Dynamic Colour X-ray Computed Tomography Imaging

X-ray Computed Tomography imaging is a widely used technique that has been applied across many different applications; from fundamental materials science at large facilities to medical diagnosis in hospitals. CT imaging is carried out by measuring transmitted X-rays at a variety of angles through a sample and then computationally reconstructing individual 2D slices to produce a full 3D density contrast image at a particular point in time. This traditional CT imaging simply measures the intensity of transmitted X-rays through a point in a sample at a given time, potentially-significant chemical information contained within the colour (energy) of the X-rays isn't measured. In time-resolved colour Computed Tomography, 5D-CT, the energy of each X-ray interaction is recorded in addition to the standard temporal and spatial information. The measurement of this colour information enhances existing techniques by providing simultaneous imaging of the chemical nature of a sample in addition to the time-resolved 3D imaging of its structure. The biggest challenge to realising this imaging capability has been the availability of high frame rate, energy resolving, X-ray imaging technologies.

Working as part of the UK's National Research Facility for lab-based X-ray Computed Tomography (NXCT), the University of Manchester are the problem holder. One of the key aims of the NXCT is to provide access for researchers in the UK to state-of-the-art X-ray instruments and the research expertise that enables the design and implementation of complex experiments. The University of Manchester have already identified colour X-ray CT as a strategically important technique and have received funding from EPSRC to pioneer its development. The delivery of a Dynamic Colour X-ray Computed Tomography imaging technology would provide a transformative capability which would benefit a broad range of disciplines. Initial target application areas include, the study of the role of nanomaterials in catalysis [1], observing the dendritic growth in eutectic alloys [2], characterising the charging-discharging of novel battery types [3] and the diffusion of heavy element stains through biological tissue [4].

STFC, the solution provider, have been developing colour X-ray imaging cameras for over a decade [5]. The frame rate of their existing systems currently limit both the temporal resolution and photon fluxes at which colour X-ray imaging can be applied. STFC have recently developed a new HEXITEC-MHz ASIC which operates at a continuous frame rate of 1 MHz enabling colour imaging to be carried out at CT-relevant fluxes of >10^6 photons s-1 mm-2. In this project STFC will deliver the world's first deployable detector of this type to the University of Manchester who will characterise the imaging performance of the system and develop the reconstruction algorithms for these novel 5D data sets pushing this technology quickly to a state where it can benefit many researchers addressing many important applications enabled through the NXCT access routes.

[1] Beale et al, Phil. Trans. R. Soc. A.3762017005720170057 http://doi.org/10.1098/rsta.2017.0057

[2] Feng et al, Acta Materialia, 221, 2021, 117389, https://doi.org/10.1016/j.actamat.2021.117389

[3] Connolley et al, J. Appl. Cryst. (2020). 53, 1434-1443, https://doi.org/10.1107/S1600576720012078

[4] Jayarantha et al, IEEE Access, vol. 9, pp. 49912-49919, 2021, https://doi.org/10.1109/ACCESS.2021.3069368

[5] M. Veale et al, Synchrotron Radiation News, 31:6, 28-32, https://doi.org/10.1080/08940886.2018.1528431

The HEXITEC-MHz ASIC represents a step-change in X-ray detector technology. The ASIC runs at a continuous frame rate of 1MHz while at the same time delivering per-pixel X-ray spectroscopy in the energy range 2-200keV with a resolution of <1keV. For the first time this allows colour X-ray imaging to be carried out at CT relevant fluxes of >10^6 ph s-1 mm-2.

To achieve these ambitious targets has required a completely new ASIC architecture. While the output of the original HEXITEC ASIC was analogue and limited to frame rates of <10kHz, the new ASIC has 'in-pixel' 12-bit time-to-digital converters (TDCs) that digitise the analogue output of the pixels. The output of these TDCs are then Aurora 64b/66b encoded and serialised over 20 lanes of differential current mode logic (CML), all operating in parallel at 4.1Gbps at 1MHz.

The ASIC design and fabrication was funded by STFC and was delivered in January 2022. The testing and characterisation of the ASIC is now underway and initial results suggest a noise performance of 100e- RMS at a frame rate of 1MHz, meeting the target specification. The aim of this project is to move from the current ASIC test system (TRL1) and build a fully functioning camera that can then be applied to the 5DCT imaging technique (TRL2-3). To do this will require the development of a new compact readout PCB, detector housing and DAQ system that can be easily integrated in to the NXCT colour X-ray bay at Manchester.

In parallel to these hardware developments, a PDRA at the University of Manchester will lead the development of the algorithms that will be required to reconstruct the 5D data sets produced by the camera system. These algorithms will be validated against test data sets of imaging of phantom objects before applying the 5DCT imaging technique to systems of scientific interest such as imaging of the charge-discharge cycle of batteries, the diffusion of heavy element stains in biological tissue and the dynamics of alloy formation.