Calculating shielding requirements in diagnostic X-ray
departments
M PETRANTONAKI, MSc, C KAPPAS, PhD, E P EFSTATHOPOULOS,...
room should be determined. The recommendation
of NCRP 49 [1] that for the range of energies used
in diagnostic radiology, ...
di¡erent approach, where the above assumption
would be ignored.
Consider a single radiation source emitting pri-
mary, sca...
thickness x2 of material 2. The same transmission
of the X-ray beam through thickness x1 of material
1 corresponds to a th...
for one sample case, are shown in Figures 3, 4
and 5.
Discussion
Transmission through a barrier is determined as
the ratio...
deserves a di¡erent approach, where the above
assumption would be ignored.
The methodology described in this paper allows
...
lead, it is possible to replace this with a precise and
accurate technique which assumes little and allows
a variety of mo...
Upcoming SlideShare
Loading in …5
×

Calculating shielding requirements in diagnostic xray departments

2,929 views

Published on

Published in: Education
1 Comment
0 Likes
Statistics
Notes
  • A very impressive article. Well prepared. Very motivating!! Set off on to way. Thanks for sharing nice info.
       Reply 
    Are you sure you want to  Yes  No
    Your message goes here
  • Be the first to like this

No Downloads
Views
Total views
2,929
On SlideShare
0
From Embeds
0
Number of Embeds
7
Actions
Shares
0
Downloads
79
Comments
1
Likes
0
Embeds 0
No embeds

No notes for slide

Calculating shielding requirements in diagnostic xray departments

  1. 1. Calculating shielding requirements in diagnostic X-ray departments M PETRANTONAKI, MSc, C KAPPAS, PhD, E P EFSTATHOPOULOS, MSc, Y THEODORAKOS, BSc and G PANAYIOTAKIS, PhD Department of Medical Physics, School of Medicine, University of Patras, 265 00 Patras, Greece Abstract. Structural radiation protection for diagnostic X-ray facilities is most commonly per- formed following the recommendations of the National Council on Radiation Protection and Measurements Report No. 49. A number of analytical methods have already been developed to improve the design of these facilities. Speci¢cally, these methods reassess shielding calculations in X-ray areas with respect to the methodology of the calculation of the barrier thickness and the number of sources considered in the area. Thus, they generate an overall solution for the cases met at the medical radiation structural design. This paper presents an extension of an existing method for calculating shielding requirements, for multiple X-ray tubes in a room operated at var- ious beam qualities. The methodology computes the required shielding thickness such that the exposure behind it stays below a desired value. The presented method eliminates the overestimation of added shielding thickness which may occur using the other methods already mentioned. A user- friendly windows-based program has also been developed to assist shielding computations. Shielding requirements for diagnostic radiation protective barriers are usually speci¢ed following the calculation concept of the National Council on Radiation Protection and Measurements (NCRP) Report No. 49 [1]. The methodology pro- posed by NCRP calculates the exposure levels of primary, scatter and leakage radiation emitted from an individual X-ray source, giving no speci¢c guidance to account for the combination of all dif- ferent radiation £uxes through the barrier, except the general ``add one half value layer (HVL)'' approximation. This has been proved by Archer et al [2] to be an arbitrarily conservative addition, leading the weekly exposure of personnel to be reduced more than required according to the designed maximum permissible dose (MPD) lim- its. The design and shielding barriers in a diagnos- tic X-ray department generally follows the ALARA principle. That means that, in practice, the exposure levels are kept ``as low as reasonably achievable'', taking into consideration economical and technical factors. Additionally, the calculation of barrier requirements include many uncertainties (e.g. the workload, the actual kVp used etc.). In an e¡ort to improve and extend the methodol- ogy described in NCRP 49 [1], Archer et al [2] have describeda¢rstapproachtodeterminethe``precise'' secondary barrier thickness required to meet the designcriteria.Theyhavetriedtoeliminatetheresul- tant overshielding barrier of NCRP 49 [1] and to allow greater accuracy in the computation proce- dure.Theirapproachhasbeenextendedandgeneral- ized by McGuire [3], who illustrated methods for performing shielding calculations for multiple sources of radiation in a diagnostic room, operating atthe same tube potential (kVp) values. McGuire [3] further extended his general solution to enable addi- tional shielding to be calculated and added to an existing protective barrier, while Simpkin [4] pro- posed a method to account for more than one X-ray sourcebeinglocatedinaroom,operatingatdi¡erent maximum tube potential (kVp) values. This paper generalizes these techniques, propos- ing a method to solve the problem of accurate calcu- lation of the required thickness of shielding material to be added in front of an existing protective barrier which provides insu¤cient shielding in a diagnostic X-ray room. Additionally, a user-friendlyWindows- based program has been implemented, based on the proposed method, to compute shielding require- ments of a diagnostic X-ray room. Methods and materials Theoretical background The required protective barrier thickness in a diagnostic X-ray room can be calculated comple- tely from a single equation proposed by McGuire [3]. The method requires that the weekly exposure radiation levels from primary, scatter and leakage radiation emitted from each X-ray source in the Received 13 November 1997 and in revised form 7 August 1998, accepted 24 August 1998. Address correspondence to George Panayiotakis. This work has been supported by the Greek Ministry of Health (K.E.S.Y. contract E/133/95). The British Journal of Radiology, 72 (1999), 179^185 E 1999 The British Institute of Radiology 179The British Journal of Radiology, February 1999
  2. 2. room should be determined. The recommendation of NCRP 49 [1] that for the range of energies used in diagnostic radiology, scatter radiation is essen- tially of the same quality and penetrating ability as primary beam, has been followed and the required shielding is then predicted by working out each weekly exposure contribution to the bar- rier, taking into account the di¡erences in trans- mission characteristics of leakage and primary/ scatter radiation. These have been generalized by the equation: P…x† ˆ ˆn iˆ1 PijTij…x† …1† where P(x) is the total weekly exposure emitted from n X-ray sources in the room at a point behind a barrier of thickness x, which should be equal to the design MPD for adequate protection and Pij is the exposure from source i, radiation type (beam quality) j. The i sources of radiation exposure include primary, scatter and leakage radiation from all the X-ray tubes in the room. The radiation type j concerns the maximum operating potential of the X-ray source and does not vary in the above equation, providing the constraint that all the X- ray tubes in the room operate at the same maxi- mum tube potential (kVp) values. Tij(x) represents the transmission characteristics of source i, radia- tion type j, through the barrier material of thick- ness x. It is determined, for primary and scatter radiation, by the use of Archer's non-linear ¢tting routine for the K curves of NCRP 49 by normal- izing them with Ko, the radiation output (mSv mA^1 min^1 at 1 m) of the primary X-ray beam in free space. K characterizes attenuation of X-rays for a certain material. The K curves for lead and concrete and for potentials of 50 kV to 3 MV exist in appendix D of NCRP 49 [1]. Transmission of leakage radiation is expressed in terms of HVLs, with a ¢rst order exponential equation of the form: TLj…x† ˆ exp…{xln2aHVLj† …2† TLj…x† represents the transmission characteristics of leakage radiation of radiation type j. The above method has the advantage of approaching the problem of additional shielding calculation, when structural shielding already exists, providing an estimate for a material 1 of thickness x1 necessary to be added in front of a barrier, constructed from a material 2 of known thickness x2, so as to satisfy the shielding requirements. The beam is considered to be transmitted through the ¢rst material of thickness x1 and, afterwards, it is assumed to be hardened so much that the passage of all the three types of radiation through the material 2 of thick- ness x2 can be described in terms of number of HVLs. Therefore, knowing the existing thickness x2, x1 can be approximated by solving the equa- tion: The algorithm Shielding requirements of a room with many X- ray sources of di¡erent radiation types Equation (1) accounts for more than one X- ray source in a room, of the same radiation type j. However, for a room with M X-ray sources oper- ating at various maximum tube potential (kVp) values and for the simplest case of no structural shielding in place, Equation (1) must be extended and rewritten as: P À x Á ~ ˆM jˆ1  PPjTPj…x Á zPSjTSj À x Á zPLjTLj À x Áà (4) or, analytically and using Equation (2), P À x Á ~ ˆM jˆ1 n PPj  Kuxj À x Á aKoj à zPSj  Kuxj À x Á aKoj à zPLjexp À {xln2aHVLj Áo (5) where PPj, PSj and PLj are the unshielded expo- sure levels from primary, scatter and leakage radiation emitted from each tube of radiation type j present in the room and de¢ned by McGuire [3], including the ``use factor'' contribu- tion. Kux are the values of K parameterized to ¢t to Archer's model and are material dependent as well as a function of material thickness x and kVp values. The calculation problem is then to ¢nd the necessary barrier thickness x which would satisfy the above equation when P(x)5MPD. Shielding requirements when existing protection is in place For the case of shielding already in place, according to McGuire's methodology, the beam is assumed to have been highly ¢ltered by passage through the ¢rst material of thickness x1 and being hardened, it is transmitted through the next material of thickness x2 in order that the weekly exposure can meet the protection criteria. However, the appropriate material thickness for a beam to be hardened cannot be exactly known and the method leads to an overestimation of added material, as will be shown in the results section. The problem therefore deserves a P…x1†~  PPjTPj À x1 Á zPSjTSj À x1 Á zPLjTLj À x1 Áà exp À {x2ln2aHVL2j Á (3) M Petrantonaki, C Kappas, E P Efstathopoulos et al 180 The British Journal of Radiology, February 1999
  3. 3. di¡erent approach, where the above assumption would be ignored. Consider a single radiation source emitting pri- mary, scatter and leakage radiation. Since the unshielded weekly exposures PP, PS and PL in the area to be protected are known, one can calculate, using Equation (5), the thickness of material x needed to reduce the total weekly exposure to MPD. Referring to Figure 1, for passage of primary and scatter radiation through two adja- cent slabs of identical material and with the thick- ness x2 of existing shielding material to be known, the thickness x1 of the same material to be added in front of x2 can be estimated through a di¡erent approach from that described by McGuire. The transmission of primary or scatter radiation PP/S through x1 can be expressed as: P'PaS~TPaS À x1 Á PPaS~  Kux À x1 Á aKo à PPaS …6† where P9P/S is the radiation exposure behind x1 thickness. Leakage radiation exposure behind the slab thickness x1 would be equal to: P'L ˆ exp…{x1ln2aHVL1†PL …7† After passing the ¢rst thickness x1, the primary and scatter radiation has already been attenuated from Ko to Kux(x1) and they will continue to be transmitted through x2 up to Kux(x1+x2). The exposure levels of primary, scatter and leakage radiation behind the total barrier then equals to: P''PaS~TPaS À x2 Á P'PaS ~  Kux À x1zx2 Á aKux À x1 Áà P'PaS …8† P''L~TL…x2†P'L~exp…{x2ln2aHVL2†P'L …9† and the total exposure to the occupied area would be: PT ~P''PzP''SzP''L ~ À Kux À x1zx2 Á aKo ÁÀ PPzPS Á zexp  { À x1zx2 Á ln2aHVL à PL (10† where HVL is the half value layer at high attenua- tion, common for both slabs which is dependent on both material and tube potential. Knowing x2, the thickness x1 can be calculated so that PT is equal to the design MPD. To extend the above concept, consider the e¡ect of putting a slab of material 1 in front of the exist- ing barrier of di¡erent material 2. Referring to Figure 2, suppose that x1 is the thickness of mate- rial 1 required to be placed in front of an existing Figure 1. Qualitative representation of the transmis- sion of primary and scatter radiation through two adja- cent slabs of the same material with thickness x1 and x2. x is the total thickness shielding requirement. PP/S is the primary or scatter radiation exposure to the bar- rier. P9P/S is the transmitted radiation exposure of PP/S through the wall slab of thickness x1 and P0P/S is the transmitted radiation exposure of P9P/S through the wall slab of thickness x2. Figure 2. Qualitative representation of the primary and scatter transmission curves through two di¡erent materials. x1 is the thickness of material 1 that is required to be placed in front of an existing barrier of thickness x2 of material 2, so that the exposure at a point behind the protective barrier is equal to the maxi- mum permissible dose (MPD) limit. The same trans- mission of the X-ray beam through thickness x1 can be obtained by a thickness x3 of material 2. Shielding in X-ray departments 181The British Journal of Radiology, February 1999
  4. 4. thickness x2 of material 2. The same transmission of the X-ray beam through thickness x1 of material 1 corresponds to a thickness x3 of another material 2. The beam then can be considered to pass ¢rst through width x3 of material 2 and to continue to be transmitted through thickness x2 of the same material. Each type of radiation exposure behind the entire wall, according to Equations (6), (7), (8) and (9), would then be: P''PaS~  Kux À x3zx2 Á aKux À x3 Áà |  Kux À x1 Á aKo à PPaS …11† P''L~exp…{x2ln2aHVL2† |exp…{x1ln2aHVL1†PL …12† where Kux(x1) equals Kux(x3) being the transmis- sions of the primary or scatter beam through thick- ness x1, material 1 and thickness x3, material 2, respectively. In summary, when M X-ray tubes emit radiation of various beam qualities in a room with or without existing protection, the exposure at a point behind the protective wall can be expressed by combining Equations (4), (10), (11) and (12) as: PT ~ ˆM jˆ1 È Kuxj À x1 Á aKoj à T2j À PPzPS Á zexp À {x1ln2aHVL1j Á |exp À {x1ln2aHVL2j Á PL É …13† where T2j is the transmission of primary or scatter radiation of beam quality j through the second slab of material and it is equal to 1 when no protective barrier is present (i.e. x250). For the case of adding a thickness x1 in front of an existing thickness x2 of the same material, transmission equals Kux(x1+x2)/Kux(x1), while when a di¡erent mate- rial is in place to that to be added, transmission equals Kux(x3+x2)/Kux(x3). Using this equation, the additional protective barrier thickness x1 required to set PT to MPD can be calculated. Implementation The described method has been developed in Microsoft Visual Basic 4.0 for Microsoft Windows 95 (Microsoft Corp., Redmond, Washington, USA). It allows for input of room type and it is concerned with various diagnostic installations in the room (£uoroscopic, common radiographic, angiographic, mammographic, CT scanner etc.). The algorithm includes a simple searching routine starting from x150 and search- ing for the value of x1 that satis¢es the equation to within some acceptable error (PT ^ MPD50.000001 mSv). The values of Kuxj(x) and Koj for tube voltages of 50, 70, 100, 125 and 150 kVp and for lead, gypsum, steel and plate glass constructed materials, have been calculated with the use of Archer et al's mathematical model [2]. For this calculation the experimental determina- tion of three parameters (alpha, beta and gamma) is needed, which has been already reported by Archer et al [5]. Transmission data, for voltages of 30 and 35 kVp have been taken from Simpkin [6, 7]. Published transmission data by Simpkin [6] for concrete material have also been used. The high attenuation HVLs have been taken from Archer et al [5]. HVL values which are not provided in that paper, were estimated by calculating the value of (1n2)/a using the values of ``a'' provided by Archer et al [5] and Simpkin [6]. Parameter a, as stated by Archer et al [2, 5], is a ¢tted parameter unique for each K curve and material. Results The shielding requirements for a barrier in a general X-ray room are speci¢ed completely by Equation (13). This equation takes the form of Equation (14) of Simpkin [4] when multiple X-ray tubes in a single room and operating at di¡erent tube potential (kVp) values are considered and no existing protection is present. The two formulae are then in agreement, providing the same results. The situation that is examined draws attention to a method which allows more precise values of additional required material to be given in case of structural shielding in place. Note that the result obtained by Equation (10), when the same material thickness x1 needs to be added to the existing con- structed barrier of thickness x2, is the same as if only one slab of thickness x5x1+x2 had been con- sidered, providing a true description of the trans- mission through material 2 after passage through material 1. Moreover, Equation (11) shows that the transmission of primary or scatter radiation (through thickness x1, material 1 and thickness x2, material 2) is reduced to the one obtained when two slabs of thickness x3 and x2 from material 2 have been used. Results of the method proposed here are com- pared with that of McGuire's, for the parameter values shown in Table 1. These results are pre- sented in Tables 2 and 3, when the same or a di¡er- ent material is added to the existing one, for concrete and lead. More results are presented in Tables 4^7 for many combinations. The calcula- tion methodology and the data sources have been mentioned above. Lead and concrete materials are examined for a barrier at 2 m distance exposed to primary, scatter and leakage radiation, emitted from a radiographic tube operating at 100 kVp. The corresponding computer program screens, M Petrantonaki, C Kappas, E P Efstathopoulos et al 182 The British Journal of Radiology, February 1999
  5. 5. for one sample case, are shown in Figures 3, 4 and 5. Discussion Transmission through a barrier is determined as the ratio of exposure with to exposure without the barrier. It does not, however, express the scattering power or energy spectrum passed through it. McGuire's method and the proposed method in this paper obtain identical results, when no structural shielding is in place. However, when existing protection is present and additional shielding of the same material is required, McGuire's method leads to an overestimation and disagreement with itself. For example, a con- crete barrier of 156.1 mm is required when no exist- ing protection is present while in the case that a protective barrier of 78.1 mm being already in place, the additional shielding is calculated to be 85.5 mm requiring a total concrete barrier of 163.6 mm thickness (Table 2). Generally, as the existing barrier thickness increases, McGuire's method necessitates greater thickness than the appropriate one to be added to the existing wall, resulting in a continuously increased overestima- tion of the total required protection barrier. Even in the case of an existing barrier with thickness equal to that being calculated when no existing protection is present, McGuire's method results in additional shielding. The calculations result in zero requirements at existing thicknesses higher than that required previously. Similar remarks arise from the calculated values presented in Table 3. However, the results of both methods depend on the speci¢c order of placement of two di¡erent materials when constructing a barrier. This is due to di¡erences in attenuation character- istics of a beam as it passes through a lead^con- crete barrier compared with a concrete^lead one constructed of identical lead and concrete thick- nesses. In the case of already existing shielding, according to McGuire's methodology, the beam is assumed to have been highly ¢ltered by passage through the ¢rst material of thickness x1 and being hardened, is transmitted through the second mate- rial of thickness x2. Subsequently, x2 is calculated in order that the weekly exposure meets the protec- tion criteria. The required material thickness that would result in beam hardening is not known, and McGuire's method leads to an overestimation of added material, as has been shown in the results section (Tables 2^7). The problem therefore Table 1. General parameter values used for the calcu- lation of the results given in Tables 2 and 3 Parameter Value Exposure limit (mSv) 0.1 Use factor 1/2 Field size (cm2 ) 1000 Workload (mA min week^1 ) 1000 Tube current (mA) 4 Table 2. Examples of additional same material thick- ness required to be added (a) at an existing concrete barrier and (b) at an existing lead barrier, as calculated by McGuire's method and that proposed in this article Existing protection McGuire [3] Proposed (mm) method (a) Concrete barrier 0.0 156.1 156.1 78.1 85.5 78.1 156.1 29.1 0.0 212.1 0.0 0.0 (b) Lead barrier 0.0 2.5 2.5 1.2 1.2 1.2 2.5 0.3 0.0 3.4 0.0 0.0 Table 3. Examples of additional di¡erent material shielding requirements, as calculated by McGuire's method and that proposed in this article Existing protection McGuire [3] Proposed (mm) method (a) Additional lead (mm) to concrete barrier 37.5 1.9 1.8 78.1 1.3 1.1 156.1 0.3 0.0 212.1 0.0 0.0 (b) Additional concrete (mm) to lead barrier 1.2 83.7 83.2 2.0 45.1 37.5 2.5 26.7 0.0 3.4 0.0 0.0 Table 4. Examples of di¡erent material shielding requirements, when no existing material is present. All thicknesses are given in mm Added material kVp McGuire Proposed [3] method Lead 70 1.0 1.0 100 2.5 2.5 125 2.9 2.9 Concrete 70 90.0 90.0 100 156.1 156.1 125 215.1 215.1 Gypsum wallboard 70 119.4 119.4 100 219.3 219.3 125 313.6 313.6 Steel 70 6.5 6.5 100 17.5 17.5 125 30.4 30.4 Plate glass 70 52.1 52.1 100 86.9 86.9 125 122.1 122.1 Shielding in X-ray departments 183The British Journal of Radiology, February 1999
  6. 6. deserves a di¡erent approach, where the above assumption would be ignored. The methodology described in this paper allows more accurate shielding requirements to be speci¢ed, when existing protection is present, fol- lowing no assumption on beam hardening caused by transmission through any material. While prac- tical shielding of diagnostic X-radiation is most often achieved with gross increments of sheets of Figure 3. Main window of the program, where neces- sary data inputs for the barrier(s) and the tube(s) of a diagnostic X-ray room are entered, in order to be used in a radiation protection study. Figure 4. Window for introducing existing shielding material and thickness. The example corresponds to barrier 2. Figure 5. Window presenting the calculated shielding requirements for several materials. The example corre- sponds to the additional shielding required for barrier 2. Table 5. Examples of di¡erent material shielding requirements, when 78.1 mm of concrete as existing material is present. All thicknesses are given in mm Added material kVp McGuire Proposed [3] method Lead 100 1.3 1.1 125 1.7 1.7 Concrete 100 85.4 78.0 125 137.8 137.0 Gypsum wallboard 100 122.0 113.6 125 196.6 195.6 Steel 100 8.9 8.0 125 17.8 17.6 Plate glass 100 49.8 46.3 125 78.5 78.0 Table 6. Examples of di¡erent material shielding requirements, when 100 mm of Gypsum wallboard as existing material is present. All thicknesses are given in mm Added material kVp McGuire Proposed [3] method Lead 100 1.9 1.1 125 2.3 1.8 Concrete 100 121.0 74.7 125 181.1 144.0 Gypsum wallboard 100 167.7 119.3 125 258.9 213.6 Steel 100 13.3 7.6 125 24.8 18.9 Plate glass 100 67.8 47.1 125 102.1 83.5 Table 7. Examples of di¡erent material shielding requirements, when 50 mm of plate glass as existing material is present. All thicknesses are given in mm Added material kVp McGuire Proposed [3] method Lead 100 1.6 0.8 125 2.1 1.5 Concrete 100 105.5 56.4 125 163.1 123.0 Gypsum wallboard 100 147.3 92.3 125 232.1 182.8 Steel 100 11.4 5.4 125 21.9 15.5 Plate glass 100 59.9 36.9 125 92.1 72.1 M Petrantonaki, C Kappas, E P Efstathopoulos et al 184 The British Journal of Radiology, February 1999
  7. 7. lead, it is possible to replace this with a precise and accurate technique which assumes little and allows a variety of more cost e¡ective materials to be con- sidered for protective barriers. The di¡erences in the results between the two methods will be even more pronounced when less attenuating materials such as gypsum wallboard [8, 9] are used for a bar- rier. This is shown in Tables 5, 6 and 7, which pro- vide an extended presentation of the results arising from the two methods, for several combinations of ¢ve materials. Computer programs for shielding computations are not widely available. In attempting to face this de¢ciency, a Windows-based program has been designed and implemented, based on the method presented. It provides the accuracy of the method presented, as well as assisting the physicist in shielding computations in a user-friendly manner. Conclusions A method, which allows for accurate determina- tion of the shielding material thickness required to attenuate radiation beams to some prescribed allowable dose limit, has been described. It takes into account the case of multiple sources operating at di¡erent maximum potentials in a diagnostic X- ray room and it deals with the case of existing shielding in place, e¡ectively removing all the uncertainties inherent in previous work found in the relevant literature and providing a useful extension to the NCRP 49 [1] formalism. Based onthismethod,acomputerprogramhasbeendevel- oped. The program can be used for the studies of diagnostic radiation protection enabling one to calculateeasilythebarrierthicknessrequiredtopro- tect areas around a source of X-ray radiation and to estimate the e¡ectiveness of an existing barrier. Acknowledgments The authors wish to thank Dr Douglas J Simpkin, for his invaluable comments and sugges- tions, and Dr E Lynn McGuire for the helpful information provided. References 1. National Council on Radiation Protection and Measurements. Shielding design and evaluation for medical use of X-rays and gamma rays of energies up to 10 MeV, NCRP Report 49. Bethesda NCRP1976. 2. Archer BR, Thornby JI, Bushong SG. Diagnostic X- ray shielding design based on an empirical model of photon attenuation. Health Phys 1983;44:507^17. 3. McGuire EL. A revised schema for performing diag- nostic X-ray shielding calculations. Health Phys 1983;50:99^105. 4. Simpkin DJ. A general solution to the shielding of medical X and c rays by the NCRP report No 49 methods. Health Phys 1987;52:431^6. 5. Archer BR, Fewell TR, Conway BJ, Quinn PW. Attenuation properties of diagnostic X-ray shielding materials. Med Phys 1994;21:1499^507. 6. Simpkin DJ. Transmission data for shielding diagnos- tic X-ray facilities. Health Phys 1995;68:704^9. 7. Simpkin DJ. Shielding requirements for mammogra- phy. Health Phys 1987;53:267^79. 8. Christensen RC, Sayeg JA. Attenuation characteris- tics of gypsum wallboard. Health Phys 1979;36: 595^600. 9. Glaze SA, Schreiders NJ, Bushong SC. Use of gypsum wallboard for diagnostic X-ray protective barriers. Health Phys 1979;36:587^93. Shielding in X-ray departments 185The British Journal of Radiology, February 1999

×