Photons herald the next revolution in computed tomography

Photon counting detectors count and discriminate the energy of X-ray quanta at photon level, providing better spatial resolution and lower radiation doses for patients.

The history of computed tomography is packed with innovations. In the 50+ years of its existence, there have been many technological improvements but the latest innovation for CT examinations, namely photon counting detectors (PCDs), is truly groundbreaking. These novel devices count and discriminate the energy of X-ray quanta at photon level, providing better spatial resolution and lower radiation doses for patients.

As a leader in computed tomography and digital X-ray sensor solutions, ams OSRAM supplies manufacturers of medical equipment with leading-edge imaging components. Roger Steadman, Head of System Solution Engineering at BL Mixed Signal Products, is a renowned computed tomography expert and has witnessed many developments in more than 20 years in the industry.  

Whenever bone fractures, swellings or any other structural changes or symptoms have to be examined and diagnosed, the most usual investigative method is computed tomography. Roger, can you explain what this method is all about?

Computed tomography is well known, as it has been around for over 50 years. It is a special type of X-ray examination that captures 3D images of the body. With the help of an X-ray tube that rotates around the body, several thousand images of a region of the body or a specific organ are taken and then combined into a single 3D volume.
Between 1990 and 2013, various advances and developments revolutionized CT, particularly the “slice race” in which vendors competed to make detectors larger and larger with the goal of covering entire organs, such as the heart, in a single rotation. Since then, much time and effort has been dedicated to reducing the X-ray dose to the patient, including developments in detectors for better image quality and reconstruction algorithms.    

You have seen several CT innovations throughout your career. Why do you consider photon counting to be different?

Photon counting will help to conquer a challenge of great importance for society. Over the last few years, the clear objective of every scientist and expert in the field has been to reduce the radiation dose and to enhance early diagnosis with the aid of additional information. The step to photon counting is considered as significant as the transition from black-and-white to color television in terms of the extra information it delivers, in this case for better diagnoses. It also allows higher resolutions at significantly lower radiation dosages. There are a number of additional factors that contribute to the ability of photon counting to reduce X-ray dosage by 40% to 80% percent, depending on the clinical protocol.

Photon counting also aims to exploit the inherent energy information of the photons, making it possible to distinguish and quantify different tissue types in the body. It’s great to witness technologies that have been researched in the field for many years finally being incorporated into a product and coming to market in the first medical certified photon-counting-based CT scanners in 2021.

Can you explain photon counting in simple language?

I can at least try. Conventional CT detects a signal which is proportional to the total energy of the X-ray photons, neglecting the fact that the attenuation of the body or tissue is energy dependent. Photon counting removes that limitation. Because it provides a measure of not only the number of photons but also the energy of those photons, it can ultimately distinguish between substances present in the body. In other words, photon counting provides information that can be used to resolve unknown variables, 

This leads to substantial benefits beyond tissue identification. It also improves image contrast as now each individual photon contributes equally to the image. Another aspect of photon counting that is receiving a lot of attention from radiologists is its significantly better spatial resolution. Because different sensors are required in this technology, spatial resolution can be up to four times greater than that of conventional CT, depending on system parameters driven by the CT OEMs.

In your opinion, what exactly sets us apart here? What makes our approach so special?

We provide our customers with a full solution. Not only is our readout IC capable of fulfilling the strict requirements of clinical photon-counting spectral CT at very low noise and power consumption but it is also delivered in an innovative Buttable System in Package, or BSIP, in which several ICs and passives are embedded in a package. The BSIP is inherently “tileable”, in other words BSIPs can be used as tiles in arrangements that can extend in all directions to form the large detector area necessary for clinical CT. This advanced solution provides ease of use for our customers and state-of-the-art performance.

Semiconductor technology brings novel features to medical imaging. That’s what drives us: technology that makes life better for everyone.