Performing shielding calculations for diagnostic radiology based on NCRP Report 147 Methodology

Structural radiation shielding calculations for diagnostic X-ray facilities is most commonly performed using the recommendations of National Council on Radiation Protection and Measurements (NCRP) Report No. 49 which continues to be the primary guide for diagnostic x-ray structural shielding design for a while. Many changes have occurred over the years that have caused the NCRP Report 49 calculation methodology to become essentially obsolete in that it did not address technology advances in Radiology. The methodology was remedied with the release of NCRP Report No. 147 by enabling shielding designers to, in part, specify effective barriers to diagnostic radiation environments. The NCRP Report 147 methodology for calculating radiation shielding requirements depend greatly on the shielding design goals (P) where a proposed design limit for controlled and uncontrolled areas is reduced to NCRP Report 49 levels. Further, the methodology most likely uses the concept of “dose constraint” in radiation installations as shielding design goals for the purpose of safety and protection optimization for occupational workers and the public. The previous NCRP Report 49 uses a very conservative approach in the assumption and methodology, which in return yielded with barriers much thicker than what is required in diagnostic facilities. In this context, Federal Authority for Nuclear Regulation (FANR), the nuclear and radiological regulator for the United Arab Emirates, recently published software which developed by authors for performing radiation shielding calculations based on an algebraic computation model and the given fi tting factors provided by NCRP Report No. 147. The International Atomic Energy Agency (IAEA), has taken interest to independently validate the codes of the software; and praise the functionality of the tool. The software performs shielding calculations in an effective, easy, and reliable way while being a cost-effective and a timesaving tool. Review Article Performing shielding calculations for diagnostic radiology based on NCRP Report 147 Methodology Mustafa Majali* and Ali Al Remeithi Federal Authority for Nuclear Regulation, Abu Dhabi, P.O. Box 112021, United Arab Emirates Received: 03 March, 2020 Accepted: 02 November, 2020 Published: 05 November, 2020 *Corresponding author: Mustafa Majali, Federal Authority for Nuclear Regulation, Abu Dhabi, P.O. Box 112021, United Arab Emirates, Email:


Introduction
Structural shielding design for diagnostic X-ray installations is widely performed following the publications of National Council on Radiation Protection and Measurements (NCRP), specifi cally Report No. 49 [1] as it has remained the main guidance for performing designs of structural shielding since its issuance. Many changes and advances in technology have emerged in the last decade that gradually rendered NCRP report 49 to be obsolete. Examples of such technological advances include: computerized tomography, mammography and digital imaging that have come into widespread use. Moreover, several reports have analytically examined the conservatism approach of the NCRP Report 49 calculations methodology and the signifi cant changes in the radiology department environment [2]. Further, there has been signifi cant developments in imaging techniques such as screen intensifying and fi lms that have caused reductions in both radiation exposures and real workload while improving image quality. The revised radiation shielding methodology found in NCRP Report 147 [3] allows shielding designers to identify barriers that are safe and cost effective in diagnostic radiation settings; taken into account new technology, real workload and developments in imaging techniques. This can apply easily to new facilities, as well as, existing facilities and hence, retrofi tting of existing structural shielding become inevitable and unavoidable.

Theoretical and methodology background
In medical diagnostic X-ray imaging installations, the radiation in that environment consists of primary and secondary radiation. Primary radiation is emitted directly from the X-ray source to a primary barrier. Secondary radiation consists of radiation scattered from the patient and other Citation: Majali  surroundings objects, as well leakage radiation from the X-ray tube. The primary and secondary radiation exposures depend on the radiation amount produced by the source, distance from the source to the exposed area, time that an individual occupied the irradiated area. Protective shielding between the radiation source and the irradiated area is one that limits the air Kerma from primary or scattered and leakage radiations generated by the radiographic unit to the appropriate shielding design goal or less.
The concepts of radiation shielding calculation found in NCRP Report 147 depend on shielding design goals (P) where proposed design limits are reduced by a factor of ten for controlled areas and by a factor of fi ve for uncontrolled areas. Traditionally, the conservative assumption of NCRP Report 49 ignores the fact that the medical exposures are perform over a wide spectrum of X-ray kVp and remains performed workload at single kVp that is usually the maximum for all diagnostic procedures. In shielding design, the distribution of kVp is more important than the magnitude of the workload (mAmin) and the same or more signifi cant for leakage radiation.
The signifi cant reduction in leakage radiation with kVp is not considered in the single kVp model [2]. Simpkin [5] provides fi ve representative workload spectra to be used as a new method to the shielding design of medical X-ray rooms. The average spectra obtained from the survey of AAPM Task Group 9 provides a more realistic and accurate estimated approach that is representative of the radiation produced in a diagnostic X-ray room.
The use factor (U) is the fraction of the primary beam workload projected toward a given primary barrier. The NCRP Report 147 methodology has made several changes on use factor values based on the survey results of AAPM Task Group 9. These results suggest that the primary beam projected to the non-chest walls are in fact much less often than the fraction previously recommended by NCRP Report 49 [2]. In addition, the X-ray tube can be rotatable, in which case it is possible for the primary beam to be directed to other barriers.
The value of U will depend on the type of radiation installation and the barrier of concern and always assumes a unity value for secondary radiation. The new approach for the use factor should be considered reasonable in shielding calculations.
Typical radiation shielding materials in facilities are lead and concrete. Other materials have been used for shielding purposes such as Gypsum, Steel, and Wood wherein the evidence shows that these materials have proven to be suffi cient to reduce doses to required levels thus avoiding costly and wasteful over shielding. Unfortunately, NCRP Report 49 does not provide guidance or attenuation data for such materials. For this reason, it is prudent to use a more realistic and accurate approach for estimates of the required radiation shielding and cost effectiveness. In this regard, NCRP Report 147 provides related data with respect to these materials that may be used as effective shielding materials.
The concept of dose constraint is used to meet facility shielding design goals for the purpose of optimizing radiation safety and protection for occupational workers and the public. It is noted that shielding calculations using conservative dose limits and assumptions allows the calculation methodology presented in NCRP Report 49 to identify barriers that are thicker than those currently in use in diagnostic X-ray facilities. However, redesign of existing thicker shielding as accurate as possible, taken into account the cost of shielding, use of alternative additional shielding materials and apply the ALARA principle when considering monetary cost-benefi t requires to obtain an accurate estimation of the equivalent and adequate additional shielding when other shielding materials would be used.

Discussion and conclusion
The effective and effi cient use of shielding materials and the development of optimal design requires a qualifi ed expert for performing either the calculations or for evaluation and reviewing the results. The time and cost required to perform a desirable radiation shielding design must be considered seriously. Therefore, FANR developed software for performing radiation shielding calculations based on the NCRP Report 147 algebraic computation model by using given tabulated data and fi tting factors. The software enables the user to enter related parameters via simple user interface, performs the shielding calculation, and provides the user with results for the appropriate shielding thickness required to achieve safety goals and provide adequate protection to occupational workers and public from radiation. (m) to software, note that the distance should be consider not more than 0.3 m from outer surface of the barrier.
The occupancy factor (T) value used by the software is unity (1.0) for Administrative or clerical offi ces; laboratories, pharmacies and work areas fully occupied by an individual, receptionist areas, attended waiting room, children's indoor play areas; adjacent X-ray rooms, fi lm reading areas, nurse's stations, and X-ray control rooms. The value of (0.5) is used for patient examinations and treatments room. The corridors, patient rooms, employee lounges, and staff rest rooms are assigning value of (0.2) and value of (0.125) for corridor doors only. Also, the value of (0.05) is assigning for public toilets, unattended vending areas, storage rooms, outdoor areas with seating, unattended waiting rooms, and patient holding areas. The other areas such as Outdoor areas with only transient pedestrian or vehicular traffi c, unattended parking lots, vehicular drop off areas (unattended), attics, stairways, unattended elevators, janitor's closets are using the value of (0.025). The nominal value for the occupancy factor, when assuming that an X-ray unit is randomly used during the week, is the fraction of the working hours in the week that a given person would occupy the area.
The weekly workload (W) of a medical imaging X-ray tube is the time integral of the X-ray tube current over a specifi ed period usually provided in units of miliamperes-minutes. The new methodology presented by NCRP Report 147 defi nes the normalized workload as the average workload per patient. It is important to distinguish between the number of patients examined in a week (N) and the number of "examinations" performed in a given X-ray room. For clarity, an "examination" refers to a specifi c X-ray procedure. A single patient may receive several such "examinations" while in the X-ray room and that may involve more than one image receptor.
The radiation shielding designer should be aware that workload information provided by facility administrators should be stated in terms of a weekly number of "examinations", "patient examinations" or "number of patients" examined by X-ray table. The FANR software refl ects the NCRP Report 147 methodology and relies only on the "number of patients" exposed in X-ray room per week and average unshielded air The FANR software enables the user to enter the related parameters via a simple user interface. It performs the shielding calculation and provides the user with the result for an appropriate shielding thickness required to achieve desired safety goals that would provide adequate protection to occupational workers and the public from radiation [7-9].
The corrections or additions after facilities are completed and existing are usually expensive and most diffi cult. Therefore, obtain as accurate as possible the equivalent and adequate shielding required when another shielding martials would to be used. The relationship between deferent shielding metatarsals thickness (concrete, lead, steel, Plate Glass and Gypsum) has been obtained for wide spectrum of X-ray modalities at diverse setting and assumption. In addition, a conservatively safe approach in specifying radiation barriers has been applied. The In addition to assume that the staff are always in the most exposed place of the room, distances are the minimum possible and leakage radiation is the maximum all the time.