H*(10) in μ Sv per 100 mas (test object 32 cm in diameter) Y = M0 + M1*x +... M8*x 8 + M9*x 9 M M M R 1 Y = M0 + M1*x +... M8*x 8 + M9*x 9 M M M R distance at 45 (m) Figure 2 presents the same data as Fig 1 for 32 cm and 16 cm test objects but is expressed in terms of DLP instead of tube loading (per 100 mgy.cm rather than per 100 mas). The figure shows that for a given diameter the dose of scattered radiation does not depend on the collimation and register a very weak dependence with high voltage (3% difference between 120 and 140 kv). The figure indicates 4

5 also that the dose of scattered radiation, at 1 m for a 32 cm test object, is 31 μsv/100 mgy.cm (54 μsv/100 mgy.cm with the NCRP model and 240 μsv/100 mgy.cm with the DIN model); for a 16 cm test object it equals 10.5 μsv/100 mgy.cm (13.5 μsv/100 mgy.cm with the NCRP model and 60 μsv/100 mgy.cm with the DIN model). A difference of a factor of 3, instead of 4 as indicated above, is found between the 32 cm and 16 cm test objects. This could be explained by the partial re-absorption of the scattered radiation within the 32 cm phantom. Figure 2: Dose of scattered radiation per 100 mgy.cm, at an angle of 45, scanned phantoms of 32 cm (120 and 140 kv) and 16 cm (140 kv) Test object 32 cm, at 45, beam 40 mm kv Test object 32 cm, at 45, beam 40 mm kv Test object 32 cm, at 45, beam 20 mm kv Test object 16 cm, at 45, beam 40 mm kv Sv /100 mgy.cm distance at 45 (m) Figures 3 and 5 present the isodose curves in terms of H*(10) of scattered radiation obtained at a high voltage of 120 kv around Plexiglas cylindrical phantoms of 32 cm (Figure 3) and 16 cm (Figure 5). The results at 140 kv are identical to those at 120 kv. The measurements were performed each 30 cm in a Cartesian system of coordinates up to a distance of 2.1 m. The H*(10) dose profiles as a function of distance both along the axis of the scanner and at an of 45 are presented in Figure 4 for the 32 cm phantom and in Figure 6 for the 16 cm one. The figures vary slightly with the direction of the measurements. The measurements are interpolated by the following functions of the distance (x): 32 cm phantom, along the axis: H*(10) (μsv/100 mgy.cm) = x cm phantom, at an angle of 45 : H*(10) (μsv/100 mgy.cm) = x cm phantom, along the axis: H*(10) (μsv/100 mgy.cm) = 9.64 x cm phantom, at an angle of 45 : H*(10) (μsv/100 mgy.cm) = x According to these results, the scattered radiation around the CT decreases as 1/x 1.95 for the 32 cm phantom and as 1/x 1.85 for the 16 cm one. The exponents being close to each other, a variation with the inverse of the square of the distance will be assumed. In Figures 4 and 6, the results obtained using the NCRP and DIN formalisms for a distance of 1 m are plotted for comparison. The figure obtained by NCRP model is close to the measured data. 5

6 Figure 3: H*(10) isodose curves obtained at a high voltage of 120 kv with a 32 cm test object Figure 4: H*(10) as a function of distance along the axis of the scanner and at an angle of 45 ( 32 cm test object and 120 kv) 6

7 Figure 5: H*(10) isodose curves obtained at a high voltage of 120 kv with a 16 cm test object Figure 6: H*(10) as a function of distance along the axis of the scanner and at an angle of 45 ( 16 cm test object and 120 kv) 7

8 3 Conclusions In Switzerland the tube loading needed for a CT slice, expressed in ma.min, for a given anatomical region to be examined is used to establish the dose of scattered radiation and thus to design the necessary shielding. If this method was appropriate for single-detector computed tomography (SDCT), with sequential scanning, it already became problematic with the introduction of spiral mode acquisition, since with a volume scanning as many slices as desired can be reconstructed. With the steady increase of the X-ray beam collimation width in modern CT units, the current method needs to be replaced with a more robust one in order to assure sufficient shielding. The DLP should be used since it leads to results independent of the collimation and the high voltage, and which depend only on the size of the scanned object. Acknowledgements The authors would like to express gratitude to the Swiss Federal Office of Public Health for supporting this work. REFERENCES [1] Sudheendra D. Diagnostic and Interventional CT Shielding: A Dramatic Decrease in Scattered Radiation for Patients. Presented at the 31st Scientific Meeting of the Society of Interventional Radiology (SIR), Toronto 30 March - 4 April (2006). [2] Neeman Z, Dromi SA, Sarin S, Wood BJ. CT fluoroscopy shielding: decreases in scattered radiation for the patient and operator. J Vasc Interv Radiol. 17(12): (2006). [3] Endo M, Mori S, and Tsunoo T. Magnitude and effects of x-ray scatter in a 256-slice CT scanner. Med. Phys. 33(9): (2006). [4] Burrage JW, Causer DA. Comparison of scatter doses from a multislice and a single slice CT scanner. Australas Phys Eng Sci Med. 29(3):257-9 (2006). [5] Simpkin DJ. Transmission of scatter radiation from computed tomography (CT) scanners determined by a Monte Carlo calculation. Health Phys. 58(3):363-7 (1990). [6] Harpen MD. An analysis of the assumptions and their significance in the determination of required shielding of CT installations. Med. Phys. 25(2): (1998). [7] Brunette JJ. Structural Shielding Design for Medical X-ray Imaging Facilities. Health Phys 89(2):183 (2005). [8] Lewis M. Radiation dose issues in multi-slice CT scanning. ImPact, London UK (2005). [9] Sheahan N. Shielding of multi-slice computed tomography facilities. Presented at the 8th meeting of the CT Users Group. Queen's Medical Centre, Nottingham, 16th November [10] National Council on Radiation Protection and Measurements (NCRP). Structual Shielding Design for Medical X-Ray Imaging Facilities. NCRP Report No. 147, Bethesda (2004). [11] Martin MC. Diagnostic X-Ray Shielding Multi-Slice CT Scanners Using NCRP 147 Methodology. Presented at the AAPM Annual Meeting, Continuing Education Refresher course. Orlando, August 3, [12] Deutsches Institut für Normung. Medizinische Röntgenanlagen bis 300 kv - Regeln für die Auslegung des baulichen Strahlenschutzes. DIN 6812 Entwurf [13] Noto K, Sota T, Koshida K, Suzuki S. Comparison of shielding calculations for diagnostic X-ray equipment. Nippon Hoshasen Gijutsu Gakkai Zasshi. 59(8): (2003). [14] Larson SC, Goodsitt MM, Christodoulou EG, Larson LS. Comparison of the CT scatter fractions provided in NCRP Report No. 147 to scanner-specific scatter fractions and the consequences for calculated barrier thickness. Health Phys. 93(2): (2007). [15] Smyth J. The black art of CT room design: comparison of BIR and NCRP shielding methods for two multislice CT scanners. Presented at the 8th meeting of the CT Users Group. Queen's Medical Centre, Nottingham, 16th November [16] Van Every B, Petty RJ. Computer tomography radiation scatter. Australas. Phys. Eng. Sci. Med. 15:15-24 (1992). [17] Langer SG, Gray JE. Radiation shielding implications of computed tomography scatter exposure to the floor. Health Phys 75(2):193-6 (1998). [18] Swiss Ordinance on Medical X-ray Units (ORX), Federal Office of Public Health, Bern (1998). 8