Airport Passenger Screening Using Backscatter X-Ray Machines Compliance with Standards (2015) / Chapter Skim
Currently Skimming:
7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems
7 Measurements, Dose Calculations, and System Design for X-Ray Backscatter Advanced Imaging Technology Systems
Pages 86-154
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 86... ...
Evaluation of effective dose from an unknown X-ray source, such as an AIT scanner, can be accomplished by measuring HVL, depth dose, and air kerma and then using the HVL and depth dose data to calculate the photon spectrum. With the photon spectrum and a suitable mathematical model of the person being scanned, 1 Effective July 1, 2015, the institution is called the National Academies of Sciences, Engineering, and Medicine.
|
From page 87... ...
The ANSI reference effective dose (EREF) is determined through a simple mathematical relationship2 based on measurements of the HVL and air kerma.
|
From page 88... ...
Although the X-ray source and associated beam-forming equipment generates a pencil beam, the continual horizontal scanning at a fixed vertical location results in irradiating a vertical line or band across the face of the ion chamber. Although only a portion of the chamber is exposed, the same portion is exposed for each attenua tor thickness, and all measurements are referenced to the same geometry exposure with no attenuator present.
|
From page 89... ...
The beam was further collimated by utilizing a sheet of lead that was custom fitted to the chamber and located behind the aluminum at tenuators to reduce scatter contributions from the aluminum attenuators and to provide narrow beam geometry. 4 This is the case for the Rapiscan Secure 1000; for the AS&E AIT system, the manufacturer's sug gested position (footprints)
|
From page 90... ...
Because the PDD measurement is a relative measurement, the chamber was not calibrated ahead of time, although the response at 50 kV was well characterized by the NRC subcontractor and compared to other parallel-plate ion chambers used in this study. To develop the PDD curves, it is necessary to have a tissue-equivalent mate rial of appropriate thicknesses.
|
From page 91... ...
Experimental Approach The general procedure utilized for performing the PDD measurements closely followed the procedure used for performing the HVL measurements where the tissue-equivalent material was used in place of aluminum sheets and with no lead collimation for the ion chamber. In order to maximize the signal-to-noise ratio for this set of measurements, the AIT system was operated in a partially fixed scanning mode (described in detail under the section "Half-Value Layer Measurements")
|
From page 92... ...
Corrections for temperature and pressure, relative to the chamber calibration conditions, are applied to ensure the precision of these measurements. Experimental Approach Air kerma measurements were performed utilizing a Keithley model 96020C 150 cc parallel-plate ion chamber in conjunction with a PTW Unidose electrometer.
|
From page 93... ...
The charge was subse quently converted to air kerma using the appropriate chamber calibration factor and making air temperature and pressure corrections relative to the chamber's calibration conditions. For each of the air kerma measurements, the 150 cc parallel-plate ion chamber was centered at x = 0 cm, y = 0 cm, with the vertical dimension (z)
|
From page 94... ...
Experimental Approach For each set of measurements, a large-area Keithley model 96020C 150 cc parallel-plate ion chamber interfaced with a PTW Unidose electrometer was used. The electrometer was utilized in integration mode13 for each of these measure ments.
|
From page 95... ...
The parallel-plate ion chamber was positioned at the two positions along the edge of the AIT system exit on the side toward the operator position. The detector was centered at a height z = 120 cm for each scatter measurement, essentially even with the midpoint of the vertical extent of the phantom.
|
From page 96... ...
4.9 cm3 chamber and illustrated graphically in Figure 7.3. Again, a second set of PDD data was collected using the larger 15 cc parallel plate ion chamber with a 32 mg/cm2 window thickness.
|
From page 97... ...
, they are not commonly characterized for energies at which AIT systems operate. This presents a unique challenge because any peak dose for 50 kV systems is expected to occur at very shallow tissue depths and requires a specialized ion chamber having a very thin entrance window, which 15 Uncertainties in the tissue depth are substantially smaller than the size of the symbols in the accompanying figures.
|
From page 98... ...
, thicker-window ion chamber. This data set provided enhanced measurement ac curacy over all but the smallest range of tissue depths.
|
From page 99... ...
maximum, the PDD curves from the two ion chambers match within the mea surement uncertainties for tissue depths greater than 1 mm. At depths less than 1 mm, the larger-volume chamber does not resolve the dose peak due to the thicker window that generates secondary electrons prior to the X-ray beam entering the sensitive volume of the ion chamber.
|
From page 100... ...
Each measurement was made with the ion chamber positioned as previously described, and a normal subject scan was made along the central axis (x = 0 cm, y = 0 cm) at several vertical positions (z)
|
From page 101... ...
The Fluke 451B was operated in integration mode and never recorded any values above 0 nGy. Results presented are based on the Keithley model 96020C 150 cc ion chamber with charge collected by the PTW Unidose electrometer.
|
From page 102... ...
This should not be surprising because the X rays produced by a 50 kV beam are readily attenuated by quite modest thicknesses of lead that may be expected to be incorporated into the scanning system. Scatter Radiation Geometry Measurement of the scatter radiation fields was performed at a very conserva tive position (the very edge of the AIT system)
|
From page 103... ...
THE RAPISCAN SECURE 1000 SYSTEM MEASUREMENT RESULTS AND SYSTEM DESIGN A Rapiscan Secure 1000 (serial number S51023005) became available for mea surements in July 2014.
|
From page 104... ...
m. In the event of a malfunction, the system shall In this report it is shown that the computed effective terminate radiation exposure rapidly enough so that doses for a normal scan are about an order of no location on the subject's body shall receive an magnitude lower than the recommended ANSI standard ambient dose equivalent (H*
|
From page 105... ...
For any X-ray system that normally keeps high Confirmed on AS&E prototype AIT system at TSIF. voltage applied to the X-ray tube at times other than during a scan, there shall be at least one lighted "X ray on" indicator at the control console where X rays are initiated indicating when X rays are being produced.
|
From page 106... ...
m. In the event of a malfunction, the system shall terminate radiation exposure rapidly enough so that no location on the subject's body shall receive an ambient dose equivalent (H*
|
From page 107... ...
HVL1 and HVL2 were 0.92 and 1.47 mm Al for the anterior unit and 0.85 and 1.42 mm Al for the posterior unit, respectively. Percent Depth Dose Results Depth dose measurements were at the same height as the HVL measurements, z = 85 cm, with x = 0 cm and y = 0 cm.
|
From page 108... ...
the posterior unit. Error bars are not shown since they are approxi mately the size of or smaller than the symbols.
|
From page 109... ...
ion chamber at tissue thicknesses (a)
|
From page 110... ...
ion chamber at tissue thicknesses (a)
|
From page 111... ...
chamber chamber chamber chamber 0.00 98.98 100.00 100.00 100.00 0.02 100.00 -- 96.19 -- 0.20 96.87 98.17 96.77 97.96 0.34 98.36 97.47 98.62 97.69 0.51 98.30 96.76 94.77 96.48 0.71 96.70 94.49 90.92 95.49 0.99 102.36 92.43 85.79 93.32 2.03 91.69 86.74 81.72 86.25 3.12 88.31 81.47 75.16 80.43 4.24 80.39 76.56 73.94 74.37 5.87 78.45 68.58 69.50 66.98 7.95 66.29 58.45 57.93 59.68 9.65 55.12 56.29 56.99 54.78 15.55 37.99 40.96 36.06 41.12 20.35 34.52 32.58 31.95 31.96 26.25 25.40 24.97 24.38 25.34 31.05 22.23 20.36 20.57 18.68 40.70 14.82 14.15 10.81 12.91 51.40 13.03 9.79 9.65 8.96 62.20 5.67 7.59 9.65 6.45 PDD data were collected using the larger 15 cc parallel-plate ion chamber (32 mg/cm2 window thickness)
|
From page 112... ...
the posterior unit.
|
From page 113... ...
at Several Vertical Positions (z) for the Rapiscan Secure 1000 Anterior Unit Posterior Unit Air Kerma Standard Air Kerma Standard per Scan Deviation per Scan Deviation Vertical Position z (cm)
|
From page 114... ...
The Fluke 451B was operated in integration mode and never recorded any values above 0 nGy. Results presented are based on the Keithley model 96020C 150 cc parallel-plate ion chamber with the charge collected by the PTW Unidose electrometer.
|
From page 115... ...
Leakage -- Rear exterior 4.0 6.0 x = 130 cm, y = 0 cm, z = 171 cm Scatter -- Center of exit 0.0 16 x = 0 cm, y = 72 cm, z = 120 cm Scatter -- Operator side of exit 7.0 4.0 x = −44.8 cm, y = 72 cm, z = 120 cm the public documentation available, from both public sources and TSA, for the Rapiscan Secure 1000. The committee also inspected the Rapiscan Secure 1000 AIT system that was made available to the committee at NIST.
|
From page 116... ...
; • Length of the scan time, in seconds; • If the operator instructions are available; and • General condition of the AIT system. It is not specified what the exact safety interlock test referred to above is.
|
From page 117... ...
, again where a potential bystander could be. The data on Forms R-0646 and R-0685 were compared to the exposure limits set, and if results were within the administrative integrated exposure limits, the AIT system was considered to meet the ANSI/HPS N43.17-2009 standard with respect to limits for reference effective dose and X-ray leakage.
|
From page 118... ...
The committee agrees with this recommendation but was unable to determine if this was being done because of the current lack of X-ray backscatter AITs in the field at commercial airports. Committee Review of the Interlocks on the Rapiscan Secure 1000 System The committee considered potential failure mechanisms that could result in X-ray overexposure of the person being screened or bystanders such as the opera tor.
|
From page 119... ...
In this report, the committee shows that for a standard screening of approximately 6 seconds, the computed effective doses are about an order of magnitude lower than the recommended ANSI standard of 250 nSv/screen. The person being screened would have to stand in the AIT system with the X-ray source
|
From page 120... ...
Committee believes image quality motion of the X-ray beam relative to the subject shall would prevent operation in these conditions when be interlocked and the exposure shall terminate when advanced imaging technology is not used. the rate of motion of the beam in any direction falls below a preset minimum speed.
|
From page 121... ...
For any X-ray system that normally keeps high Confirmed on NIST AIT system. voltage applied to the X-ray tube at times other than during a scan, there shall be at least one lighted "X ray on" indicator at the control console where X rays are initiated indicating when X rays are being produced.
|
From page 122... ...
(nSv) Rapiscan anterior 0.92 1.47 ~11 30.6 3.5 Rapiscan posterior 0.85 1.42 ~11 29.8 3.2 AS&E 1.1 1.7 ~12.5 113 15.5 NOTE: EREF, reference effective dose; HVL1, first half-value layer; HVL2, second half-value layer; PDD, percent depth dose .
|
From page 123... ...
Crayton Pruitt Family Department of Biomedical Engineering, University of Florida, applied state-of-the-art computational techniques to assess the organ-absorbed dose and whole-body effective dose received by adults, children, and the developing fetus of pregnant females scanned by a computationally mod eled X-ray backscatter AIT system and by variations on that system that include some current and anticipated engineering designs. Passengers were simulated using a suite of hybrid digitized phantoms.
|
From page 124... ...
The idea is that with such a model system it is possible to compare how differences in design and settings affect dose for not only previous systems but also potential future systems. Thus, a Monte Carlo simulation model of a ref erence X-ray backscatter AIT system was implemented for the computations.
|
From page 125... ...
The effect of variations in organ and effective dose due to changes in the horizontal location of the screened passenger between the anterior and posterior units; 23 This study was conducted to enable evaluation of and comparisons to AIT systems that may not include angular variations in the vertical and horizontal scan, which is created by tilting the X-ray source as it moves from the bottom to the top of the scanning region and fixed horizontal position of the tube anode. The study also intends to do comparisons with previous computations that made the parallel beam assumption due to modeling limitations of their Monte Carlo approach.
|
From page 126... ...
The results of the Monte Carlo process are initially normalized to the absorbed dose per incident photon.24 The objective is to obtain the effective dose per scan, which in turn requires the number of incident photons per scan. This "normaliza 24 Because Monte Carlo calculations follow the histories of individual photons and average the results over large numbers of photons, the result is the energy deposited (or absorbed dose)
|
From page 127... ...
SOURCE: Figure 10 from NIST Report to DHS, Assessment of the Rapiscan Secure 1000 Single Pose (ATR version) for Conformance with National Radiological Safety Standards, Jack L
|
From page 128... ...
The results presented in the following sections are normalized to a value of 9.203 × 108 photons per scan, which corresponds to an air kerma of 68 nGy/ scan representing a simulated scanning protocol for the reference AIT system. All results relating to organ absorbed dose and effective doses received during pas senger screening will scale linearly with the value of air kerma.
|
From page 129... ...
Energy Sampling The softened energy spectrum shown in Figure 7.11 was incorporated into the computational X-ray source term. Standard Monte Carlo methods based on random numbers were used to sample photon energies for the emerging scanning beam with probabilities represented by this distribution.
|
From page 130... ...
All results from a scan simulation obtained in this manner must be multiplied by a "normalization factor" to convert, for example, from dose-to-passenger per starting photon to dose-to-passenger per scan. Once the X-ray source term was constructed 29 Ray tracing is a technique for generating an image by tracing the path of light (or X rays)
|
From page 131... ...
Additionally, using MCNPX, the fluence into that volume of air 30 The value of 68 nGy was selected based on an earlier measurement made by NIST because mea surements by the NRC subcontractor of air kerma per scan were not available when it was necessary to begin the Monte Carlo computations. All results relating to organ absorbed dose and effective doses received during passenger screening will scale linearly with the value of air kerma.
|
From page 132... ...
The detailed geometry of the parallel-plate ionization chamber did not need to be modeled in the virtual simulations because the software was able to estimate tissue absorbed dose directly. Simulated doses were quantified as a function of depth in the virtual tissue phantom and used to generate a PDD curve.
|
From page 133... ...
scan width was approximately 90 cm at this point, according to measurements detailed in the NIST report discussed in this section. Based on visual inspection of a Rapiscan Secure 1000 unit at TSIF, the source vertically traverses (z-axis)
|
From page 134... ...
, facing the anterior unit, and (2) between the anterior and posterior units (center of phantom at a distance of 43.5 cm from the front plane of the AIT system)
|
From page 135... ...
Lateral and anterior-posterior views of the adult male and female phantoms that were used in these simulations are shown in Figures 7.16 and 7.17, respectively. The colored features provide visual distinction of tissues and organs.
|
From page 136... ...
views of the adult male phantoms used for the passenger screening simulations set at 50th percentile for height and varied by body mass index: (A) 5th percentile, (B)
|
From page 137... ...
views of the adult female phan toms used for the passenger screening simulations set at 50th percentile for height and varied by body mass index: (A) 5th percentile, (B)
|
From page 138... ...
FIGURE 7.19 Left lateral and anterior-posterior views of the pediatric female phantoms used for screening simulations set at 105-cm height and varied by body mass index: (A) 5th percentile, (B)
|
From page 139... ...
Dosimetry Results for Standard Screening Conditions All phantom simulations presented in this report were performed in MCNPX v2.7 using the custom X-ray source term described earlier and the passenger po8. Organ and effective doses to representative pregnant female passengers The UF pregnant female phantom series provides highly-detailed anatomical representation for eight fetal ag sitioning described above in Figure 7.14.
|
From page 140... ...
Bolch, University of Florida. Absorbed doses (nGy)
|
From page 141... ...
Complete lists of absorbed doses for all male and female organs are provided in Appendix C The effective dose is the value obtained by averaging the results from both male and female phantoms.
|
From page 142... ...
Per anterior scan 12 10 9 7 4 Per posterior scan 3 3 3 2 2 Per screen 15 13 12 9 6 NOTE: For comparison, the ANSI reference effective dose for an HVL1 of 1.18 mm Al and an air kerma of 68 nGy per scan is 20 nSv. TABLE 7.13 Summary per AIT Screening of Critical Organ Absorbed Dose and Effective Dose to Pediatric Passengers of ~105 cm in Total Height and Three Different Weight Percentiles Weight Percentiles, U.S.
|
From page 143... ...
15 Weeks Post-Conception 25 Weeks Post-Conception 38 Weeks Post-Conception Anterior Posterior Screen Anterior Posterior Screen Anterior Posterior Screen Whole body 7.2 1.3 8.5 3.4 0.6 4.0 3.4 0.9 4.3 Brain 3.7 2.3 6.0 0.8 1.4 2.2 0.5 2.7 3.2 Lungs 8.3 0.9 9.2 5.1 0.2 5.3 2.6 0.2 2.7 Thyroid 5.8 1.4 7.2 2.5 0.4 2.9 1.3 0.3 1.6 Active bone marrow 14 2.5 16 6.4 1.8 8.1 8.4 2.7 11 143
|
From page 144... ...
were scanned using each geometry, and the resulting doses are summarized in Table 7.15. Passenger Position Within the Unit Variations in the dose to organs and effective dose received by the passenger due to position variations between the anterior and posterior units was investigated by placing the adult male phantom closer and further from each scanning unit TABLE 7.15 Absorbed Doses to Critical Organs for Males and Females and Effective Doses for 50th Percentile of U.S.
|
From page 145... ...
Table 7.17 provides a summary of the resulting organ absorbed doses and effective dose received by the passenger.
|
From page 146... ...
Breast 23 26 29 32 35 37 39 Thyroid 17 22 26 31 35 39 42 Skin 44 46 49 52 54 56 57 Eye Lens 43 45 47 49 50 52 53 Effective dose (nSv) Per anterior scan 9 11 13 15 17 19 21 Per posterior scan 3 3 5 6 8 10 11 Per screen 12 14 18 22 25 29 32 Ratio (to column 1)
|
From page 147... ...
The second failure mode assumed a stationary vertical beam position and a nonfunctional chopper wheel resulting in a stationary pencil beam. A circular beam cross section was assumed for the second failure mode with an area equivalent to a 3 mm × 3 mm square beam, the approximate dimensions of the properly collimated scanning beam under normal TABLE 7.18 Comparison of the Absorbed Dose per Scan to Either the Total Skin Volume (Dermis and Epidermis)
|
From page 148... ...
37 and eye lens (0.5 Gy or 500,000,000 nGy) .38 The skin determines the threshold limit for the breast due to the sensitivity for necrosis of the skin compared with clinical issues involving breast tissue.39 Summary The NRC subcontractors and the committee performed a detailed compu tational assessment of the doses received during a security screening process in volving X-ray backscatter AIT systems.
|
From page 149... ...
The model represents a dual scan–single-pose configuration with an anterior unit providing an anterior image and a posterior unit producing a posterior image. The spatial dimensions were obtained from the previous NIST report evaluating a Rapiscan Secure 1000.
|
From page 150... ...
• The absorbed doses to individual tissues and organs for the reference adult phantoms located midway between the anterior and posterior AIT system units were all greater for a plane-parallel incident-X-ray beam compared with the exposure conditions in an actual AIT system. • The sex-averaged effective dose for the adult phantoms increased as the phantom was located closer to the anterior unit.
|
From page 151... ...
FINDINGS AND RECOMMENDATIONS Measurements Key Finding: Using appropriate detectors, the estimated values of the radiation outside the inspection area that might affect a bystander are so low as to be statistically indistinguishable from the background radiation. System Design Although the radiation measurements and dose computations were performed for the committee in a detailed manner, the committee was unable to unequivo cally determine whether the X-ray backscatter AIT systems studied have adequate operating safety interlocks that will prevent the AIT system from exceeding the ANSI/HPS N43.17-2009 standard under every imaginable situation for the fol lowing reasons: • The committee was not given an opportunity to independently verify how all of the interlocks would perform in different situations, with the exception of simple functions such as termination of operation if a door was opened.
|
From page 152... ...
Key Recommendation: Any future testing procedures should at a minimum continue to follow the indicators, controls, and safety interlocks require ments of the ANSI/HPS N43.17-2009 standard, or similar testing procedures, and include daily verification of safety parameters by a test piece. Dose Computations The committee's approach in examining the dose to the individual being screened differs from that of previous investigations in two ways: • It made use of sensitive detectors with tissue-equivalent phantoms to verify beam intensity, X-ray quality, and penetration; and • It performed computations using estimates of beam intensity, scanning geometry, and digitized human phantoms that have realistic dimensions and morphology.
|
From page 153... ...
This included details of absorbed dose distributions in a wide variety of body types, including children and the developing fetus, as well as specific organs in each of those body types. Under standard operating conditions, the committee found that: Key Finding: • No person, regardless of age and weight modeled, would exceed the effective dose limit per screen (i.e., 250 nSv/screen)
|
From page 154... ...
This fact points out how important the previous recommendation is that there is some independent mechanism to ensure that the AIT does not screen any person for longer than the time needed to acquire the appropriate image. Key Finding: The agreement between the estimated dose results from the NRC subcontractor and the results from previous studies confirms that the calcula tions performed in previous studies were adequate to establish compliance with effective dose limits recommended in ANSI/HPS N43.17-2009.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.