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It’s being used to develop bespoke computed tomography (CT) solutions that integrate third party analysis software and control external hardware. The 4D (three dimensions plus time) CT laboratory experiments have the potential to open up new avenues in industrial environments.

Parmesh Gajjar is a research associate at the imaging facility who has been discovering the possibilities of the programmable IPC (inter-process communication) interface to Nikon’s X-ray control software and how it can be harnessed to perform temporal (time-related) CT for scientific, non-destructive observation and quantification of processes that change structure over time in 3D.

He commented: "Nikon Metrology's programmable CT systems are a gold mine for researchers and manufacturers alike, as it gives users the flexibility to do whatever they choose."

Andrew Ramsey, a consultant at Nikon Metrology with experience of developing special CT applications in industry, added: "In the aerospace industry for example, when studying accelerated fatigue crack propagation in fan blades, time-lapse CT can be used to replicate years of work in a fraction of the time.”

Gajjar and colleagues together with Ramsey have recently written a scientific paper entitled ‘New software protocols for enabling laboratory based temporal CT’ (https://doi.org/10.1063/1.5044393), which was published on 5th September 2018. In it they offer an insight into how similar technology can be used in industrial environments.

The fully programmable IPC software interface allows users to write their own code and implement individual functions in Inspect-X, from as simple a task as turning the X-rays on and off to high-level actions like initiating a CT scan with previously stored acquisition parameters, automatically reconstructing a CT volume using stored settings, and running an automatic analysis using stored macros while providing progress feedback throughout, all without further human intervention. The IPC program can create simplified user interfaces for previously cumbersome tasks and acquire data over time highly productively for non-destructive examination of a 3D sample.

Temporal CT can be grouped into two main classes: time-lapse and continuous acquisition. Time-lapse CT is the 3D analogy of its photography equivalent and involves taking traditional CT scans at specific intervals, whereas continuous acquisition CT involves continually collecting projections of an object as it changes and subsequently reconstructing subsets to form several volumetric time-series. As is shown in the paper, the advantage of IPC is that it provides flexibility for implementing both of these classes on standard CT machines.

Time-lapse CT

To investigate the in-vitro sprouting of a mung bean, Gajjar uses polyimide tubing with wet tissue to create a sample holder with a moist micro climate in which the bean can germinate. After soaking the bean in hot water to initiate sprouting, it is placed within the sample holder inside the CT machine. As the bean germinates, 54 CT scans are automatically taken at two-hour intervals over five days throughout the sprouting process. Uninterrupted time-lapse scanning creates a 4D video of bean germination that allows the internal changes to be visualized and can also be used for quantitative analysis.

He also uses time-lapse CT to visualise the Brazil nut effect, which is the tendency for large objects to rise to the top of a shaken mixture. A special shear-cell that fits on the stage of the CT system was developed for agitating a mixture of glass beads (nuts) of two different sizes - 6mm beads at the bottom and 3mm beads on top. The mechanism applies a motion force to its contents at intervals and the CT system takes automated scans after each event. The resulting scans can be stitched to create a movie that depicts the natural ordering effect that takes place.

Continuous acquisition CT

The other class of temporal CT involves collecting a stream of projections continually as the object changes. Here, the spatial and temporal resolutions can be jointly optimised. Gajjar implements the ‘golden ratio’ method for angular sampling, which allows the number of projections in a reconstruction to be changed optimally as sample evolution occurs. The advantage of the golden ratio (Φ ≈ 1.618) to CT scanning is that it allows projection angles to be distributed so that the projections collected are as independent from each other as possible, unlike if regular incremental angles (eg 10, 20, 30 degrees etc) had been chosen. It enables the number of projections in a reconstruction to be changed as evolution of the sample occurs, allowing a full picture to be built up more quickly.

This golden ratio approach has been successfully implemented before, in MRI scans, with neutron tomography and at synchrotron X-ray facilities. For laboratory based CT systems though, until now it has not been technically possible. However, in Gajjar's experiment, the IPC interface enables golden ratio sampling to be implemented for the first time in a laboratory.

4D CT in a manufacturing environment

The impact temporal CT could have on manufacturing industry and quality assurance (QA) departments now and in the future is significant. The possibility of synchronised CT scanning opens the door to tests that could not be achieved before. In smart factories, the technique could be the holistic solution for inspection of life-critical components, meeting the demands of Industry 4.0 and taking quality control to the next level.

Ramsey explained that in a production environment, the introduction of temporal CT could prove to be highly beneficial. In the automotive sector and especially in aerospace, failure is not an option. Due to the demand for unwavering quality, components, assemblies and mechanisms are subjected to extensive and vigorous tests. Incorporating 4D CT into these tests gives manufacturers the ultimate inspection tool for obtaining accurate results quickly.

QA departments often use CT to see inside components, including those that have been additively manufactured, without slicing or destroying them. They also use various simulations and tests of materials, components, parts and assemblies. The introduction of temporal CT can combine these procedures, allowing unparalleled insight to be gained during tests into the smallest details of critical components and parts with the tightest tolerances.

4D CT can show where, why, when and how a component has failed, providing a complete understanding, which is vital for product development and priceless in terms of quality control. Both time-lapse and continuous acquisition protocols could soon be a part of smart Industry 4.0 factories worldwide, fulfilling jobs like mechanical testing, following battery degradation and studying chemical reactions.

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