X-ray inspection systems for cargo containers have now become a standard feature in many ports. This rapid adoption has been accelerated by the needs of port security, but made practical by the systems’ unique ability to non-destructively penetrate entire containers and generate images of the contents in just a few seconds.
Even at this large scale, the resulting images are comparable to those obtained through traditional baggage scanning at airports and capable of identifying objects smaller than a baseball Consequently, these systems are commonly employed for manifest verification and contraband interdiction in a variety of environments.
In a prior article (see Port Technology 35), I reviewed the component technologies that contribute most to X-ray image quality in cargo screening systems. Those included a stable source of high-energy X-rays (usually a linear accelerator) and a number of design parameters associated with the detection process such as detector modularity, dynamic range, sensitivity, pixel size, etc. This article extends the prior review by looking at the broader system considerations in selecting and implementing a cargo screening system.
Beginning with differences due to inspection objectives, the discussion covers several variations in available X-ray inspection systems, each with certain advantages. From that point, actual system performance is driven by the effectiveness of the ‘imaging chain,’ which is described in terms of the interaction of its core components, with special attention toward the multifaceted nature of imaging software.
Inspection objectives are important considerations for the design and implementation of a cargo screening system and will impact the type and utilisation of screening equipment and the process of interpreting the resulting images. X-ray cargo inspection is typically performed for three different (but not mutually exclusive) objectives; manifest verification, homeland security, and contraband interdiction. Screening equipment is often specified or operated differently, depending on the screening objectives.
Manifest verification is often performed for tariff enforcement with the objective to confirm the contents of a container, both in type and quality. Here, operators may choose to examine the container for overall compliance with their requirements, with less focus on small anomalies.
Containers with homogeneous contents can be cleared quickly, especially when prior images of similar shipments are available for comparison and verification. Image inspection for detecting contraband involves a more detailed analysis assuming that the contraband is likely to represent a small percentage of the overall container volume. In addition, perpetrators are expected to intentionally disguise or hide the materials of interest. In these cases, inspectors must examine both
the cargo itself and the common vehicle hiding areas such as the frames or tanks. Because some contraband can be disassembled into component parts and distributed among permissible cargo, a combination of operator training and advanced software features provides a robust capability to help identify such violations.
The newest and perhaps most urgent need for screening cargo containers comes from the threat of smuggled special nuclear materials. The rarely expected occurrence of this event is offset by its potential impact should such material be used for terrorist purposes. Fortunately, the densities of special nuclear materials make them clearly visible on X-ray images in contrast to virtually all permissible products. Additionally, the advent of dual-energy screening systems adds a second level of material discrimination, allowing software to clearly identify the unusual nature of this threat.
At the equipment selection stage, author ities consider whether mobile, relocatable, or fixed installations will offer the best implementation for their needs. Fixed installations arepermanent sites that are built for a specific screening system. Typically these are shielded concrete buildings where containers enter at one end and leave through an opposite exit. This allows their throughput to be limited only by the amount of time it takes to move a container through the installation.
Mobile (truck based) systems offer the advantage of being quickly deployable to different ports or locations but generally operate with a lower total throughput than fixed sites. A screening truck must first extend its detector boom, and then drive past one or more aligned subject containers. However, pre-staging the containers can improve the overall scanning rate.
Finally, the emergence of relocatable scanning systems has provided a third choice. Relocatable systems take advantage of a fixed infrastructure (control room, x-ray bay, etc.) yet, are designed to be readily moved to alternate sites based on user needs. Once installed, a relocatable system operates very much like a fixed site.