Technical Bulletin on Airport Backscatter X-ray Systems
Who decides what’s safe with regard to radiation? Guidance on radiation safety comes from two recognized authorities: the International Commission on Radiological Protection (ICRP) and the National Council on Radiation Protection and Measurements (NCRP), both of which issue recommendations to regulatory and advisory agencies on the fundamental principles on which appropriate radiological protection can be based 1. The Food and Drug Administration (FDA) considers such authorities (e.g., NCRP Commentary No. 16 (2003) Screening of Humans for Security Purposes Using Ionizing Radiation Scanning Systems) when evaluating radiation protection issues concerning exposure to ionizing radiation from electronic products used for non-medical security purposes.
What standards cover these machines? National consensus safety standards for x-ray systems are published by the American National Standards Institute (ANSI) Accredited Standards Committee N43, Equipment for Non-Medical Radiation Applications. The N43 committee is administered by the Health Physics Society (HPS). International standards are published by the International Electrotechnical Commission (IEC). The Transportation Security Administration (TSA) has required that the backscatter x-ray systems approved for deployment conform to the ANSI/HPS standard, ANSI/HPS N43.17-2009.
ANSI/HPS N43.17-2009 (revision of 2002) Radiation Safety for Personnel Security Screening Systems Using X-Ray or Gamma Radiation applies to systems used to expose people to x rays for the purpose of security screening. The standard provides guidelines specific to radiation safety in the design, performance, and operation of these systems, and covers dose to subject, interlocks, operational procedures, information to provide to subjects, training for operators, etc. Both the ICRP and NCRP recommend limiting the annual effective dose from all sources to members of the public to 1 mSv (100 mrem). The NCRP further recommends that if all sources are not known, the annual effective dose to members of the public should be limited to 0.25 mSv (25 mrem) from any single venue. This standard defines a full-body general-use x-ray screening system as one that delivers less than 1/1,000 of this dose per screening (0.25 μSv (25 μrem)); e.g., the limit for the dose per screening under N43.17-2009 is 0.25 μSv (25 μrem). There is a comparable international standard, IEC 62463-2010 Radiation Protection Instrumentation - X-Ray Systems for the Screening of Persons for Security and the Carrying of Illicit Items, which was published summer, 2010.
How can we be sure that systems comply with standards? Manufacturers of electronic products (Title 21 1000.3(j)) that emit radiation are responsible for compliance with the Federal Food, Drug and Cosmetic Act (FFDCA), Chapter V, Subchapter C - Electronic Product Radiation Control. Manufacturers of Personnel Security Screening X-Ray Systems are also responsible for compliance with all applicable requirements of Title 21 Code of Federal Regulations 1000 through 1004, and 1005.252. As discussed above, ANSI/HPS N43.17-2009, which is a consensus standard and not a requirement by law or regulation, establishes a variety of requirements for manufacturers of and organizations using these products. TSA supports independent safety and performance testing and evaluation (T&E) as described below.
What did NIST do? Under an interagency agreement with the TSA, the National Institute of Standards and Technology (NIST) evaluated the x-ray emissions from full body scanners such as those to be used for the screening of passengers. The goal was to estimate the effective dose to human subjects, operators and bystanders resulting from the operation of such screening equipment. The measured doses were compared to the limits stipulated by existing radiation safety standards, particularly ANSI/HPS N43.17-2009 Radiation Safety for Personnel Security Screening Systems Using X-Ray or Gamma Radiation. Effective dose is an estimate of the combined effects of radiation on various body tissues and organs as defined by the ICRP and NCRP. The Monte Carlo program, PCXMC3, was used to estimate absolute individual organ doses and to calculate effective dose4. The input information required by the PCXMC program included the 1) x-ray tube anode angle, 2) anode voltage, 3) total filtration, 4) x-ray field size, 5) location of the field on the body, 6) focus-to-skin distance, and 7) entrance skin exposure. All of these parameters were measured, calculated, or verified by indirect measurements.
Exposure measurements were made using a radiation monitor system (Radcal 9015), an electrometer, and a cylindrical ionization chamber (model 10X5-1800). A 1 cm2 solid-state detector (RTI model R100B) was used when good spatial resolution was required, as in the determination of scan field size. The ionization chamber and solid-state detector were calibrated at appropriate x-ray beam energies. The background exposure level was measured and subtracted when necessary. An AS&E SmartCheck scanner was evaluated.
The photon energy spectrum (to determine the end-point energy and the x-ray tube kilovoltage) was obtained by means of a Canberra Inspector-2000 spectrometer system and a 6-cm diameter, high-purity germanium detector. The energy scale was calibrated using the 14.1 keV and 122.1 keV gamma energies from a calibrated 57Co source. A Technical Associate model P8-Neon survey instrument was used to locate leakage radiation. The instrument, consisting of an array of eight Geiger Muller pancake probes, was designed for quick, qualitative surveys of the shielding. The Radcal 9015 monitor system was used for follow-up and quantitative measurements of any detectable leakage.
All results confirm that the radiation dose from the scanner studied was below that set by the American National Standards Institute standard for safety. The effective dose to a subject being screened varies depending on the age and size of the person. For the AS&E SmartCheck scanner, an adult would receive an effective dose of about 6.2 μrem per frontal scan. A small child would receive an effective dose of about 7.4 μrem per frontal scan. An infant would receive a dose of about 7.2 μrem per frontal scan. In order to be compliant with the ANSI/HPS N43.17-2009 standard, the effective dose should not exceed 25 μrem per screening (which may involve more than one scan) at the point of maximum exposure but no closer than 30 cm from the “beam exit surface.” All exposure measurements outside of the primary beam, due to scatter from the screened individual or leakage from the cabinet, were below the ANSI/HPS N43.17-2009 limits for dose to bystanders and operators.
Summary: The TSA funded NIST to do a study of an AS&E Inc. backscatter x-ray screening system that is undergoing the Transportation Security Laboratory (TSL) certification process. Under the agreement, NIST performed measurements of the amount and spatial distribution of x-rays emitted by this single advanced imaging unit. NIST found that the scanner tested did comply with the relevant safety standard, ANSI/HPS standard N43.17, and that the radiation emissions from the scanner studied were below the limits set in this consensus standard. It should be noted that the researchers who conducted these tests are physicists, and their expertise is in measuring radiation emissions and calculating doses.
1 Dose limits in ICRP and NCRP recommendations are set with the understanding that the general public includes individuals who may be more susceptible to radiation-induced health effects, such as pregnant and potentially pregnant women, children, and persons receiving radiation treatment for medical conditions. These consensus limits are the distillation of eight decades of research into the health effects on humans of ionizing radiation and are assigned by impartial panels of radiation physicists, medical physicists, and radiation biologists worldwide who consider all available relevant data and modern analysis techniques.
2 Further information on radiation-emitting products used in security screening of people is available from the FDA.
3 Certain commercial equipment, instruments, or materials are identified in order to specify adequately the procedure. Such identification does not imply recommendation nor endorsement by the National Institute of Standards and Technology, nor does it imply that the material or equipment identified is necessarily the best available for these purposes.
4 Absolute individual organ doses are normalized to tissue-based cancer risk, to give the whole body “effective dose,” which can be properly compared to the effective dose from other sources of radiation that have different spectral distributions.