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Innovation culture

Why computed tomography was redefined

Implementing the groundbreaking idea of a photon-counting detector1 called for a complete redefine of established CT technology.

8min
Andrea Lutz
Published on 16. November 2021

When asked about the significance of photon counting for computed tomography (CT), Stefan Ulzheimer, Program Manager for photon-counting CT, says: “This development is comparable with the move from pixelated black-and-white television to HD color TV.”

Nevertheless, the establishment of the photon-counting detector is about much more than simply producing improved images using CT scanners. It also involves using these “colors” to perform more nuanced evaluations and to “obtain valuable clinical information that may allow physicians to make conclusive diagnoses at an earlier stage and to discuss possible treatments,” says Professor Thomas Flohr, Head of CT Concepts. With a view to providing rapid access to this additional information, researchers embarked on a 20-year journey in order to completely reinvent the established technology in the field of computed tomography.
Professor Thomas Flohr, PhD, Head of CT Concepts, Siemens Healthineers

It’s about obtaining valuable new clinical information that may allow physicians to make conclusive diagnoses at an earlier stage and to discuss possible treatments.

Professor Thomas Flohr, PhD, Head of CT Concepts, Siemens Healthineers

The whole is greater than the sum of its parts

For almost 50 years, computed tomography has been an indispensable tool for detecting and identifying diseases quickly and at an early stage. In some cases, however, CT scans do not allow physicians to make a clear diagnosis, because the level of detail in a conventional CT scan is limited by the existing technology, which has reached the extent of its capabilities. This has to do with the conventional detection principle, whereby an X-ray tube on one side emits radiation consisting of a multitude of tiny, energetic particles – or quanta. This radiation becomes weaker as it passes through a part of the body. In other words, a certain number of quanta get stuck in the body tissue. The energy lost by the end of the particle’s journey depends on how dense or transparent a tissue is and which materials the particle encounters on its way.

Opposite the X-ray source lies the target – that is, the detector, which is responsible for absorbing as many of the quanta that pass through the tissue as possible and then converting them into an electrical signal for image generation. The problem is that a conventional detector can only record the cumulative effect of these quanta and therefore only outputs an “intensity value” for the incoming signal. An important piece of information is therefore lost, because technology based on this principle does not record the lower-energy section of the X-ray quanta. In practical terms, this means that although a conventional CT scan can indicate that a certain tissue is present – a tumor in the liver, for example, or a problem in the coronary vessels – it does not allow physicians to reach further conclusions about what that means for the patient. 

What are quanta?

In physics, quanta are the very smallest units of energy, which cannot be broken down any further. The quanta of an electromagnetic field are known as photons.

Counting photons: a new dimension in computed tomography

More information from each particle

A photon-counting detector can convert incoming X-ray quanta directly into an electrical signal. Furthermore, as the name suggests, it counts each individual photon – in other words, each packet of energy that has passed through the tissue – and communicates the energy level with which it arrives at the detector. As the energies of the quanta provide us with information about the material that the X-ray beam has passed through, it’s now possible to break down a CT image into different materials. Up to four different energy levels can be identified and color-coded. In the case of bone metastases, for example, this may reveal a heterogeneity that was not possible to visualize in the past.

What kind of materials?

For example, in different tissue types or in contrast agents such as iodine or barium sulfate.

Electronic noise eliminated

Measuring the energy of the photons allows the technology to differentiate the real signal from any electronic noise. This paves the way for another key advantage: For the first time, disruptive electronic noise can be eliminated altogether, leading to improved image quality in all applications where it is necessary to manage with just a low detector signal. Examples include screening examinations, which are carried out with a low level of X-ray radiation from the outset in order to protect the patient.
Stefan Ulzheimer, PhD, Program Manager for photon-counting CT, Siemens Healthineers

The development is comparable with the move from pixelated black-and-white television to high-definition color TV.

Stefan Ulzheimer, PhD, Program Manager for photon-counting CT, Siemens Healthineers

Higher spatial resolution

In photon-counting detectors, the size of the detector pixels can be reduced considerably so that the measurement object can be scanned with a higher spatial frequency. This leads to an increase in spatial resolution compared with conventional detectors and is beneficial for cardiac diagnosis, for example, where physicians want a clear view of the coronary arteries. Alternatively, it can also be used to reduce the radiation dose during scans, which is a major advantage for preventive and follow-up examinations.

To leverage all of these capabilities, it was necessary to rethink several aspects of computed tomography. After all, photon counting is about more than just installing a new detector – and so Thomas Flohr and Stefan Ulzheimer embarked on an almost 20-year journey back in 2001. Their work would be defined by the motto “Every photon counts!”

Much more than just a new detector

Redefined: the basic detection principle

A CT scanner features an X-ray source and a detector system that move around the patient on a circular path. The conventional detector systems are known as “energy-integrating detectors” and contain a scintillator layer that first converts the X-rays into visible light signals, which are then converted into electrical signals by photodiodes. These signals are ultimately digitalized, allowing a computer to generate images of the inside of the body without superimposition. By contrast, a photon-counting detector consists of a semiconducting material that is able to convert X-rays directly into electrical signal pulses. As a result, no more energy information from the X-ray quanta is lost – and this direct conversion dispenses with an intermediate step of the process. The new detection principle is therefore exceptionally efficient and produces more data – and therefore clinical information – than a conventional CT scanner.

How do scintillators work?

Molecules in a scintillator are excited by the passage of photons and emit the resulting energy in the form of light.

Redefined: unprecedented computing power in the world of CT imaging

A photon-counting detector therefore delivers more information in a short time. To take advantage of this additional information, researchers have overhauled the entire concept of the CT system – the hardware, the software, and data transmission. Computers with previously unseen performance in the field of medical technology are now able to generate three-dimensional images in just a few seconds. Speed is of the essence, because in routine clinical practice physicians are keen to see the levels of performance that they are used to– in other words, the images must be available as quickly as possible after the end of the examination, even if this now entails enormous levels of computing power. The engineers therefore found themselves facing the mammoth task of developing a computer that is able to process this volume of data in a short time while remaining within a marketable price range.
Christian Schröter is head of the crystal center.

We started to generate more than 400 square meters of space to learn and install the crystal growth processes.

Christian Schröter, PhD, Project Manager for direct conversion materials, Siemens Healthineers

Redefined: detector material

Another huge subproject related to producing the material from which the detector is made: cadmium telluride (CdTe). Although CdTe was already identified as a promising substance for the detector back in 2001, the material available in those days didn’t meet the high standards needed for medical CT imaging. As the use of photon-counting X-ray detectors is now becoming a reality in routine clinical practice, it is vital to manufacture sufficiently large quantities of the necessary crystals for thousands of systems. Over the course of 20 years, this elaborate process has been perfected in close collaboration with key partner companies.

After all, nothing short of perfection will do when it comes to manufacturing crystals. The process involves filling carbon-coated ampoules with cadmium and tellurium and sealing them over an open flame. Next, the material is placed into a new ampoule with a piece of a perfect, previously grown crystal that serves as the seed for the actual crystal-growing process. Over a period of ten weeks, these ampoules pass through a heating element as the crystal slowly grows to produce cadmium telluride with a purity level of more than 99.9999 percent.

How to produce large quantities of crystals?

To produce the crystals, a dedicated laboratory known as the Crystal Center was established in Forchheim, Germany, and opened in 2020.
Read more
Björn Kreisler, PhD, Senior Key Expert for detectors, Siemens Healthineers

The capabilities that we’ve gained from the new detector make it possible to distinguish different contrast agents from one another. In turn, this will encourage our partners to produce new developments and innovations.

Björn Kreisler, PhD, Senior Key Expert for detectors, Siemens Healthineers

Redefined: sparing use of radiation and contrast agents

The resulting cadmium telluride crystals enable an exceptionally fine detector structure. This paves the way for high image resolution and, in turn, lower radiation doses and reductions in the use of contrast agents. These benefits open computed tomography up to groups of people where there would once have been concerns regarding exposure to radiation or contrast agents, such as children or people suffering from kidney diseases. As contrast agents can be detected more effectively, it is now also possible to visualize other contrast materials such as bismuth or gold.

There may be further improvements in image quality if, in the near future, it becomes possible to use new contrast agents that are adapted to the properties of the quantum-counting detector. “As there were previously no CT scanners that were able to distinguish between different contrast agents, manufacturers had no incentive to develop new contrast agents. We’re now seeing a great deal of interest and research activity in this area,” says Stefan Ulzheimer.

Why are new contrast agents important?

Radiologists must inform patients of the risks and potential for kidney damage before administering iodinated contrast agents.

Sharing knowledge and building stronger networks in the clinical world

Several stages were involved in the process of completely reinventing computed tomography. So far, hundreds of patents have been produced in relation to photon-counting technology. “We want to focus our efforts on launching sensible products as quickly as possible and without getting in each other’s way,” says Stefan Ulzheimer. For Siemens Healthineers, “sensible” also means that the clinical benefits should be quantifiable. Describing one vision of the future, Ulzheimer says: “In five to ten years, there may no longer be a need for diagnostic catheterization of the coronary arteries in order to rule out significant stenosis … we now have a plan to identify the benefits of this technology and make them quantifiable.”
By Andrea Lutz
Andrea Lutz is a journalist and business trainer specialized on medical topics, technology, and healthcare IT. She lives in Nuremberg, Germany.

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  1. 1 The products/features (mentioned herein) are not commercially available in all countries. Their future availability cannot be guaranteed.