Leon Kaufman, PhD

Add to the safety and privacy questions about airport whole-body x-ray scanners one more: are they really effective?

A phantom study conducted using a Monte Carlo simulation suggests otherwise and that a substantial amount of explosive could be hidden on a terrorist’s body and escape detection by today’s backscatter x-ray scanners.

Some background: While x-ray backscatter units continue to be deployed at U.S. and foreign airports (

“Senators seek more study of whole-body x-ray airport scanner safety

), concerns remain unanswered by those with a stake in the deployment of the technology. In a previous note (

Radiation risks: Are airport body x-ray scanners ‘a great public health experiment’? 

) we briefly argued that contrary to assertions by the manufacturers, government, and assorted apologists, there are issues with penetration, dose and, separately, privacy.

Finally, the justification for all this: The flying public’s safety. But that claim may be questioned as well. We have modeled the machine’s performance using publicly available information on the units and a Monte Carlo program developed by investigators for CERN, the European Council for Nuclear Research. We concluded that only a very poorly informed malefactor would be foiled by these machines. This conclusion has been expressed by others familiar with the technology. (Airport body scanners would be "unlikely" to detect many of the explosive devices used by terrorist groups, 

; Government Accountability Office says airport body 

scanners may not have thwarted Christmas Day bombing

Full-Body Scanners Offer a Sneak Peek at TSA Bumbling

Figure 1 (black and white), shows x-ray backscatter images, on the left from a 125-kVp x-ray source, on the right from a 50-kVp source. The top images are from the Monte Carlo simulation. Depending on assumptions about geometry and detector efficiency in the backscatter unit, the single-view entrance exposure is from 10 microrem (best case) to 70 microrem (more realistic case). In the simulation using the Geant4 human phantom, the subject is carrying 320 grams of PETN plastic explosive. For context, the “shoe bomber” was carrying 40 grams of PETN, and the “Christmas bomber” was carrying 80 grams of PETN. Both amounts were considered sufficient to blow a hole in an aircraft’s skin. The amounts simulated here are four to eight times larger.

The bottom images are provided on the web and for government documents by the manufacturers. Notice at left how conveniently the subject has tied a bag of drugs to the outside of his shoulder and on his sides at the waist, and how he’s placed a gun against a background of tissue. Looking at this image, one could reasonably conclude that the placement of drugs and gun should have been reversed. On the image at right, the low-Z contraband (it does not matter whether it is drugs or explosive or even the same material as the underlying tissue) is packed in a hard edged “brick.” It is detected because of a well-understood hard-edge effect that these machines exhibit.



Figure 2 (color) shows the simulated “bomber.” He is carrying a small (35-ml) water bottle against his body, shown in blue, which is easily seen in the simulation. He is also carrying a 10 x 1-cm piece of iron taped to his abdomen (red in the figure), also easily seen. Finally, he has taped to his abdomen a tapered pancake of PETN explosive, 20-cm in diameter and 1-cm thick at the center, tapering to the edge of the pancake (orange in the color image). The composition of the pancake is approximately 320 grams of PETN explosive, and shows in the simulated images as a slight elevation of intensity, easily confused with normal anatomy (see the images below the simulations). A side image of the phantom (Figure 3) makes the point clearly: All we have done is added a bit of “belly” and exposure. A 40-g pancake would be just 1.25 mm thick at the center and invisible in the image.




Leon Kaufman is professor of physics, emeritus, at the University of California, San Francisco. Joseph W. Carlson is a former professor of physics at the same institution. They have collaborated on many articles and patents. 

Articles

A phantom study suggests whole-body x-ray scanners are not effective. Backscatter scanners unlikely to detect substantial explosive amounts on the body.

Phantom study suggests airport x-ray body scans could miss bombs

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

, 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.”

; and William Baukus, Manager, Technical Marketing of AS&E, “...backscatter x-rays do not need to penetrate all objects in the beam path...”




Let us look at the data. According to a president report prepared by the National Council on Radiation Protection and Measurements (NCRP),

 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,

 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 Rapiscan

 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.
 

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.

 The AS&E Bodysearch permits hard-copy printing and storage of images for “file reference.”



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 

, 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?



1. “Beyond metal detectors,” March/April 2005 issue of 

, 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.

Airport X-ray backscatter machines exposing adults, pregnant women, and children to radiation without knowing the effects for 20 years.

Leon Kaufman, PhD

Articles

Phantom study suggests airport x-ray body scans could miss bombs

Radiation risks: Airport body x-ray scanners ‘a great public health experiment’?