Unique scanner unveils art and other secrets

A Chinese puzzle ball is a richly decorated ivory hollow sphere, the size of a tennis ball, with up to twelve other hollow spheres inside. Inwardly the spheres get of course smaller and smaller, but they also become less and less richly decorated. You can move the spheres independently of each other, and the puzzle consists in matching the holes of all the spheres neatly. The spheres are ingeniously crafted from a single solid piece of ivory – a truly artisanal piece of art that must have taken months of work.

Chinese puzzle balls were a status symbol in China three hundred years ago. The Rijksmuseum in Amsterdam has one in its collection and wanted to know exactly how it looks inside without having to open the ball physically. The answer was provided by CWI’s unique FleX-ray scanner. Among other things, the scanner showed that one of the inner spheres had circular grooves, showing exactly how wide and high the blade used by the artist was. It also turned out that the ball was made from African ivory instead of from Asian ivory, which means that there must have been ivory trade between China and Africa centuries ago.

Bookcases of Hugo de Groot

Since the FleX-Ray scanner went into operation in 2017, it has already revealed several such cultural heritage secrets. In November 2021, CWI-led research won the NWO Team Science Prize for scanning and dating two wooden bookcases that were both attributed to the 1621 escape of Dutch intellectual Hugo de Groot. The scanning of such cultural artifacts was, however, not the primary goal of CWI, but was primarily needed to test the newly developed algorithms, examine their limitations and solve the resulting challenges.

Tristan van Leeuwen, group leader of the Computational Imaging group that operates the scanner, explains: “Image processing algorithms have hardly changed since the 1980s, which limits their possibilities. If you want to scan with higher speed, more accuracy or less radiation dose, you need fundamentally different algorithms. CWI is typically a place where such new algorithms can be developed.”

Scanning objects in real-time

The main motivation of Van Leeuwen’s predecessor Joost Batenburg to develop the FleX-ray scanner, was being able to scan objects in real-time. “While the object is still being scanned, the scanner can already show a preliminary three-dimensional image”, tells Van Leeuwen. “That has the great advantage that if you see something interesting, you can immediately zoom in and you don’t have to wait until the full three-dimensional scan is finished. That is particularly interesting for art and cultural heritage experts because they often don’t know what is inside an object.”

What also makes the FleX-ray scanner unique in the world, apart from the real-time aspect, is its flexibility, high resolution and the different types of materials the scanner can see through. Furthermore, both the X-ray source, the detector and the object to be scanned can move in relation to each other. The development of the FleX-ray scanner, including its algorithms and software, fits perfectly into the synergy between mathematics and computer science that is unique to CWI.

From the mathematical point of view CWI needed to develop methods to scan objects that are very different in shape, dimensions and materials. However, it was the computer science challenges that were most difficult, says Van Leeuwen: “How do you make sure that all the imaging data is in the right place at the right time? That is the biggest challenge. Normally you need something like a thousand different X-ray scans to build a full three-dimensional image. What we do is send the data from a single scan directly to the algorithm so that it can show an intermediate three-dimensional image. Gradually the algorithm collects more and more data with every new image that is added.”

Exploring new applications

The challenge in terms of mathematics and computer science that Van Leeuwen wants to solve in the near future, is to make optimal use of the flexibility offered by the scanner, and to develop algorithms that will automatically and autonomously scan an object in an optimal way, like the automatic mode does for a digital camera, for example.

On the side of new applications of the FleX-ray scanner, Van Leeuwen and his group are exploring applications in the food industry (like high speed fruit quality checking), the chemical process industry (like studying process dynamics in a reactor), and even medical applications (especially trying to make medical scanning much faster). In all these research project CWI collaborates with various Dutch universities as well as with industrial partners.

Author: Bennie Mols