Microwaves vs. X-Ray Transmission

8/3/2019 Microwaves vs. X-Ray Transmission

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A COMPARATIVE STUDY OF NEW IMAGING SECURITY TECHNOLOGIES FOR

INSPECTING PEOPLE: PART I. ACTIVE MILLIMETER WAVE RADAR VERSUS X-

RAY TRANSMISSION SCANNER

by Iouri Emelianov (ADANI, Belarus)

May 18, 2007

Abstract: Two different security systems, namely X-ray transmission scanner and millimeter wave

radar portal, are compared to reveal their limits to detect dangerous objects hidden on the human

body or inside it. The concerns upon potential long-term adverse effects of microwave radiation on

human health are raised.

Key words: security systems, microwave radiation, X-ray transmission, detection ability, health

effects

1. Introduction

In recent years new imaging security technologies (passive and active terahertz imaging, active

millimeter wave imaging, X-ray transmission imaging, X-ray backscattering imaging) have been

developed to detect and identify dangerous and prohibited objects concealed on the human body or

inside it.

The purpose of this study was to reveal the detection limits of different imaging technologies as

well as to investigate their potential risks for human health. The X-ray transmission technology,

which is better established, is used here as reference to estimate other technologies. In Part I, X-ray

transmission scanner and active millimeter wave radar system are compared. X-ray backscattering

and passive terahertz imaging technologies will be the subject of the following parts of this study.

The comparison is mainly based on the technical information available for the DRS SecureScan

(ADANI, Belarus) [1], which is the X-ray digital transmission scanner, and SafeScout 100/360 (L-3

Communications Safe View, USA) [2], which is an active millimeter wave scanning portal.

2. Risks for human health

2.1.Microwave radiation


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Obviously the manufacturer of SafeScout claims that microwave radiation used in their scanner

is absolutely safe for people. This conclusion is predicated upon the following facts:

1) The power density of radiation emitted by the source is very low (0.265 W/cm2), which is

much lower than from mobile phones.

2)

The microwave radiation is emitted in the wide frequency range 24.25 to 30 GHz, which is

totally reflected by the skin layer of the body.

3) No any harmful effects on human health were reported.

The power density of microwave radiation used in SafeScout complies with many international

and national standards imposing both general limitations on all (1 mW/cm2, [3-6]) or specific

sources (10 W/cm2, [7-9]) of electromagnetic fields in the frequency (SHF) range from 0.3 to 300

GHz.However, this compliance does not mean that microwave radiation is absolutely safe because

aforesaid limits were established by accounting for the thermal effect only. Long-term non-thermal

biological effects, which were found to exhibit cumulative character[10] (harmful effects on the

eyes, such as cataracts), can be more important and dangerous for the human health even at much

lower power densities as it has been evidenced by many scientific studies [11-15]. Microwave

radiation lies in the frequency range of vital biological processes and can interfere with them

causing various adverse effects on human health [16]. The Commission on Science and Technology

of European parliament states in the recent report concerning mostly the mobile telephony [17] that

the adverse biological effects of microwave radiation at power density of tenth parts of 1 W/cm 2

have been convincingly demonstrated and suggests lowering the commonly adopted limitation on

power density to 10 nW/cm2. This is 26.5 times lower than the power density used by SafeScout.

Some scientists have given the evidence of adverse effects of microwave radiation on adults at

power density


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communications [22], and currently in security systems [2]. In previous studies various adverse

effects on the health were found at power density >20 W/cm2[19]. It means that adverse effects

could be observed at lower power density, but no through researches were done with respect to

long-term cumulative effects. Yet, the small penetration depth of microwaves in K/Ka band does

not mean that radiation makes no effects on living organism either. Skin contains numerous

periphery sensorial nerve endings that are closely interrelated with other vital biological systems,

and the eyes surface layer is very susceptible to radiation. In [20], general conclusion was made

that neither the laboratory nor human research literature is sufficient at this time to make a

definitive assessment of the health risk of long-term, low-level exposure to microwaves, e.g. which

may have occurred for some police officers using traffic radars in the past.

Thus the innocence of microwaves even at low power densities remains in question due to

lack of proper scientific studies of long-term biological effects of this non-ionizing radiation. Untilit is proven to be safe, the active microwave scanners should not be considered as absolutely safe

and used freely, and the record of number of exposures should be restricted and controlled like it is

normally done for X-ray scanners.

2.2.X-ray radiation

X-rays are classified as ionizing radiation of which properties and biological effects are well

known. Deterministic effects of ionizing radiation can only occur at very high dose that is far

beyond the medical and security equipment useful range. Therefore, only stochastic effects (cancer

risk and hereditary diseases) should be of concern. The effective dose [23] was introduced to limit

the risk of stochastic effects of radiation exposure and is used for regulatory purposes worldwide.

The Table 1 shows the annual effective dose constrains for radiation workers and general public

from all non-medical man-made sources adopted by International Commission on Radiation

Protection [23] and national authorities (National Council on Radiation Protection and

Measurements (USA) [24] and Ministry of Health (Russia, Belarus) [25-26]).


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Table 1. Limitations for annual effective dose from different standards

ICRP (1991,

2005)

NCRP (USA, 1993);

ANSI 43.17

Ministry of Health (Russia, 1999;

Belarus, 2000)

Occupational

exposure1

20 mSv 1 mSv 20 mSv

General public

exposure2

1 mSv 1 mSv or

250 Sv

1 mSv or

300 Sv

1 For pregnant women (operators) the equivalent dose on the surface of abdomen must not exceed 2

mSv (ICRP), 0.5 mSv/month (NCRP) or 1 mSv (Russia, Belarus) for pregnancy period after its

declaration.

2 If information relating to other sources of radiation exposure is not available, lower limits should

be applied for any given radiation source

For general public there is no difference in dose limits for men and women, but some additional

limitation is recommended for pregnant women who are the radiation workers.

It is recommended that the X-ray facility for scanning people for non-medical reasons shall be

operated to ensure that no individual scanned receives from the facility an effective dose in excess

of 0.25 mSv [24, 27], i.e. 25% of the annual effective dose limit as mentioned above. However,

assuming the individual did not receive radiation exposure from any other non-medical source, the

annual quote of the dose for one person in case of repeated inspections of him/her can be as high as

1 mSv.

It follows from these recommendations that two different subject dose limits are applicable to

the DRS SecureScan as the situation requires. If the user of the scanner is able to make a record

of doses received by the individuals from all man-made, non-medical radiation sources during any

twelve-month period, e.g. in prisons, the maximum annual subject dose from DRS SecureScan

can be as high as 1 mSv. If such the control is not possible, e.g. at the airports, the annual effective

dose limit of 0.25 mSv shall be applied. If the dose per scan is 0.1 Sv or less, the number of scans

is not limited [27]. Table 2 gives the limitation for annual permissible number of scans which

depends on the dose per scan. These data are typical for DRS SecureScan.


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Table 2. Permissible number of scans per annum for DRS SecureScan

Permissible number of scans per annum as

function of dose per scan

Annual

limit,

mSv 2.0 Sv 1.5 Sv 1.0 Sv 0.5 Sv

Typical applications

0.1 Sv

1 500 666 1000 2000 unlimited Prisons, military sites, nuclear power

stations, precious stones mines

0.25 125 166 2502 500 unlimited Airports, customs offices, public

places

Sometimes questions are asked on how big the dose per scan is or what is the risk to get cancer.

The diagram in Fig.1 compares the typical effective dose received by a person from 1 scan at DRS

Secure Scan with other typical doses from different sources. It can be easily seen how small it is.

100 Sv(1 chest X-ray

exam)

61.6 Sv(London-Los

Angeles, 10 h)

(6.5 Sv/h onaverage at10000 m

altitude)

6.6 Sv(average natural

background, 24 h)

(typical range -2.7-27 Sv, 24 h) < 2 Sv

(1 scan, DRS SecureScan)

1 scan = 7.3 h natural background1 scan =18 minut es (air flight, 10000 m altitude)50 scans = 1 chest X-ray examination

Fig.1. Dose comparison from different sources

Typical doses for air flights were taken from the US Federal Aviation Administration (FAA)

and National Research Center for Environment and Health (GSF, Germany) reports [28-29]. Some

missing data were interpolated by using dose per hour values for similar air flights from the above


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mentioned reports. Cosmic radiation dose depends upon departure and destination geographical

locations, flight time, flying path and typical altitudes along path.

Natural radiation background varies with geographical location on the Earth. Worlds average

data were taken from UNSCEAR (United Nations Scientific Committee on the effects of Atomic

Radiation) 2000 Report [30].

Data for cancer risk, which were adapted from [31-32], are shown in Fig.2. One chest X-ray

radiography (100 Sv) is equivalent to 50 scans with DRS SecureScan in cumulated effective dose,

from which we can assume the risk of cancer for 50 scans with DRS SecureScan to be

approximately the same.

50 scans(DRS Secure Scan)

1 chest X-ray radiography

1.5 cigarettes

40 tablespoons peanut butter

30 cans diet soda

100 charcoal steaks

Fig.2. Risk of cancer inducing due to different reasons (1 person per 1,000,000 persons)

In summary, one can conclude that the dose from DRS SecureScan is very low and the

corresponding risk of stochastic effects is negligible and is compared well with risks from non-

radiation sources.

3. Detection thresholdsConcealed objects3.1.

Both scanners are declared to detect all kinds of materials and objects concealed on the human

body. Unfortunately, it is impossible to compare the detection ability of the scanners with respect to

the object dimensions, object materials and its location on the human body in a scientifically right

manner due to the lack of the unified checking procedure and test phantoms. Currently work is

going on to come up with a new standard [33] for quality assessment of X-ray and gamma-ray

personal scanners, which will establish the scientific background for comparison of different


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technologies using ionizing radiation. X-ray backscattering technology is in some way similar to

microwave technology, and this standard could be probably used for comparative studies.

Nevertheless, the two systems exhibit the evident difference in their ability to detect objects

concealed inside of the body (swallowed or hidden in natural anatomical cavities) and inside of the

prosthetic devices and casts. The microwave scanner cannot detect such the objects and the X-ray

transmission scanner certainly can as illustrated in Fig.3.

Swallowed drugs

Fig.3. X-ray transmission images of swallowed drugs and artificial arm with cavity that can be

used to conceal something.

Artificial

arm


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Moreover, microwaves in the frequency range specified are reflected by natural leather,

especially when it is impregnated with water. Therefore, it is no problem to make some case from

this or another disguising material, which would follow the landscape of the human body, and place

any objects inside of it. Microwave scanner will never detect any objects hidden in this manner,

while X-ray scanner will see through any casing easily. Besides, the microwave scanner will

hardly detect the objects hidden inside shoes, and the X-ray scanner can detect such objects (see

Fig.4).

Fig.4. Images of mens and womens shoes taken with DRS SecureScan

2D versus 3D3.2.

SafeScout makes the 360 scan of the human body and 3D surface image is mathematically

reconstructed. It can help to recognizing a limited number of objects by their particular shapes, but

many objects are very similar by their shapes, and some others, e.g. plastic explosives can be cast in

either shape. The objects can also be concealed in casual disguising casings making it impossible to

see their contours. Thus, there are many situations when 3D analysis is helpless.

DRS SecureScan makes 2D image of the whole body, which shows the inner structure of the

objects. The inner structure can tell more about the objects to classify them as potential threat or

innocent stuff. Fig. 5 demonstrates how the radio receiver was classified as innocent object.


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antenna

loud speaker

Fig 5. The image of the radioreceiver taken with DRS SecureScan.

Wire detection3.3.

The capability of detecting thin wires is very important for any security system because wires

usually take part of typical and improvised explosive devices. Such information is not available for

microwave scanners, but there is significant doubt that their ability to detect wires is less than that

of X-ray scanners (copper wire as thick as 0.2 mm in diameter can be detected, Fig.6). Furthermore,

microwave scanners will not see the wires inside of the device.

Fig.6. Wire detection ability of DRS SecureScan


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4. Peoples privacy

Since the human body is highly reflective to microwave radiation, the system easily captures

detailed images of a person's anatomy. FAA officials still say they're not sure the 3-D scanner is the

ideal solution for its security needs because they are concerned over the extremely detailed images

the system gives of each person's anatomy under clothing [34]. The bodily detail is probably more

than most travelers would feel comfortable with, said an FAA official.

In contrast, the images taken with X-ray transmission scanners do not show too detail of the

human anatomy which could be offensive. This technology does not undress people like surface

sensitive technologies, and the appearance of intimate areas can be tolerated (see. Fig.7).

male female

Fig.7. Appearance of male and female intimate areas on the images taken with DRS SecureScan

5. Conclusions

As it follows from the literature review, the innocence of microwaves in the frequency range

of 20 to 40 GHz remains in question even at low power densities due to lack of proper scientific


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studies of long-term biological effects of this non-ionizing radiation. Until it is proven to be safe,

the active microwave scanners should not be considered as absolutely safe and used freely, and the

record of number of exposures should be restricted and controlled like it is normally done for X-ray

scanners.

X-ray transmission scanners use very small exposure doses per scan (typical range is 0.1-2 Sv),

that are comparable with average natural radiation background level and are much less than doses

that aircraft passengers receive from cosmic radiation during intercontinental flights. The

assessment of the cancer risk shows that it is negligible. The 50 scans (dose per scan of 2 Sv) on

DRS SecureScan are equivalent in cancer risk to smoking 1.5 cigarette or drinking 30 cans of diet

cola.

The unified checking procedure and test phantoms are needed to compare the detection ability

of the scanners with respect to the object dimensions, object materials and its location on the humanbody in a scientifically correct manner. However, some limitations of microwave technology are

evident and are summarized in the Table 3.

Table 3. Detection thresholds for different technologies

Can the system detect this?Detection parameter

DRS SecureScan SafeScout

Swallowed objects Yes No

Objects concealed in natural anatomical cavities Yes No

Objects concealed under casing made of disguising

materials (natural leather, especially impregnated with

water)

Yes No

Objects in shoes Yes ?

Copper wire 0.2 mm (diameter) Yes ?

Peoples privacy issues are important for microwave scanners whereas there are no such

concerns about X-ray transmission scanners.

References

1. Technical information on DRS SecureScan, available at

http://www.adani.by/products/security/securescan/.

2. Technical information on SafeScout 100/360, available at

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