Computed tomography: Digital signals with photon-counting CT

Does medicine get digital when we scan in diagnostic findings and digitize them in the process? It obviously is more efficient to record data directly in digital form, but not all diagnostic tools have this option. Computed tomography has now made enormous progress in this area: Unlike conventional CT technology, the new photon-counting CT directly creates digital image data.

In this MEDICA-tradefair.com interview, Professor Nikolaou explains how photon-counting computed tomography (PCCT) works, details its advantages, and reveals how research and the healthcare industry benefit from digital CT images.

Prof. Nikolaou, what is the technology behind photon-counting CT?

Prof. Konstantin Nikolaou: Unlike conventional CT scanners, photon-counting CT technology uses an entirely different detector material. These semiconductor detectors are made of high-purity cadmium telluride crystals, which directly transform x‐ray photons into electrical signals. In contrast, current CT detectors first convert the X‐rays into visible light and subsequently convert the light into an electrical signal using photodiodes. That means the photon-counting CT signal always remains fully digital. The first scanners for clinical application featuring the innovative detectors have been on the market since 2021.

What are the benefits?

Nikolaou: The technology enables physicians to control the X-ray radiation dose more accurately, which also leads to better image quality. An X-ray source emits photons with different energy intensities that we can now filter more precisely – examples include low energy photons that do not contribute to the image structure. We can also take away individual photons and virtually split up the X-ray spectrum, thus distinguishing and visualizing different materials from others in the image.

Take cardiac imaging, for example: we can subtract calcified plaque in the heart's arteries, i.e., the build-up within the walls of the coronary arteries. Prior to this, severe calcification made it difficult to view the affected vessel. If we subtract the calcification, we get a better view of the remaining vessel lumen, allowing a more accurate assessment of any narrowing in calcified blood vessels. Early research is available, but studies have not yet been extensive enough. We still need additional multicenter research trials to demonstrate the potential.

However, research has already shown that PCCT facilitates a radiation dose reduction since the technology counts photons more efficiently. Obviously, this benefits patients since it allows us to reduce CT scan radiation exposure while maintaining image quality.

The detector material also enables better contrast on CT images because the detector is no longer divided into individual detector elements but can count the photons on every square millimeter of the detector. Current CT scanners have a spatial resolution of 0.5 millimeter, whereas PCCT allows a higher spatial resolution by a factor of two or more. This is especially exciting and beneficial when you examine smaller vessels such as the coronary arteries, tiny bone structures in the inner ear, or small fibrotic changes in lungs or infiltrates.

Is the technology geared toward specific applications?

Nikolaou: Computed tomography is a medical imaging technique used to obtain detailed images for nearly all diagnostic indications. I do not think that PCCT will turn into a special scanner dedicated to the analysis of certain organs or entities, such as hearts or tumors, or be limited to examining children on account of the lower radiation dose. The scanner has been installed here in Tübingen since October 2021 and our objective is to examine the full spectrum of patients. The scanner is not explicitly made for one CT application, since it can be used to visualize nearly all parts of the body and diagnose diseases.

The tool stands to benefit all patients: smaller people thanks to a lower radiation dose, cancer patients because it allows us to characterize the tumor structure and different tissue constructs more accurately, and patients with cardiovascular diseases as we can get a better view of the coronary arteries.

The "Photon Counting CT Consortium" (PC3) comprises the University Medical Center of Freiburg, Tübingen University Hospital, Mannheim University Hospital, and project partners Siemens Healthineers and BIOPRO Baden-Württemberg GmbH. The group plans to build a technology ecosystem based on this system - what does that mean?

Nikolaou: Within the context of the Consortium, we plan to jointly establish the scanner technology, compile our CT data, and generate added value for the region. The idea is to create a complex value chain to drive both scientific applications and commercial exploitation and development. The new, fully digital CT datasets are very large – simply because higher resolution and intrinsic spectral information create large-scale datasets. We reap significant benefits if we compile and combine them.

This supports academic institutions, including the three aforementioned medical centers and future partners who plan to train artificial intelligence algorithms for imaging applications, for example. They will have faster access to image data from three different locations. The mere fact that we can build a digital infrastructure for scientific collaboration across multiple universities is already a great advantage.

Apart from that, we also want to target companies or startups that develop software tools in this field. In doing so, we also foster economic connectivity.