Photon Counting: Evolution in Computed Tomography

Photon counting is the (not so) new star in the sky in radiology. The detector technology is used in computed tomography (CT) scanners and has taken imaging a significant step forward.

In conventional CT detectors, X-rays are first converted into visible light signals by a scintillator layer. Then, photodiodes convert these light signals into electrical current signals yielding the X-ray image on the monitor. The difference in photon counting is the detector material used: These are semiconductors, more specifically cadmium tellurite (CdTe) or cadmium zinc tellurite (CdZiTe) crystals which directly read the photons of the X-rays and convert them into electrical signals. The detector can detect each of the photons produced and measure its energy. Thus, the signal in the photon counting CT always remains completely digital, so that the yield of converted radiation is significantly higher even with high-resolution detectors. The first scanners with CdTe detectors for clinical use have been on the market since 2018.

Advantages

Due to the direct conversion of the X-rays, photon counting detectors (PCD) work particularly efficiently, which is reflected, for example, in a higher spatial resolution. This very high spatial resolution allows structures to be seen with a much-improved level of detail. As a result, even smallest structures such as vessels, fine bone structures or even small fibrotic lung changes or infiltrates can be reliably diagnosed.

Another advantage of the PCD is its 100% dosing efficiency. In addition to better imaging, it also means that radiologists can work with a lower radiation dose without having to accept a lower image quality.

The first CT devices with photon counting technology used in clinical practice confirm the advantages mentioned above. In addition, various experts expect that the new technology will enable even better characterization of tissues in the future and thus contribute to improved early and differential diagnostics as well as optimized therapy response monitoring. Examples include more accurate characterization of bones or vascular plaques. Photon counting also opens new possibilities for research: The high-resolution images provide extremely detailed data material for artificial intelligence applications and radiomics analyses. Apart from delivering images with richer details, photon counting technology can also be used to obtain information about materials that would remain hidden with conventional detectors. They allow spectral analysis of the detected X-rays, which enables, for example, differentiation between coronary calcification and the contrast agent.

Limitations

Today, the many advantages are not offset by any disadvantages. However, there is one drawback: Since the images are much more detailed, they also produce a much larger volume of data. The computer and IT infrastructure of a facility must be prepared for this. Only suitably powerful computers are capable of processing and reconstructing the recordings in an acceptable time. In particular, the connections to the PACS and the respective workstations should be designed for large data volumes in order to avoid delays in processing.

Conclusion

Photon counting technology is ready for clinical use. It results in an increase in efficiency and quality in CT imaging. Until it will be possible to produce detectors cost-effectively and in good quality, they will demonstrate their strengths for specific requirements and indications – as it is the case with our innovative breast CT nu:view.