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Feb 22, 2022

Interview: Professor Dr Franz Pfeiffer on Bringing Dark-field Imaging to Clinical CT

- By York Haemisch

Professor Dr Franz Pfeiffer and his team at the Technical University of Munich (TUM) have integrated dark-field X-ray technology into a clinical CT scanner. Although this technology is in its early stages, pre-clinical studies with mice have demonstrated clear benefits from dark-field CT scans, especially for capturing images of lung tissue. The use of photon counting X-ray detectors in the system could bring further benefits. Direct Conversion’s Dr York Haemisch finds out more.

Professor Pfeiffer, what motivated you to undertake the challenge of bringing dark-field X-ray imaging into a clinical CT?

The main motivation for this project was, originally, pure scientific curiosity, combined with the fundamental desire to make new technical innovations and research findings usable for medical applications. As the project continued, the first promising results with tissue samples and small animals (and corresponding lung disease models) became, of course, a very compelling reason to take on the considerable task of translating this to the human scale, and to continue along this path until we achieved our objective. Finally, the first successful results we achieved with the initial radiographic dark-field images of patients encouraged us to make this innovation usable for computed tomography as well.

Can you outline the major obstacles you had to overcome when implementing dark-field imaging in such a system?

The main difficulties in transferring dark-field X-ray technology to a clinical computed tomography system are essentially threefold. First, a major technical challenge is, of course, the fabrication of suitable X-ray grating optics needed for dark-field imaging. It is important to consider that, for computed tomography, large areas have to be equipped with homogeneous X-ray optics, which must be adjusted with great precision and centre focused. The second challenge is mainly related to the fact that modern computed tomography rotates extremely fast, and ensuring the stability of the optical components, which is in the micrometer range, is therefore extremely difficult. This aspect alone has required multiple iterations in the technical design of the adaptation of the CT gantry. The final major challenge has been the development of completely new and appropriate data acquisition, processing, and multi-contrast X-ray reconstruction algorithms. This is complex because, ultimately, all previously existing methods in the software pipeline for normal CT applications have to be extended to include the additional dark-field contrast modalities.

As you see this system moving from lab to medical practice, what is the additional information you would expect a clinician to gain from using it with real patients?

From a clinical perspective, the new dark-field X-ray technology primarily provides additional information about the microstructure of tissue materials. While ultimately helpful for most clinical diagnostic tasks, its importance is most immediately apparent with respect to the example of the lung, which is built up on the micrometer scale, primarily by examination of the alveoli. Current computed tomography cannot resolve fine structural changes on the length scale of the pulmonary alveoli, and thus cannot provide medically relevant information about the early stages of diseases that typically begin at this scale. With the dark-field method, information about the nature of the alveoli can be obtained, and this is of high diagnostic value, for example, for chronic obstructive pulmonary disease, lung cancer, pulmonary fibrosis, inflammatory lung diseases, and not least COVID-19. We expect to see significant diagnostic added value, in particular, in follow-up examinations after therapy, for example after irradiation of the lung, or after a COVID-19 infection.

What are the limitations of the current system and how do you think they could be overcome or, in other words, what would you like to do to further enhance the existing method?

The current technological implementation is, of course, not yet perfect in this first prototype. One limitation, for example, is that the X-ray optics used do not work ideally over the complete energy range. This means that certain energy ranges provide significantly more contrast than other energy ranges. A further optimization of the grating structures, or the use of energy resolved X-ray CT methods, could bring notable improvements here. Another limiting aspect of dark-field technology is that, due to the very complex reconstruction procedures and the multi contrast information, the image quality is more strongly influenced by the noise, for example in the detector. Here, technological improvements in X-ray detection would help significantly.

What enhancements do you expect to achieve when using photon counting detectors for this approach?

Photon counting X-ray detectors are mainly characterized by the possibility of energy resolution into different energy bins, since the individually detected photons can be sorted with respect to their energy and counted separately in energy bins. As mentioned above, this energy information would be very helpful for dark-field imaging, as it could help to weight the optimal energy ranges – where the X-ray optics provide particularly high dark-field contrast – more strongly over other energy ranges, where the dark-field contrast is less pronounced. This should allow significantly higher quality dark-field CT images. Another advantage is the virtual absence of noise in photon counting detectors. By using far more sophisticated reconstruction and image processing algorithms, dark-field imaging is somewhat more sensitive to background noise, and in particular electronic noise that arises in actual scintillator-based detectors. Noise-free, purely Poisson statistics limited, single photon detection would of course offer valuable improvements here. Of these characteristics, the last would also further contribute to significantly reduce the dose.