X-Ray Limitations: Getting the most out of our equipment

There was much scepticism when X-rays were first trialled for screening passenger cabin baggage. Manchester Airport was one of the early pioneers and there were those who said it would never catch on as an inspection tool. Of course there was also much scepticism when Wilhelm Conrad Röntgen first demonstrated the potential uses of ray tubes in imaging back in 1896. Amazingly, although we have made considerable scientific advances to Röntgen’s discovery, the core principle of X-ray examination hasn’t changed since and it will always have its limitations. Paul Quellin sets out to explore some of those limitations, specifically as they relate to conventional X-ray screening systems.

X-ray has proven an invaluable tool over the years. It has provided us with a better means of screening baggage whilst keeping up to speed with passenger numbers which have grown steadily over the decades since the technology’s introduction. Terrorists’ tactics have also evolved and X-ray screening equipment has had to evolve in tandem in an effort to keep apace; that hasn’t been easy. Whilst the technique has its limitations, unless we were to shelve the technology or deploy advanced variations of the technology (such as computed tomography) across the board, we can’t do a great deal about many of those which are latent in conventional X-ray applications.

It is worth going back a little to consider early screening equipment in order to help understand how we have got to where we are now.

Hijacking hasn’t stopped, but its peak was between the late 1950s to the mid-1980s, by which time it was a large-scale problem. Many of those early hijacks were effected with handguns and grenades and searching every item of cabin baggage by hand was never going to be something we could sustain for long. X-ray seemed ideal to counter these ‘simple’ threat items. No need for colour, just a need to be able to pick out denser materials amongst the largely lower density background of the baggage and its other contents. Terrorists, however, were not put off by the new screening regime and began concealing weapons in more elaborate ways, often breaking them down into their component parts. A demand for better image quality became inevitable.

Aircraft bombings go back a long way, but it was really the 1980s that saw the extensive use of IEDs against aircraft and elaborate efforts to infiltrate them on board. The threats became harder to locate as explosives lacked the substantial difference in density (which firearms had) necessary to distinguish them from the ‘background’ items in the bag. The demand for ‘materials discrimination’ was born.

Colouring the Picture
Modern multi-energy colour X-rays enable us to group materials by density more readily than we could with monochrome images. There are however limits to what can be achieved here. In an item of cabin baggage there isn’t so much scope for confusion between density and quantity; experienced screeners should be able to report with some certainty what material an object is made from. They will do this by using the colours, and shades of those colours, displayed, the size and shape of the target object and their experience (their mental image library).

Compare that situation with an image of a large and complex item of air freight and it can become a little more challenging. It doesn’t have to be a complex image to cause some confusion. If we take a 15kg sack of a low density organic material and put it through an X-ray machine, it will appear mostly as being of an organic nature (typically displayed in orange); it may be a little dark in the centre, but there will be no real mistaking what it is. Now take a pallet load of the same sacks and it is quite likely that, as in the picture we have of the sacks, you will see organic material, but you will also see composite, inorganic and X-ray opaque areas. The quantity of the material has changed significantly pushing us beyond the limits of the materials discrimination process.

 

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