The Present State of the Art in X-ray Screening
Current X-ray screening systems can be classified into four different but familiar applications, each having different system requirements, purposes and parameters.
1. Health screening
2. Baggage screening
3. Cargo screening
4. Mail screening
Health Screening
Health screening provides a frame of reference with which we are all familiar. Health screening X-ray technology concerns itself with high image resolution, which is needed to ensure the accuracy of X-ray interpretation. At the same time, it is designed to maintain low X-ray exposure to the patient.
To balance these two needs, most health screening X-ray systems use between 20 and 80 kilovolts of anode voltage to produce images with resolution of about three line pairs per millimeter (lp/mm), as shown in this image of the hands. The term lp/mm represents the number of equally spaced lines that can be seen clearly in one millimeter of space. Both kilovolts and lp/mm are important parameters that help us compare the various X-ray technologies available today.
In security screening, we are not concerned with X-ray dose. However, high image resolution is critical in order to achieve precise image detail and the best possible degree of interpretation certainty.
Baggage Screening
Most people are familiar with the X-ray screening procedures in use today at airports and access-controlled facilities such as courthouses. The X-ray technology employed at these baggage inspection stations requires that an examination of a large field, four to six square feet, is accomplished quickly. Typically, these systems are designed to detect specific types of objects: relatively large, metallic or "high-density" materials with shapes that suggest a threat such as guns, knives, pipes, etc.
The X-ray imaging technology for this application, which has been established for many years, employs a linear array X-ray detector (a linear strip of light-sensitive elements coated with an X-ray luminescent phosphor) and X-ray backscatter detectors. The linear array detector as well as the backscatter detector signals are video composited to form a complete but reduced picture of the baggage or package.
Most baggage screening systems employ X-ray sources operating in excess of 80 to 160 kilovolts to achieve the necessary power to obtain satisfactory images. These devices are characterized by their abilities to image large fields at resolutions limited by the pixel pitch of the photosensitive elements (less than two line-pairs per millimeter of space: i.e., two lp/mm).
Cargo Screening
The X-ray imaging technology used for cargo inspection employs either very large transmission X-ray imaging panels or backscatter technology using X-ray voltages in excess of 300 kilovolts to achieve the power necessary to penetrate a shipping container or palletized shipment. Of necessity, due to these high operating voltages and very large image fields, severe limitations in resolution must be anticipated.
Mail and Parcel Screening
At present, most mail and parcel screening systems use conveyorized X-ray screening devices that employ linear array X-ray detector technology as well as stationary fluoroscopic screening X-ray cabinets. The fluoroscopic screen was first developed by Thomas Edison at the turn of the 20th century and has been virtually unchanged since that time. These systems operate at anode voltages between 80 and 140 kilovolts. Due to the detectors' large fields and their limitations in X-ray imaging resolution, this type of X-ray imaging technology is limited to, at most, two lp/mm.
Some Technical Aspects of X-ray Imaging
The X-ray Shadow Image
The X-ray shadow image for a typical X-ray screening application is shown schematically at the top of the next column. The X-ray shadow is governed by the same geometric principles that govern the light shadow. The relative sharpness and resolved detail of the light shadow is determined by how small and/or by how well collimated the light source is and the smoothness of the shadow plane. As previously discussed, the resolved detail of both the light and X-ray shadow is measured in terms of the number of equally spaced lines that can be clearly seen in one millimeter of space.
The X-ray Screening Shadow Plane
Smoothness as Determined by the Detector
The X-ray screening shadow plane for all X-ray screening systems is the type of X-ray detector employed in the particular system design. The majority of these detectors are linear array detectors or less commonly "flat panel" detectors. The resolution specifications of all these detectors determine the "smoothness" of the shadow plane. In every case, this resolution is low; it does not exceed three lp/mm.
The Degree of Smoothness of the Shadow Plane Determines the Resolution of the Image.
It is important to recognize this fact since it is low resolution that is the limiting factor in present mail-screening technology. The following two images are actual shadows falling on two different shadow planes.
Other Characteristics of the Shadow Image.
One characteristic of the shadow plane that can make interpretation of the X-ray image uncertain is that many different juxtapositions of objects can result in the same X-ray image.
Additional uncertainty in X-ray image interpretation results from the fact that the farther the object is from the shadow plane, the more blurred it is.
Some X-ray image enhancement technology makes use of the fact that the X-ray absorption and scatter of different materials depends on the density of the materials and the spectrum of the X-rays. X-rays contain a spectrum of energies, just as visible light contains a spectrum of colors. When the black-and-white X-ray image is computer colorized according to the absorption of the material, it can differentiate organic (low density) materials from inorganic (higher density) materials. However, the resolution or detail of the image is not increased but remains at less than 2 lp/mm. ·
Mail Threats and the Technology to See Them
Mail Bombs
A mechanical letter bomb, known as "The Bekar Valley" bomb due to its popularity in the Middle East, employs a mechanical detonator imbedded in Semtex-H. It is easy to misinterpret an X-ray image of such a package, since the image can appear as a pen wrapped in organic packing material. Semtex, being putty-like, has a distinctive mottled appearance. This detail can only be seen with high-resolution X-ray technology.
The Unibomber package bomb was a thin plywood box, paper wrapped, 101/2 inches by 43/4 inches by 13/4 inches in dimension. The bomb contained four nine-volt batteries, a copper pipe containing black powder, coil springs, a pressure release switch and a bulb filament igniter. It would have been easy to detect using most current X-ray screening devices.
However, some designs have made use of "Polapulse" film batteries, button batteries, cell phone batteries, Semtex-H, Detasheet, hair wires and bulb filament detonators.
Designs such as these have demonstrated their abilities to elude existing mail-screening technology. The simulated mail bomb below/left was constructed with a musical holiday greeting card, replacing the speaker with Semtex-H. This design passed through the mail X-ray screening process of a government agency.
Other Mail Threats
The Anthrax letters mailed after 9/11 have made the public and government agencies particularly concerned about mail contents and have raised a critical question: Can an X-ray detect the presence of Anthrax spores in an envelope?
We have a few facts. The amount of particulates in Senator Patrick Leahy's letter was as much as one gram, a sugar packet's worth. Not all of this material was Anthrax spores. Some of it was a particle carrier to help the spores become airborne.
Is there an X-ray technology that can detect powders? The answer is yes, depending on the nature of the powder. X-rays are absorbed to a greater degree by inorganic compounds as compared to organic compounds. This can be seen below in the pair X-ray images of different granules.
Other Types of Biological and Toxic Threats
There are many other biological and toxic threats that come through the mail. One example consists of a padded envelope containing hypodermic needles filled with HIV positive blood, with a plunger actuator mechanism. Due to the low density of the plastic and thinness of the needles, a conventional X-ray image of these contents can miss the threat potential.
Enhanced X-ray Technology for Present and Future Threats
MXRA (Ultra-high resolution) X-ray Imaging Technology
MXRA X-ray Imaging Technology introduces the ability to perform X-ray screening at 10 times or more the resolution previously achieved and, at the same time, magnify a particular area of an object up to 25 times. The technology achieves this through the development of an extremely smooth shadow plane, with 10 times better resolution than the linear arrays or flat panel detectors discussed previously. Further, all other technologies reduce the size of the object, making interpretation more difficult. In effect, when viewing a 50 millimeter diameter object field, the MXRA camera acts as an "X-ray microscope" that allows the image to be enlarged up to 25 times the size of the original. Finally, due to the · compact nature and extremely high sensitivity of the MXRA X-ray camera, X-ray screening systems designed around it are compact and light weight. They require only 20 to 40 kilovolts of anode voltage and minimal shielding protection, yet produce high-resolution images that can be magnified with internal optics up to 25 times.
The health screening X-ray of hands (below/ left) was produced with a flat panel detector having typical mail-screening resolution (three lp/mm). The X-ray of the fingertip on the right showing the blood vessels, was produced with MXRA X-ray camera imaging resolution (22 lp/mm).
An Effective Mail X-ray Screening Strategy
A serious mail X-ray screening strategy must recognize both the low-resolution capabilities of existing mail screening systems and the subtle nature of mail threats. These systems have been designed for luggage screening with the X-ray imaging technology available and, as such, reduce the X-ray image in size and present a low-resolution X-ray image to interpret. In addition, many of the operators of these screening systems are untrained in image interpretation.
We have seen that potential mail threats are subtle in nature. To effectively screen for them, one must look for any hint of wires, batteries, unidentifiable plastic or putty-like material and granular material. Mail should be X-ray inspected in smaller lots and arranged horizontally. It has been observed that X-ray screening of mail that has been arranged vertically in a mail trays has permitted suspicious contents to pass through.
Operators of this level of X-ray screening security need to know how, for example, a floppy disk or CD appear in an X-ray image. Any detected material other than staples and paper clips should invite a second look. This may slow down the screening process, but it does represent a serious attempt at secure mail screening.
Summary
In the course of this overview, we have found that the X-ray technology being employed today in mail screening is essentially the same as that being used in luggage screening. This technology employs low resolution, large field X-ray imaging that reduces the size of objects to one-third their original size and obscures details. Conversely, we have seen that the types of subtle mail threats experienced in the past and anticipated in the future can be detected only with systems that produce highly detailed and sensitive images, capable of revealing any hint of wires, batteries, unidentifiable plastic or putty-like material and granular material. Today, a serious mail screening program must recognize this requirement. To accomplish this, conventional large field, low-resolution imaging must be augmented by, or replaced with, ultra-high resolution X-ray imaging technology that can achieve the precision necessary to identify a mail threat. Such proprietary technology exists. Although it is new to the security field, it is proven technology. For more than a decade, it has been used worldwide to provide quality assurance of critical electronic components.
Gil Zweig is chairman and founder of Glenbrook Technologies, Inc. He received the 1994 Inventor of the Year Award, Inventors Congress and Hall of Fame; New Jersey Institute of Technology Awarded for the invention of Glenbrook's proprietary X-ray camera technology.
Glenbrook Technologies is a leading supplier of high-resolution X-ray inspection systems to the worldwide electronics manufacturing industry. This X-ray technology is now being introduced to the mail industry. For additional information, you may contact Gil by phone at 973-361-8866, by fax at 973-361-9286 or on the Web at www.glenbrooktech.com.
