Presentation on theme: "IAEA International Atomic Energy Agency RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY L12: Shielding and X Ray room design IAEA Training."— Presentation transcript:
IAEA International Atomic Energy Agency RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY L12: Shielding and X Ray room design IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
IAEA 12: Shielding and X Ray room design2 Introduction Subject matter: the theory of shielding design and some related construction aspects. The method used for shielding design and the basic shielding calculation procedure
IAEA 12: Shielding and X Ray room design3 Topics Equipment design and acceptable safety standards Use of dose constraints in X Ray room design Barriers and protective devices
IAEA 12: Shielding and X Ray room design4 Overview To become familiar with the safety requirements for the design of X Ray systems and auxiliary equipment, shielding of facilities and relevant international safety standards e.g. IEC.
IAEA International Atomic Energy Agency Part 12: Shielding and X Ray room design Topic 1: Equipment design and acceptable safety standards IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
IAEA 12: Shielding and X Ray room design6 Purpose of Shielding To protect: the X Ray department staff the patients (when not being examined) visitors and the public persons working adjacent to or near the X Ray facility
IAEA 12: Shielding and X Ray room design7 Radiation Shielding - Design Concepts Data required include consideration of: Type of X Ray equipment Usage (workload) Positioning Whether multiple tubes/receptors are being used Primary beam access (vs. scatter only) Operator location Surrounding areas
IAEA 12: Shielding and X Ray room design8 Shielding Design (I) Equipment What equipment is to be used? General radiography Fluoroscopy (with or without radiography) Dental (oral or OPG) Mammography CT
IAEA 12: Shielding and X Ray room design9 Shielding Design (II) The type of equipment is very important for the following reasons: where the X Ray beam will be directed the number and type of procedures performed the location of the radiographer (operator) the energy (kVp) of the X Rays
IAEA 12: Shielding and X Ray room design10 Shielding Design (III) Usage Different X Ray equipment have very different usage. For example, a dental unit uses low mAs and low (~70) kVp, and takes relatively few X Rays each week A CT scanner uses high (~130) kVp, high mAs, and takes very many scans each week.
IAEA 12: Shielding and X Ray room design11 Shielding Design (IV) The total mAs used each week is an indication of the total X Ray dose administered The kVp used is also related to dose, but also indicates the penetrating ability of the X Rays High kVp and mAs means that more shielding is required.
IAEA 12: Shielding and X Ray room design12 Shielding Design (V) Positioning The location and orientation of the X Ray unit is very important: distances are measured from the equipment (inverse square law will affect dose) the directions the direct (primary) X Ray beam will be used depend on the position and orientation
IAEA 12: Shielding and X Ray room design13 Radiation Shielding - Typical Room Layout A to G are points used to calculate shielding
IAEA 12: Shielding and X Ray room design14 Shielding Design (VI) Number of X Ray tubes Some X Ray equipment may be fitted with more than one tube Sometimes two tubes may be used simultaneously, and in different directions This naturally complicates shielding calculation
IAEA 12: Shielding and X Ray room design15 Shielding Design (VII) Surrounding areas The X Ray room must not be designed without knowing the location and use of all rooms which adjoin the X Ray room Obviously a toilet will need less shielding than an office First, obtain a plan of the X Ray room and surroundings (including level above and below)
IAEA 12: Shielding and X Ray room design16 Radiation Shielding - Design Detail Must consider: appropriate calculation points, covering all critical locations design parameters such as workload, occupancy, use factor, leakage, target dose (see later) these must be either assumed or taken from actual data use a reasonable worst case more than typical case, since undershielding is worse than overshielding
IAEA International Atomic Energy Agency Part 12: Shielding and X Ray room design Topic 2: Use of dose constraints in X Ray room design IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
IAEA 12: Shielding and X Ray room design18 Radiation Shielding Parameters (I) P - design dose per week usually based on 0.3 mSv per year
IAEA 12: Shielding and X Ray room design19 Radiation Shielding Parameters (II) Film storage areas (darkrooms) need special consideration Long periods of exposure will affect film, but much shorter periods (i.e. lower doses) will fog film in cassettes A simple rule is to allow 0.1 mGy for the period the film is in storage - if this is 1 month, the design dose is mGy/week
IAEA 12: Shielding and X Ray room design20 Radiation Shielding Parameters (III) Remember we must shield against three sources of radiation In decreasing importance, these are: primary radiation (the X Ray beam) scattered radiation (from the patient) leakage radiation (from the X Ray tube)
IAEA 12: Shielding and X Ray room design21 U - use factor fraction of time the primary beam is in a particular direction i.e.: the chosen calculation point must allow for realistic use for all points, sum may exceed 1 Radiation Shielding Parameters (IV)
IAEA 12: Shielding and X Ray room design22 Radiation Shielding Parameters (V) For some X Ray equipment, the X Ray beam is always stopped by the image receptor, thus the use factor is 0 in other directions e.g.: CT, fluoroscopy, mammography This reduces shielding requirements
IAEA 12: Shielding and X Ray room design23 Radiation Shielding Parameters (VI) For radiography, there will be certain directions where the X Ray beam will be pointed: towards the floor across the patient, usually only in one direction toward the chest Bucky stand The type of tube suspension will be important, e.g.: ceiling mounted, floor mounted, C-arm etc.
IAEA 12: Shielding and X Ray room design24 Radiation Shielding Parameters (VII) T - Occupancy T = fraction of time a particular place is occupied by staff, patients or public Has to be conservative Ranges from 1 for all work areas to 1/20 for toilets and 1/40 for unattended car parks
IAEA 12: Shielding and X Ray room design25 Occupancy (NCRP 147) Area Administrative or clerical offices; laboratories, pharmacies and other work areas fully occupied by an individual; receptionist areas, attended waiting rooms, children indoor play areas, adjacent X ray rooms, film reading areas, nurse stations, X ray control rooms Room used for patient examinations and treatments Corridors, patients rooms, employee lounges, staff rest rooms Occupancy factor T 1 1/2 1/5
IAEA 12: Shielding and X Ray room design26 Occupancy (NCRP 147) Area Corridor doors Public toilets, unattended vending areas, storage rooms, outdoor areas with seating, unattended waiting rooms, patient holding areas Outdoor areas with only transient pedestrian or vehicular traffic, unattended parking lots, vehicular drop off areas (unattended), stairways, unattended elevators Occupancy factor T 1/8 1/20 1/40
IAEA 12: Shielding and X Ray room design27 Radiation Shielding Parameters (VIII) W - Workload A measure of the radiation output in one week Measured in mA-minutes Varies greatly with assumed maximum kVp of X Ray unit Usually a gross overestimation Actual dose/mAs can be estimated
IAEA 12: Shielding and X Ray room design28 Workload (I) For example: a general radiography room The kVp used will be in the range kVp The exposure for each film will be between 5 mAs and 100 mAs There may be 50 patients per day, and the room may be used 7 days a week Each patient may have between 1 and 5 films SO HOW DO WE ESTIMATE W ?
IAEA 12: Shielding and X Ray room design29 Workload (II) Assume an average of 50 mAs per film, 3 films per patient Thus W = 50 mAs x 3 films x 50 patients x 7 days = 52,500 mAs per week = 875 mA-min per week We could also assume that all this work is performed at 100 kVp
IAEA 12: Shielding and X Ray room design30 Examples of Workloads in Current Use (NCRP 147)
IAEA 12: Shielding and X Ray room design31 Workload - CT CT workloads are best calculated from local knowledge Remember that new spiral CT units, or multi- slice CT, could have higher workloads A typical CT workload is about 28,000 mA- min per week
IAEA 12: Shielding and X Ray room design32 Tube Leakage All X Ray tubes have some radiation leakage - there is only 2-3 mm lead in the housing Leakage is limited in most countries to 1 1 meter, so this can be used as the actual leakage value for shielding calculations Leakage also depends on the maximum rated tube current, which is about kVp for most radiographic X Ray tubes
IAEA 12: Shielding and X Ray room design33 Radiation Shielding Parameters
IAEA 12: Shielding and X Ray room design34 Room Shielding - Multiple X Ray Tubes Some rooms will be fitted with more than one X Ray tube (maybe a ceiling-mounted tube, and a floor-mounted tube) Shielding calculations MUST consider the TOTAL radiation dose from the two tubes
IAEA International Atomic Energy Agency Part 12: Shielding and X Ray room design Topic 3: Barriers and protective devices IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
IAEA 12: Shielding and X Ray room design36 Shielding - Construction I Materials available: lead (sheet, composite, vinyl) brick gypsum or baryte plasterboard concrete block lead glass/acrylic
IAEA 12: Shielding and X Ray room design37 Shielding - Construction Problems Some problems with shielding materials: Brick walls - mortar joints Use of lead sheets nailed to timber frame Lead inadequately bonded to backing Joins between sheets with no overlap Use of hollow core brick or block Use of plate glass where lead glass specified
IAEA 12: Shielding and X Ray room design38 Problems in shielding - Brick Walls & Mortar Joints Bricks should be solid and not hollow Bricks have very variable X Ray attenuation Mortar is less attenuating than brick Mortar is often not applied across the full thickness of the brick
IAEA 12: Shielding and X Ray room design39 Problems in shielding - Lead inadequately bonded to backing Lead must be fully glued (bonded) to a backing such as wood or wallboard If the lead is not properly bonded, it will possibly peel off after a few years Not all glues are suitable for lead (oxidization of the lead surface)
IAEA 12: Shielding and X Ray room design40 Problems in shielding - Joins between sheets with no overlap There must be mm overlap between adjoining sheets of lead Without an overlap, there may be relatively large gaps for the radiation to pass through Corners are a particular problem
IAEA 12: Shielding and X Ray room design41 Problems in shielding - Use of plate glass Plate glass (without lead of specified quantity as used in windows, but thicker) is not approved as a shielding material The radiation attenuation of plate glass is variable and not predictable Lead glass or lead Perspex must be used for windows
IAEA 12: Shielding and X Ray room design42 Radiation Shielding - Construction II Continuity and integrity of shielding very important Problem areas: joins penetrations in walls and floor window frames doors and frames
IAEA 12: Shielding and X Ray room design43 Penetrations Penetrations means any hole cut into the lead for cables, electrical connectors, pipes etc. Unless the penetration is small (~2-3 mm), there must be additional lead over the hole, usually on the other side of the wall Nails and screws used to fix bonded lead sheet to a wall do not require covering
IAEA 12: Shielding and X Ray room design44 Window frames The lead sheet fixed to a wall must overlap any lead glass window fitted It is common to find a gap of up to 5 cm, which is unacceptable
IAEA 12: Shielding and X Ray room design45 Shielding of Doors and Frames
IAEA 12: Shielding and X Ray room design46 Shielding - Verification I Verification should be mandatory Two choices - visual or measurement Visual check must be performed before shielding covered - the actual lead thickness can be measured easily Radiation measurement necessary for window and door frames etc. Measurement for walls very slow
IAEA 12: Shielding and X Ray room design47 Shielding Testing
IAEA 12: Shielding and X Ray room design48 Records It is very important to keep records of shielding calculations, as well as details of inspections and corrective action taken to fix faults in the shielding In 5 years time, it might not be possible to find anyone who remembers what was done!
IAEA 12: Shielding and X Ray room design49 Summary The design of shielding for an X Ray room is a relatively complex task, but can be simplified by the use of some standard assumptions Record keeping is essential to ensure traceability and constant improvement of shielding according to both practice and equipment modification
IAEA 12: Shielding and X Ray room design50 Practical Questions for shielding calculations Radiography Find the necessary amount of lead to protect personnel sitting in an adjacent office, for a dose constraint of 0.30mSv per year , taking into account the following assumptions and data. You must calculate and consider all 3 types of radiation (Primary, Scattered and Leakage).
IAEA ASSUMPTIONSDATA KNOWN Leakage dose is 1mGy per hour at a distance of 1.0 meter. (Worst Case Scenario) Workload of 100 patients per day 3 films per patient 20mAs per film 7 days per week Scatter fraction (Sf) from patient is at 125kVp and 135 degrees, (Worst Case Scenario) Unit maximum current 2.0mA Average dose per unit workload of 4.72mGy [2,3] per week at 1 meter distance. Field size of 1000cm². Relate this to the standard 400cm² Focus to skin distance 80cm. (Needed to calculate the scattered dose) Use factor U=0.25. Occupancy factor T=1.0. Critical distance d=2.5 m
IAEA 12: Shielding and X Ray room design52 Solution (a) 1/8 Step1.Calculate the Workload (W) in mAmin/week W=(mAs/film) x (patients/day) x (films/patient) x (days/week) =20 x 100 x 3 x 7 =42,000 mAs/week =700mAmin/week
IAEA 12: Shielding and X Ray room design53 Solution (a) 2/8 Step 2.Calculate the primary dose per week at 1 meter P 1 =(mAmin/week) x (Average dose per unit workload in mGy/week) =700 x 4.72 =3,304mGy/week
IAEA 12: Shielding and X Ray room design54 Solution (a) 3/8 Step 3.Calculate the Primary dose per week at the critical distance. P=(P 1 x U x T)/d² =(3,304 x 0.25 x 1)/2.5² =132.16mGy/week
IAEA 12: Shielding and X Ray room design55 Solution (a) 4/8 Step 4.Calculation of scattered dose per week at the critical distance S=(P 1 x T x Sf x 1000)/(400 x d x 0.8²) =(3304 x 1 x x 1000)/(400 x 2.52 x 0.8²) =5.2mGy/week
IAEA 12: Shielding and X Ray room design56 Solution (a) 5/8 Step 5.Calculation of the leakage radiation per week at the critical distance a) Tube on time=(W)/(Tube current) for leak. calc.=700/(2 x 60) =5.83 hours b) Leakage dose (L)=(Time x U x T)/d² =(5.83 x 1 x 0.25)/2.5² =0.233mGy/week
IAEA 12: Shielding and X Ray room design57 Solution (a) 6/8 Step 6. Add the three sources of radiation together Total dose = P+S+L =( ) =137.59mGy/week
IAEA 12: Shielding and X Ray room design58 Solution (a) 7/8 Step 7.Calculate the required attenuation If the required attenuation is 0.006mGy/week (0.3mSv/year¹), then the required attenuation ( ) would be: =0.006/ =
IAEA 12: Shielding and X Ray room design59 Solution (a) 8/8 Shielding Calculation From graph below a lead of thickness approximately 2.6mm is necessary mm Lead Required Reduction factor kV kV kV
IAEA 12: Shielding and X Ray room design60 Question for shielding calculation Computer Tomography A CT scanner is placed in a room as in figure 1. The height of the ceiling is 4.0m. The walls are made of lightweight concrete (1840kg/m³), with a minimum thickness of 110mm. The scanner isocentre is located 0.9m above floor level. Isodose curves have been provided for a 120kVp, 250mAs, 10mm slice on a 320mm diameter PMMA body phantom and a 350mAs 10mm slice on a 160mm head phantom.
IAEA 12: Shielding and X Ray room design61 Figure 1
IAEA ASSUMPTIONSDATA KNOWN 1 The scatter dose per mAs from a 10mm slice through the head is half that from a slice through the body. 140 body examinations/week. Average body examination comprises of 24 slices of 10mm width with a table feed of 14mm head examinations/week. Average head examination consists of 10 slices of 10 mm width and 5 slices of 5mm width.
IAEA 12: Shielding and X Ray room design63 Solution (b) 1/9 Step1.Calculate the total workload per week a)Number of 250mAs body slices per week = 140 body examinations x 22 slices per examination =3080 slices per week b)Number of 350mAs, 10mm equivalent head slices per week. =(10 slices x 100) + [(5 slices x 100) x (5/10)] =1250 The division 5/10 was needed to normalize the five slices of 5mm to the equivalent of 10mm.
IAEA 12: Shielding and X Ray room design64 Solution (b) 2/9 c)=Equivalent number of 250mAs, 10mm head examinations. =(1250/2) x (350/250) (See assumption 1) =875 slices per week for head examination Hence: The total workload per week = =3955 body slices of 250mAs and 10mm width.
IAEA 12: Shielding and X Ray room design65 Solution (b) 3/9 Step 2.Calculation of the transmission factor, regarding wall B From figure 1, distance from the isocentre is 2.5m and the dose contour is 1.5 Gy Hence: The dose per week from 3955 slices is equal to =3955 x 1.5 Gy =5933 Gy The area behind wall B is an office, where the occupancy is estimated to be 100% The required transmission for that barrier, B B=0.3/(5.933mGy x 1 x 52) B=3.2x10-3
IAEA 12: Shielding and X Ray room design66 Solution (b) 4/9 Step 3.Calculations of coefficients, and, based on Archers et al (1997) formula, interpolated for 10kVp With reference to table 4.6 of BIR (2000)¹, the following coefficients have been calculated for lead material by interpolation for 120kVp: = = =0.7664
IAEA 12: Shielding and X Ray room design67 Solution (b) 5/9 Using the following formula the thickness of the material required (lead), x, to provide the desired transmission can be calculated
IAEA 12: Shielding and X Ray room design68 Solution (b) 6/9 x = x ln( /5.0991) x =2.35 mm of lead
IAEA 12: Shielding and X Ray room design69 Solution (b) 7/9 With reference to table 4.6 of BIR (2000)¹, the following coefficients have been calculated for concrete by interpolation for 120kVp: = = =0.7302
IAEA 12: Shielding and X Ray room design70 Solution (b) 8/9 Using the following formula the thickness of the material required (concrete), x, to provide the desired transmission can be calculated
IAEA 12: Shielding and X Ray room design71 Solution (b) 9/9 x = x ln( /2.9338) x = mm of concrete
IAEA 12: Shielding and X Ray room design72 References: 1. Radiation Shielding for Diagnostic X-Rays (2000), Ed. D.G. Sutton and J.R. Williams, Pub. BIR. 2. IAEA Training Material, Diagnostic Radiology, L.12.1, slide National Council on Radiation Protection and Measurements Structural Shielding Design for Medical X Rays Imaging Facilities 2004 (NCRP 147) 4. Diagnostic X-ray shielding design, B. R. Archer, AAPM Monograph The expanding role of medical physics in diagnostic radiology, 1997
IAEA 12: Shielding and X Ray room design73 Where to Get More Information (I) New concepts for Radiation Shielding of Medical Diagnostic X Ray Facilities, D. J. Simpkin, AAPM Monograph The expanding role of medical physics in diagnostic radiology, 1997