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Nominee 2021

Photon-counting computed tomography scanner

Photon-counting computed tomography scanner - A revolutionary way of looking inside the human body

Prof. Dr. rer. nat. Thomas Flohr (Spokesperson)
Dr. rer. nat. Björn Kreisler
Dr. rer. biol. hum. Stefan Ulzheimer
Siemens Healthineers AG, Forchheim

(f.l.t.r.) Dr. rer. biol. hum. Stefan Ulzheimer, Prof. Dr. rer. nat. Thomas Flohr,
Dr. rer. nat. Björn Kreisler

Computer tomography (CT) is one of the most important technologies used to obtain images from inside the human body. The three-dimensional images reveal fine differences or changes in organs, vessels and bones and help in the diagnosis of diseases or injuries. Yet a refinement of the previous technology for even more informative images – an added medical benefit – has reached the saturation phase. Is there any way to overcome it?

Professor Thomas Flohr, Dr. Björn Kreisler and Dr. Stefan Ulzheimer have opened the door to a new application. The three nominees have developed technical components and a number of innovative techniques to detect the X-ray light used in computer tomography. They also re-structured the CT system's digital architecture.

This makes it possible to obtain images of the human body in a detail previously impossible. The innovative detector principle is based on the counting and analysis of individual X-ray photons using crystalline materials. This produces a spatial resolution previously unattainable in computer tomography images and also provides much valuable additional information.

Medical diagnoses are thus provided with a safe and reliable foundation facilitating or even enabling the decision regarding the best possible form of treatment. At Siemens Healthineers Thomas Flohr heads the CT Physics division, Björn Kreisler is the senior key expert for detectors, Stefan Ulzheimer is program lead for the photon counting technology.

In computer tomography, an X-ray source rotates rapidly around the body, penetrating it. In the process, some of the radiation is absorbed. Depending on the type and structure of the tissue, the X-ray light is attenuated differently, the rest is recorded by a detector. As the body is scanned, countless X-ray images are produced from different perspectives. A computer algorithm creates three-dimensional images of the body region examined. Data is recorded by a conventional CT system in two steps: the detector initially converts absorbed X-ray light into visible light which is then translated into an electrical signal. However, due to the double signal conversion, information about the energy of the X-ray light - which strikes the detector in the form of individual photons - is lost. And: the sharpness of the images is limited.

The nominated team has overcome these obstacles. The nominees developed an innovative detector that registers every single X-ray photon that strikes it and converts it directly into a digital signal that can be used by the computer. Experts call this "photon counting". This capability is based on cadmium telluride - a crystalline compound that the team used for the first time as a material for whole-body computer tomography in medical diagnostics. The material has several advantages: firstly, it improves the sensitivity of measurements with the possibility of photon counting - and results in a much higher resolution and better image contrast. Much finer details can thus be seen in these images compared to those produced using conventional CT systems. Secondly, the separate detection of each X-ray photon also registers the energy the photon carries. This spectral analysis makes it easier to identify anatomic structures and differentiate different types of tissue more reliably than in the past. By integrating two separate X-ray sources and detectors - the "dual source principle" - the exposure time is also short and kept to a minimum. This means that sharp images can be produced even of moving organs like the heart. The new technology also lowers the amount of X-ray radiation the body is exposed to by up to 40 percent. In addition, it has more options in regard to the use of contrast media. These are already employed to differentiate soft tissues and make it easier to recognize blood vessels. Thanks to the spectral resolution of the photon-counting CT, different contrast media can be combined to be able to differentiate organs more clearly or use media that are tolerated better.
The key element of the innovation is the cadmium-telluride detector. The nominated team and their colleagues succeeded in growing this promising crystalline material in sufficiently high quality and prepare it to detect X-ray light. The nominees also developed a new technology for transferring data and interpreting images which allows the vast amounts of data generated to be processed rapidly.

The high resolution and additional information make it easier to diagnose and understand diseases. Even the most minute changes in the body can be identified at an early stage - and treated as necessary. Photon-counting computer tomography scanners reliably detect narrowing of the arteries - and thus an increased risk of heart attack. In oncology, the early stages of tumors and metastases are easier to detect than has been previously possible. The innovative technology also helps in the planning of operations. Lung diseases, particularly those that affect especially fine tissue structures, can be directly diagnosed. In emergency medicine, the innovation provides a particularly fast and reliable overview of injuries.

The initial prototypes of CT scanners with the new technology were built in 2008. The first clinical trials began in 2014. Since 2021, several computer tomography systems incorporating photon-counting detectors have routinely been used in clinical settings. This puts Siemens Healthineers at the cutting edge worldwide - a position the company hopes to expand on: by introducing a completely new product line. The goal is to sell several thousand of the innovative systems in the next ten years and thus generate revenue in excess of several billion euros. With this goal in mind, the company is building a new plant to grow cadmium telluride crystals and plans on expanding the production facilities for CT systems in Forchheim. This will create numerous new jobs - at Siemens Healthineers as well as at suppliers.

More Details


Prof. Dr. rer. nat. Thomas Georg Flohr

Born in Neustadt / Aisch, Germany
1970 - 1979
Qualification: High-school diploma, Friedrich-Alexander-Gymnasium, Neustadt / Aisch, Germany
1979 - 1985
Study of physics, qualification: Diploma, Friedrich-Alexander-University, Erlangen-Nuremberg Erlangen, Germany
1985 - 1989
Doctoral candidate in physics, Friedrich-Alexander-University, Erlangen-Nuremberg Erlangen, Germany
Qualification: Doctor of natural sciences (Dr. rer. nat), Friedrich-Alexander-University Erlangen-Nuremberg, Erlangen, Germany
1989 - 2000
Research position in the computed tomography section, Siemens, UB Med, Erlangen/Forchheim, Germany
2000 - 2003
Head of CT physics department, computed tomography section, Siemens, UB Med, Forchheim, Germany
Since 2004
Head of the CT physics, application predevelopment, and global clinical collaborations department, Siemens Healthcare, now Siemens Healthineers, Forchheim, Germany
Habilitation in "Medical Physics", Eberhard-Karls-University Tuebingen, Tuebingen, Germany
2007 - 2011
Qualified Professor, Eberhard-Karls-University Tuebingen, Tuebingen, Germany
Since 2011
Adjunct professor for medical physics, Eberhard-Karls-University Tuebingen, Tuebingen, Germany

Further activities

Since 2005
Founding member of the Society of Cardiovascular Computed Tomography
Since 2006
Member of the Program Committee of the Physics of Medical Imaging Conference of the SPIE
Since 2007
Member of the Scientific Committee of the Fully 3D Conference: International Meeting on Fully Three-Dimensional Image Reconstruction in Radiology and Nuclear Medicine
2016 - 2017
Conference Chair of the Physics of Medical Imaging Conference of the SPIE


230 peer-reviewed publications in scientific journals, including 46 as the first or last author
Publisher / author of five books
Ad hoc reviewer for several scientific journals, including Investigative Radiology, European Radiology, Medical Physics

Awards (a selection)

"Inventor of the Year" of Siemens AG for work on the 4-slice CT scanner "SOMATOM Volume Zoom"
Finalist of the German Future Prize of the German Federal President, for "Looking into the heart without a catheter – cardio CT” (Bernd Ohnesorge, Thomas Flohr, Richard Hausmann)
Winner of the Siemens "top+ award" in the category "Innovation" for the 16-slice CT scanner "SOMATOM Sensation 16"
Appointment as "Siemens Top Innovator"
Winner of the Siemens "top+ award" in the category "Innovation" for the dual-source CT scanner "SOMATOM Flash"
Finalist of the German Innovation Prize for the Dual Source CT Scanner "SOMATOM Force"
Award of an honorary doctorate from the Faculty of Medicine of the University of Zurich

Award-winning papers

The paper "Flohr TG et al, First performance evaluation of a dual-source CT (DSCT) system. Eur Radiol. 2006" was chosen as one of 10 papers for presentation at the "European Radiology 25th Anniversary Session" at the ECR 2017. It is among the 100 most cited works in radiology journals in the years 1945-2012 (Yoon DY et al. Citation Classics in Radiology Journals: The 100 Top-Cited Articles, 1945-2012. American Journal of Roentgenology. 2013;201: 471-481
The paper "Ohnesorge B, Flohr T, et al. Cardiac imaging by means of electrocardiographically gated multisection spiral CT: initial experience. Radiology. 2000" was included in the series "Radiology Golden Oldies" (Radiology. 2015;276(3))
DThe paper "Flohr TG, et al. Dual-source spiral CT with pitch up to 3.2 and 75 ms temporal resolution: image reconstruction and assessment of image quality. Med Phys. 2009" was chosen for the edition "Significant Advances in CT – an inaugural Virtual Issue of Medical Physics in honor of the 40th anniversary of Cormack and Hounsfield’s 1979 Nobel Prize for the development of the CT scanner" of the journal Medical Physics.

Dr. rer. nat. Björn Kreisler

Born in Frankfurt am Main, Germany
1990 - 1999
Qualification: High-school diploma, Emil-von-Behring Gymnasium, Spardorf, Germany
1999 - 2000
Civilian service, Erlangen, Germany
2000 - 2006
Studies in physics, focus "Physics in medicine", Friedrich-Alexander-University, Erlangen-Nuremberg / University of York, Erlangen, Germany / York, United Kingdom
Diploma in physics, thesis: "Influenced signals in pixelated semiconductor X-ray detectors”, Friedrich-Alexander-University Erlangen-Nuremberg, Erlangen, Germany
2006 - 2010
Research position in the Medical Physics Group at the Erlangen Center for Astroparticle Physics (ECAP) in the Institute for Physics, Friedrich-Alexander-University Erlangen-Nuremberg, Erlangen, Germany
Doctorate at the Institute for Physics: "Simulation of Medical Irradiation and X-Ray Detector Signals", Friedrich-Alexander-University Erlangen-Nuremberg, Erlangen, Germany
Since 2010
Detector physicist for CT detectors with the focus on counting detectors, Siemens Healthcare, now Siemens Healthineers, Forchheim, Germany
Appointment as senior key expert for detectors, Siemens Healthineers, Forchheim, Germany

Study grants

2002 - 2003
Erasmus grant for study year at York, United Kingdom
2006 - 2010
Doctoral studies grant of the International Max-Planck Research School (IMPRS) for Optics and Imaging

Dr. rer. biol. hum. Stefan Ulzheimer 

Born in Nuremberg, Germany
1981 - 1990
Gymnasium, Martin-Behaim-Gymnasium, Nuremberg, Germany
Qualification: High-school diploma, Martin-Behaim-Gymnasium, Nuremberg, Germany
1990 - 1991
Basic military service, Regensburg, Germany
1991 - 1994
Foundation studies in physics, Friedrich-Alexander-University Erlangen-Nuremberg, Erlangen, Germany
1993 - 1994
Erasmus student, Imperial College, London, United Kingdom
Intermediate examination in physics, Friedrich-Alexander-University Erlangen-Nuremberg, Erlangen, Germany
1994 - 1998
Advanced studies in physics, Friedrich-Alexander-University Erlangen-Nuremberg, Erlangen, Germany
Qualification: Diploma in physics, Friedrich-Alexander-University Erlangen-Nuremberg, Erlangen, Germany
1998 - 2001
Doctoral candidate in human biology at the Faculty of Medicine, Friedrich-Alexander-University Erlangen-Nuremberg, Erlangen, Germany
Doctor of human biology (Dr. rer. biol. hum.), Friedrich-Alexander-University Erlangen-Nuremberg, Erlangen, Germany
2001 - 2004
General manager, Verfahren und Apparate der Medizinischen Physik GmbH, Moehrendorf, Germany
2004 - 2009
Collaboration manager for computed tomography, Siemens Healthcare, Forchheim, Germany
2010 - 2014
Head of Global Scientific Marketing for Computed Tomography, Siemens Healthcare, Forchheim, Germany
Since 2014
Program Director for counting technology, Siemens Healthcare, now Siemens Healthineers, Forchheim, Germany


Magna cum laude prize of the ECR (European Congress of Radiology): "Improvements of cardiac CT using ECG oriented image reconstruction in subsecond spiral multirow scanning"
Winner of the Hochschulgründerpreis (prize for university spin-offs) in the Bavarian Business Plan Competition: "TomoScope – A micro-CT system for in-vivo small animal imaging"



Ulrich Künzel
Communications, Media Relations
Siemens Healthcare GmbH
Karlheinz-Kaske-Straße 5
91052 Erlangen
Mobile: +49 (0) 162 / 24 33 492


Prof. Dr. Thomas Flohr
Siemens Healthcare GmbH
Siemensstraße 3
91301 Forchheim
Mobile: +49 (0) 173 / 25 62 351

A description provided by the institutes and companies regarding their nominated projects

Photon-counting computed tomography scanner – A revolutionary way of looking inside the human body

Computed tomography (CT) is an indispensable tool of modern medicine for early and fast detection and diagnosis of diseases. It is the backbone of radiological diagnosis. Yet, not all diagnostic questions can be answered using CT imaging. For some diseases, a CT examination will not produce a conclusive diagnosis from which necessary treatment steps can be derived. In such cases, further examinations with other procedures will be required to diagnose the disease reliably. There are also various reasons why certain patient groups are not examined with fast and straightforward CT imaging, which can delay reporting. These reasons include reticence regarding X-ray dose in the case of children and young adults and the commonly used iodinated contrast media in the case of kidney disease patients.

Conventional CT imaging is a technically mature procedure, so further efforts to overcome these limitations in order to achieve earlier, more conclusive diagnosis of larger patient groups is not to be expected. Nominees Professor Thomas Flohr, Dr. Bjoern Kreisler, and Dr. Stefan Ulzheimer have overcome this obstacle. Together with their colleagues at Siemens Healthineers, they developed an innovative CT concept, tested it with clinical prototypes, and developed it for series production. The concept is based on completely new detector technology and thorough redevelopment of all the system components. Photon-counting computed tomography paves the way for the previously unattainable combination of strong image sharpness and extremely short image acquisition times with improved image contrast and more diagnostically useful image contents. At the same time, the X-ray and contrast agent dose can be reduced. This is not merely a new, improved generation of CT scanners – CT imaging has in effect been reinvented.

In a CT scanner, an X-ray tube and a detector system rotate around the patient. During this rotating movement, the detector registers the X-rays attenuated by the body and converts them into an electrical signal, which is then digitized. From this data, a computer calculates images that show the inside of the body and its organs in three dimensions and without overlap. Conventional detectors operate in two stages: The X-rays are converted to visible light in a scintillator, which then generates an electrical signal. The information about the energy of the X-rays is lost, which results in reduced image contrast and unclear image content. It is also not possible to reduce the size of the individual detector elements meaning that image sharpness cannot be significantly increased.

Significantly wider range of applications for computed tomography

As early as 2001, the Siemens healthcare division embarked on the development of new detector concepts in cooperation with partner organizations from research and industry. Cadmium telluride (CdTe) was soon identified as a very promising material. The "photon-counting detector" converts X-rays in a CdTe crystal into electrical pulses, whose magnitude is proportional to the energy of the X-ray. The energy information is not lost when the detector is read out – this increases image contrasts, the images contain more information and are clearer, making diagnosis easier. The fine structures of the detectors result in a considerable increase in image sharpness, and the X-ray and contrast medium dose can be substantially reduced with the new detection concept. As a result, photon-counting CT has extended the range of use to patient groups for whom CT imaging was previously considered unsuitable due to the X-ray or contrast agent dose. When the new technology is implemented in a Dual Source CT – that is, in a system with two X-ray tubes and two detectors – extremely short image acquisition times and fast acquisitions are achieved. As a result, the examination of moving organs, such as the lung and heart, is possible with previously unknown accuracy.

These improvements would not have been possible with conventional CT technology. The CdTe detector material available in 2001 was, however, far from meeting the high-quality requirements of medical CT. During the course of nearly 20 years of development work, Siemens Healthineers and its partners took the technology step by step to series maturity with numerous improvements and laid the foundations for series detector manufacturing. With completely redeveloped readout electronics connected to the CdTe crystals across their entire surface, any number of individual crystals can be juxtaposed to form larger detectors. In parallel to this work, the three nominees and their team overhauled and optimized the entire CT system concept, its hardware and software, to make the best possible use of the advantages of the new detector. In order to handle the significantly higher data volume and process data containing much more information, new approaches to data transfer and algorithms were developed. Three-dimensional images are now calculated in a matter of seconds on much more powerful computers being used for the first time in medical CT systems.

40 percent lower X-ray and contrast medium dose, twice as sharp images, and more refined tissue characterization

Since 2014, prototype CT scanners with these new photon-counting detectors have been installed in clinical settings to examine patients. Considerable knowledge about their potential in clinical use has been collected as a result. The clinical institutions confirmed the predicted reduction of up to 40 percent in X-ray and contrast medium dose on the patients they examined. They achieved significantly stronger image contrasts, for example, in examinations of the brain, and improved tissue characterization. With almost twice the image sharpness, new opportunities in bone and vessel imaging result, leading to more precise and patient-friendly intervention planning, for example, in partial nephrectomy in the treatment of kidney tumors. Lung and heart examinations also benefit from the combination of strong image sharpness, short acquisition times, and more refined tissue characterization. COVID-19 follow-up checks can also be significantly improved in this way. CT examinations of the coronary vessels can now be performed on significantly more patients, even those with severe calcification or stents. Previously, these patients usually had to undergo invasive diagnostic cardiac catheterization. With the new CT technology, diagnostic images of the coronary vessels with reliable visualization of stenoses can be acquired in these cases, too – without the risks of an invasive examination. In the case of polytraumas, the whole body can be examined with the highest precision in under three seconds.

In trials of the new CT technology, the nominees also recognized opportunities for further improving imaging with new contrast agents which could be developed by adapting them to the properties of the photon-counting detector. This was taken up by Bayer AG, who as part of a partnership with Siemens Healthineers is already researching new CT contrast agents.

Overall, a large number of cooperation partners has been recruited over the 20-year history of this unique research and development project. New core competencies have been established and transferred to the project. Company locations have been expanded, new employees recruited, a new production center for CdTe crystals built in Forchheim, and 30 leading clinical research partners have been involved. These include, for example, the university hospitals in Augsburg, Tübingen, Freiburg, and Hanover, as well as University Hospital Zurich and Erasmus Medical Center Rotterdam.

In future, every computed tomography scanner will be a photon-counting CT

The huge potential of this innovative CT concept offers great advantages for doctors and patients alike. Patients who previously could not benefit from this noninvasive, fast examination method can now be examined with CT imaging. Moreover, the technology offers opportunities for more precise, earlier diagnoses with fewer time-consuming and costly additional examinations, better therapy decisions, and an earlier therapy start. Another possibility is the more extensive utilization of the large volume of information extracted with photon-counting CT using artificial intelligence, both to improve cardiovascular and oncological differential diagnosis, and to plan and follow targeted therapies.

Since Siemens Healthineers is confident of the new technology's success, it will be installed in every computed tomography scanner manufactured by the company in the medium term and long term. Independent market forecasts expect an increase in global annual sales of CT systems from 6.43 billion U.S. dollars in 2020 to 9.64 billion U.S. dollars in the year 2027, which could be replaced with photon-counting CT systems. Due to its innovation and market lead of several years, Siemens Healthineers with its production and development location Forchheim will have a disproportionately larger share of this market. This development will also further accelerate contrast media research and contribute to shaping new research areas in process engineering and intelligent processing of large data volumes.

Siemens Healthineers AG (listed in Frankfurt, Germany: SHL) is shaping the future of healthcare. As a leading medical technology company headquartered in Erlangen, Germany, Siemens Healthineers enables healthcare providers worldwide through its regional companies to increase value by empowering them on their journey towards expanding precision medicine, transforming care delivery, improving the patient experience, and digitalizing healthcare. Siemens Healthineers is continuously developing its product and service portfolio, with AI-supported applications and digital offerings that play an increasingly important role in the next generation of medical technology. These new applications will enhance the company’s foundation in in-vitro diagnostics, image-guided therapy, in-vivo diagnostics, and innovative cancer care. Siemens Healthineers also provides a range of services and solutions to enhance healthcare providers’ ability to provide high-quality, efficient care to patients. In fiscal 2020, which ended on September 30, 2020, Siemens Healthineers generated revenue of €14.5 billion and adjusted EBIT of €2.2 billion. Following the acquisition of Varian Medical Systems, Inc. the company has approximately 66,000 employees worldwide. Further information is available at