L12 Shielding

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

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

Topics Equipment design and acceptable safety standards Use of dose constraints in X Ray room design Barriers and protective devices

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.

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

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

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

Shielding Design (I) Equipment What equipment is to be used? General radiography Fluoroscopy (with or without radiography) Dental (oral or OPG) Mammography CT

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

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.

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.

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

Radiation Shielding - Typical Room Layout A to G are points used to calculate shielding

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

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)

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

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

Radiation Shielding - Calculation Based on NCRP 147 Assumptions used are very pessimistic, so overshielding is common Various computer programs are available, giving shielding in thickness of various materials

Radiation Shielding Parameters (I) P - design dose per week usually based on 5 mSv per year for occupationally exposed persons (25% of dose limit), and 1 mSv for public occupational dose must only be used in controlled areas i.e. only for radiographers and radiologists

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 0.025 mGy/week

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)

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)

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

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.

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

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

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

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

Workload (I) For example: a general radiography room The kVp used will be in the range 60-120 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 ?

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

Examples of Workloads in Current Use (NCRP 147) Weekly Workload (W) mA-min at: 100 kVp 125 kVp 150 kVp General Radiography 1,000 400 200 Fluoroscopy (including spot films) 750 300 150 Chiropractic 1,200 500 250 Mammography 700 at 30 kVp (1,500 for breast screening) Dental 6 at 70 kVp (conventional intra-oral films) More realistic values include CT: see ref. Simpkin (1997)

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

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 mGy.hr-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 3-5 mA @ 150 kVp for most radiographic X Ray tubes

Radiation Shielding Parameters

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

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

Shielding - Construction I Materials available: lead (sheet, composite, vinyl) brick gypsum or baryte plasterboard concrete block lead glass/acrylic

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

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

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)

Problems in shielding - Joins between sheets with no overlap There must be 10 - 15 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

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

Radiation Shielding - Construction II Continuity and integrity of shielding very important Problem areas: joins penetrations in walls and floor window frames doors and frames

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

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

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

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!

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

Where to Get More Information (I) Radiation shielding for diagnostic X Rays. BIR report (2000) Ed. D.G. Sutton & J.R. Williams National Council on Radiation Protection and Measurements “Structural Shielding Design for Medical X Rays Imaging Facilities” 2004 (NCRP 147)

Where to Get More Information (II) 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 Diagnostic X-ray shielding design, B. R. Archer, AAPM Monograph The expanding role of medical physics in diagnostic radiology, 1997

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