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According to the TSA safety documents, AIT uses an 50 keV source that emits a broad spectra (see adjacent graph from here). Essentially, this means that the X-ray source used in the Rapiscan system is the same as those used for mammograms and some dental X-rays, and uses BOTH ‘soft’ and ‘hard’ X-rays. Its very disturbing that the TSA has been misleading on this point. Here is the real catch: the softer the X-ray, the more its absorbed by the body, and the higher the biologically relevant dose! This means, that this radiation is potentially worse than an a higher energy medical chest X-ray.

With that being said, because the scanners have both a radiation source AND a detector in the front AND back of the person in the scanner, it is actually possible for the hardware to conduct a classic, through-the-body X-ray. The TSA claims that the machines are not currently being used in that way; however, based on the limited engineering schematics released in the safety documents, they could be certainly be easily reconfigured to do so by altering the aluminum-plate (or equivalent) filter or by changing the software. So the hardware has the capability to output quite high doses of radiation, however a biological dose is a function of the time of exposure as well as the proximity to the source and the power of the power of the source. Unfortunately, it is difficult to determine which zones in the scanner are ‘hottest’ because that information is masked in the document. An excerpt of the safety evaluation from Johns Hopkins is shown below to give you sense of how much other information is being withheld. Ultimately my point is this: even though the dose may actually be low, these machines are capable of much higher radiation output through device failure or both unauthorized or authorized reconfiguration of either hardware or software.

Which brings me to how the scanner works. Essentially, it appears that an X-ray beam is rastered across the body, which highlights the importance of one of the specific concerns raised by the UCSF scientists… what happens if the machine fails, or gets stuck, during a raster. How much radiation would a person’s eye, hand, testicle, stomach, etc be exposed to during such a failure. What is the failure rate of these machines? What is the failure rate in an operational environment? Who services the machine? What is the decay rate of the filter? What is the decay rate of the shielding material? What is the variability in the power of the X-ray source during the manufacturing process? This last question may seem trivial; however, the Johns Hopkins Applied Physics Laboratory noted significant differences in their test models, which were supposed to be precisely up to spec. Its also interesting to note that the Johns Hopkins Applied Physics Laboratory criticized other reports from NIST (the National Institute of Standards and Technology) and a group called Medical and Health Physics Consulting for testing the machine while one of the two X-ray sources was disabled (citations at the bottom of the page).

These questions have not been answered to any satisfaction and the UCSF scientists, all esteemed in their fields and members of the National Academy of Sciences have been dismissed based on a couple of reports seemingly hastily put together by mid-level government lab technicians. The documents that I have reviewed thus far either have NO AUTHOR CREDITS or are NOT authored by anyone with either a Ph.D. or a M.D., raising serious concerns of the extent of the expertise of the individuals and organizations evaluating these machines. Yet, the FDA and TSA continue to dismiss some of the most talented scientists in the country…

With respect to errors in the safety reports and/or misleading information about them, the statement that one scan is equivalent to 2-3 minutes of your flight is VERY misleading. Most cosmic radiation is composed of high energy particles that passes right through our body, the plane and even most of the earth itself without being absorbed or even detected. The spectrum that is dangerous is known as ionizing radiation and most of that is absorbed by the hull of the airplane. So relating non-absorbing cosmic radiation to tissue absorbing man-made radiation is simply misleading and wrong.

Furthermore, when making this comparison, the TSA and FDA are calculating that the dose is absorbed throughout the body. According the simulations performed by NIST, the relative absorption of the radiation is ~20-35-fold higher in the skin, breast, testes and thymus than the brain, or 7-12-fold higher than bone marrow. So a total body dose is misleading, because there is differential absorption in some tissues. Of particular concern is radiation exposure to the testes, which could result in infertility or birth defects, and breasts for women who might carry a BRCA1 or BRCA2 mutation. Even more alarming is that because the radiation energy is the same for all adults, children or infants, the relative absorbed dose is twice as high for small children and infants because they have a smaller body mass (both total and tissue specific) to distribute the dose. Alarmingly, the radiation dose to an infant’s testes and skeleton is 60-fold higher than the absorbed dose to an adult brain!