Twenty years ago De Beers discovered that 10% of its uncut diamonds were being stolen by its own workers. With annual sales in excess of US$ 6 billion, this was a significant amount of money. Clearly something had to be done! Their engineers set about developing a system to screen each worker when leaving the mine every day. A key design constraint was that no person should be subjected to any physiological harm. Their solution was an imaging system based on slot scanning x-rays that produced high resolution pictures of the whole body at extremely low radiations doses. De Beers was awarded US Patent 5,404,387 in 1995.
But what exactly are slot scanning x-rays and how can they be used in the fight against breast cancer? To answer this question, we have created a short video (see below). The first step is to understand how a conventional digital x-ray image is acquired. A flat panel detector is a rectangular sensor – similar to that found in a digital camera – that converts x-rays into an image. The detector is exposed by the lilac coloured x-ray beam for a period of less than a second.
In slot scanning, a narrow image sensor moves in synchrony with an x-ray fan beam. As the tungsten blades of a collimator, located just beneath the x-ray tube, move from side to side, so the lilac x-ray fan beam sweeps back and forth. Image acquisition typically takes about three seconds.
Slot scanning has the major advantage of reducing x-ray scatter and this in turn leads to better quality images at lower dose, both key attributes for a successful mammography system. The narrow image sensor not only costs far less than a flat panel detector, the size of its picture elements (or pixels) are also much smaller, further enhancing image quality. While there are downsides – a more powerful x-ray tube is needed to offset the effects of collimation, and the breast must be stationary for a longer period – the benefits of slot scanning are considerable.
Who would have thought that a method to catch diamond thieves could have an impact on our efforts to diagnose breast cancer?