The Design of Diagnostic Medical Facilities where ... - ResearchGate

More documents

Recommendations

Info

5.2.4 Primary radiation A barrier is required to attenuate the primary beam to a level that complies with the dose constraint. Primary barriers are typically required in general radiographic rooms, dedicated chest rooms and rooms where there is a combination of radiography and fluoroscopy. For mammography, fluoroscopy, CT and DXA – the entire primary beam is normally incident on the face of the image detector which acts as a primary beam stop. Therefore for these types of rooms only secondary radiation needs to be considered. The use factor (U) is the fraction of the primary beam workload that is directed toward a given barrier (NCRP, 2004). It is used in the NCRP methodology for primary radiation shielding calculations and the value will depend on the type of installation and the barrier being assessed. Different barriers may have different use factors. For example, the workload presented for the radiographic room (Chest Bucky) in Table 5.4 is directed entirely at the wall behind the Bucky, and the use factor, U = 1 for this wall. In addition, this workload only contributes secondary radiation to the other barriers in the room. However, it should be noted that a more complex situation can arise with other configurations. Primary beam use factors published by the NCRP are reproduced in Table 5.6. The ceiling and operator’s control screen are generally considered to have U = 0. U is also zero for installations where only secondary radiation needs to be considered such as image intensifier and mammography systems. Table 5.6: Use factors from NCRP for radiographic room (based on NCRP, 2004) Barrier Use factor (U) Workload distribution Floor 0.89 Rad Room (floor or other barriers) Cross-table wall 0.09 Rad Room (floor or other barriers) Unspecified wall (other than chest bucky wall or cross-table wall) 0.02 Rad Room (floor or other barriers) Chest Bucky wall 1.00 Rad Room (Chest Bucky) To calculate the shielding required in the primary barrier, the barrier transmission factor must be determined. This is the ratio of the air kerma behind a barrier of thickness, x, to the air kerma at the same location with no barrier. In the following sections, an overview of the BIR methodology and equations are presented. Detailed equations for employing the NCRP methodology are available (NCRP, 2004). A comparison of both methods is presented in Example 1 in Section 5.3. BIR methodology for primary radiation The BIR recommends that one of two methods is used to calculate primary shielding requirements, and the method used will depend on the clinical situation. The film dose method is used where the X-ray beam is entirely intercepted by the patient and the entrance surface dose method is used where this is not the case. 56 The Design of Diagnostic Medical Facilities where Ionising Radiation is used
(a) Film dose method This approach should be used when: • the X‐ray beam is entirely intercepted by the patient. • the shielding calculation is based on incident film kerma. • the incident kerma K inc (mGy) at the barrier is calculated using the Inverse Square Law. K inc = n × K film × B film × FFD FFD + d 2 Equation 5.1 where n = number of films per week K film = film kerma (mGy) B film = transmission through the film and cassette d = distance from the film to the barrier (m) FFD = focus-film distance (m) (b) Entrance surface dose (ESD) method This approach, uses a minor adaptation of the BIR definitions, and should be used when: • the X-ray beam passes outside the patient, e.g. skull or extremity radiography. • the incident kerma K inc (mGy) at the barrier in question is calculated from the Entrance Surface Dose (ESD) and uses the Inverse Square Law. K inc = n× ESD × FFD − d FFD + d f w 2 Equation 5.2 where n = number of films per week ESD = Entrance Surface Dose per film (mGy) FFD = focus-film distance (m) d f = entrance surface to film distance (m) d w = distance from film to barrier (or, for practical purposes, the point of interest in the adjacent room/area) (m) It should be noted that in practice, the unit used for film kerma and Entrance Surface Dose is milligray (mGy) or microgray (μGy). As referred to in Section 5.1, mGy and mSv are taken as being equivalent for shielding calculations. Care must be taken to ensure that the magnitude of the unit used to denote the resulting value of K inc is consistent with that used for the dose constraint which by convention is stated in millisieverts (mSv). The Design of Diagnostic Medical Facilities where Ionising Radiation is used 57
Page 1:
RPII 09/01 The Design of Diagnostic
Page 4 and 5:
Foreword The original Code of Pract
Page 6 and 7:
4. Nuclear medicine 35 4.1 Introduc
Page 8 and 9: 1. Legal and administrative framewo
Page 10 and 11: e made as soon as possible. It shou
Page 12 and 13: It should be noted that it is the u
Page 14 and 15: Records should be kept as they will
Page 16 and 17: 2. Identify the persons (staff and
Page 18 and 19: 3.1.3 Computed Tomography (CT) CT a
Page 20 and 21: Photo 3.2: Interventional X‐ray r
Page 22 and 23: 3.3 Radiography rooms 3.3.1 General
Page 24 and 25: X Figure 3.2: Dedicated chest room
Page 26 and 27: Figure 3.4: DXA room Operator’s c
Page 28 and 29: Figure 3.5b: A dental radiography s
Page 30 and 31: Radiation shielding calculations fo
Page 32 and 33: There are large variations in the s
Page 34 and 35: e assessed by the RPA as part of th
Page 36 and 37: The trailer must be sited in an are
Page 38 and 39: It is recognised worldwide that the
Page 40 and 41: The operator console should be loca
Page 42 and 43: Figure 4.3: A possible layout for a
Page 44 and 45: The workstation should be adequatel
Page 46 and 47: uptake probe and associated equipme
Page 48 and 49: The layout of the department should
Page 50 and 51: The control/console room should pro
Page 52 and 53: 5. Shielding calculations 5.1 Gener
Page 54 and 55: Table 5.2: Suggested minimum distan
Page 56 and 57: Table 5.4: Workload data from NCRP
Page 60 and 61: The primary beam will be attenuated
Page 62 and 63: 1 χ = ln αγ B −γ 1+ + β α
Page 64 and 65: . The reduction in the dose rate fr
Page 66 and 67: R D(t) = . t D(0) × t Equation 5.8
Page 68 and 69: 5.3.1 Radiology shielding calculati
Page 70 and 71: Example 2: Dedicated chest room (BI
Page 72 and 73: Example 4: Dental X‐ray room (BIR
Page 74 and 75: Example 6: Shielding required for a
Page 76 and 77: 6. Some practical considerations 6.
Page 78 and 79: 6.1.4 Brick There are many types of
Page 80 and 81: Walls are generally shielded to ful
Page 82 and 83: 6.3.3 Doors As illustrated in Chapt
Page 84 and 85: Photo 6.4 Lead blinds protecting wi
Page 86 and 87: Chapter 2). It is becoming common t
Page 88 and 89: Gaps in shielding are more likely t
Page 90 and 91: ICRP. 2007. Recommendations of the
Page 92 and 93: Appendices Appendix A: Dose limits
Page 94 and 95: Appendix C: Radiation transmission
Page 96 and 97: Figure C.1: Transmission of primary
Page 98 and 99: Figure C.5: 2 mm lead equivalent th
Page 100 and 101: Table D.3: Physical properties & ef
Page 102 and 103: Appendix E: Schematic diagrams of r
Page 104 and 105: Figure E.7: Shielding around the ed
Page 106 and 107: Figure F. 3: Sample radiation warni
Page 108 and 109:
Notes 106 The Design of Diagnostic
Page 110:
Notes 108 The Design of Diagnostic
show all

The Design of Diagnostic Medical Facilities where ... - ResearchGate

Create successful ePaper yourself

Delete template?

Save as template?