A very few dozen of zealots and madmen have led the economically and militarily most powerful nation the world has known to, in the name of security, spend billions of dollars, waste billions of man-hours each year, disrupt travel and commerce, and now, with the use of x-ray backscatter machines, begin a great public health experiment.
Contrary to every policy enacted since the effects of radiation were understood, we are engaging in a mass experiment of irradiation of not just adults, but also pregnant women, fetuses, children, women of child-bearing age, men who may conceive after exposure—in short, everyone unfortunate enough to have to be in an airport. The results will not be known for 20 years. Imagine if, as an investigator at UCSF, I had asked my IRB to allow me to carry out an experiment involving indiscriminate exposure to x-rays of a randomly selected population with no informed consent. I would have been laughed out of the school.
We are told that the machines are safe because they do not penetrate farther than sunshine and the “effective” dose is 10 (or, variously in different documents, 2 or 3 or 5) microrem. The safety questions hinge on penetration and dose. There is enough available information to suggest that the TSA and manufacturers may not be telling the full story about penetration. I have not found an indication of how dose is computed or data on it.
The problem is easy to spot. A then-spokesperson for the TSA was quoted as saying that the radiation used is as penetrant as sunshine: “The radiation dosage is about the same as sunshine,” the spokesperson said. The TSA, in its own pages, states, “Backscatter x-ray technology uses x-rays that penetrate clothing, but not skin, to create an image.”
Two manufacturers, AS&E and Rapiscan, support this position by omission, implication, and commission: Peter Williamson, vice president of worldwide sales for Rapiscan said the machines themselves are “very low power x-ray generators,” which penetrate some substances, including clothing, but not the human body or other materials, such as metals and ceramics.”1; and William Baukus, Manager, Technical Marketing of AS&E, “...backscatter x-rays do not need to penetrate all objects in the beam path...”2
Let us look at the data. According to a president report prepared by the National Council on Radiation Protection and Measurements (NCRP),3 the AS&E unit operates at 125 kVp, and the Rapiscan at 50 kVp with 1.5 mm and 1 mm, respectively, of aluminum filtration (see chart, above). The first is equivalent to CT kilovoltage, the second above that of mammography. AS&E quotes the mean x-ray energy at 60 keV,4 which is about right for the kVp. At 60 keV, only 0.3% of the beam’s energy is deposited in the first 1 mm of tissue, and a bit over half the energy is deposited by 25 cm of depth. For a typical body thickness, half of the energy leaves the body on the side opposite the beam’s entrance point. For the Rapiscan, with an average beam energy of 30 keV, 1.4% of the energy is deposited in the first mm, and half of the energy in the first 5 cm—well beyond the skin. At 40 keV, where there is a substantial amount of flux, 0.6% of the energy is stopped in the first mm, and half penetrates beyond 10 cm.
Look at the Rapiscan4 and AS&E4 images below. In the first, we see shadows of arm and leg bones, as well as air spaces in the mouth and neck areas. In Figure 3 we also see bone shadows, as well as clear shadows from lungs and spine in a hefty individual. The reason that we do not see good internal detail is not that there is no radiation at depth, but that the backscattered radiation from deep in undergoes further scattering and appears as a diffuse image. One can see that organic explosives in the soles of the shoes would not be detected.
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| Rapiscan |
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| AS&E |
For privacy protection, it is stated that images will be erased after being examined, that they cannot be secreted out by a member of the TSA workforce, which has a turnover of about 30%, and one that has seen hundreds of its members arrested for theft. Under “features” in the Rapiscan brochure are listed archive, save, and print.5 The AS&E Bodysearch permits hard-copy printing and storage of images for “file reference.”2
If they are wrong about penetration and privacy, can they be wrong about dose? I do not know the answer, but the burden of proof is on the people who promote this technology, among them former Homeland Security Secretary Michael Chertoff, not the millions who will be exposed each year.
Leon Kaufman holds a BS in Engineering Physics, a PhD in Physics from UC Berkeley and a Master's in Pacific Rim Studies from USF. He has held positions at the Lawrence Berkeley Laboratory, Bellcomm, UCSF, Toshiba America MRI, and AccuImage.
Editor’s note: Both Rapiscan and AS&E were provided copies of the article and invited to prepare a written response. Rapiscan declined the invitation. A response from AS&E is pending.
For Diagnostic Imaging’s original report on this topic, see:
Whole-body airport scanners are basically safe—or are they?
References
1. “Beyond metal detectors,” March/April 2005 issue of Homeland Security, a McGraw-Hill publication: pages 32-34.
2. Baukus WJ. X-ray imaging for on-the-body contraband detection. 16th Annual Security Technology Symposium and Exhibition, Session V, June 28, 2000.
3. SC 01-12, President Report on Radiation Protection Advice: Screening of Humans for Security Purposes Using Ionizing Radiation Scanning Systems (2003), prepared by the National Council on Radiation Protection and Measurements (NCRP), requested by DHHS, USFDA.
4. Kassiday D, Radiation Protection Activities Related to Security Products that Use Ionizing Radiation, May 25, 2004, USFDA, Center for Devices and Radiological Health (05-25-04_1330_Kassiday.pdf). By the same author, Security Products that Use Ionizing Radiation, presented to TEPRSSC, Oct. 1, 2003.
5. Company brochure, Rapiscan Secure 1000, Rapiscan Systems, an OSI Systems company.
