Photon-counting CT: quick and non-invasive scanning

Compared with images from conventional computed tomography (CT) scanners, clinical images generated using photon-counting detectors feature a high resolution and contrast improvements. The technology also paves the way for decisive further reductions in X-ray dose during CT scans. Imaging with photon-counting detectors therefore puts less strain on patients and offers physicians a genuine alternative when it comes to screening programs and follow-up examinations, for example in cancer treatment. Patients benefit from the noninvasive detection of even slight tissue alterations, and treatment planning can then begin immediately.[1] As a result, multiple clinical disciplines may see an increase in the number of patients for whom a CT scan is recommended.

In many cases of cardiac insufficiency, the coronary vessels are affected by calcareous deposits – and this calcification in the arteries is so dominant in conventional CT scans that it prevents physicians from assessing circulation in affected vessels. This is because, in practice, calcium and a vessel filled with a contrast agent appear as gray on gray. Until now, many patients have therefore had to undergo a minimally invasive cardiac catheter examination in order to obtain a more accurate diagnosis. With a photon-counting CT scanner, however, physicians are now able to visualize the coronary vessels clearly to assess narrowing.

Professor Heinz-Peter Schlemmer, MD, Head of Radiology at the German Cancer Research Center (DKFZ) in Heidelberg, has already had the chance to work with a photon-counting CT scanner. A consistent clinical image is essential for reliably distinguishing between tissue and other materials. In cardiology, this is achieved using the dual source principle, which is also used in photon-counting CT. Indeed, the two measurement systems in NAEOTOM Alpha allow physicians to keep the exposure time for a CT scan so short that the heart can be visualized in the resting state between two beats, thereby producing a sharp image.
When it comes to postprocessing, different materials are then color-coded – for example, calcium, iodine, and the actual lumen are depicted in different tones, allowing physicians to evaluate the degree of narrowing or plaques in the coronary arteries based on the CT scan.

Computed tomography is an established method for detecting anatomical changes in lung tissue and is capable of detecting tumors measuring as little as 0.3 millimeters, as well as interstitial lung diseases. Photon-counting CT can visualize detailed structures and combine the resulting images with functional information. By way of background, research by the Robert Koch Institute indicates that conventional chest X-rays show changes in 50 to 60 percent of people with COVID-19, and conventional CT scans reveal these changes in around 85 percent of cases.[2] In patients with suspected COVID-19 pneumonia at University Hospital Rechts der Isar, Munich, Germany, 81 percent of PCR-positive patients also showed typical findings in CT scans.[3]

In all cases of cancer, it’s important to determine whether the chosen course of treatment is having the desired effect. This entails identifying even slight changes in tissues and, above all, detecting any metastases at an early stage. Among other examinations, cancer patients are now asked to undergo multiple CT scans, where the sparing use of radiation is of paramount importance.
For example, breast cancer often forms skeletal metastases – in other words, metastases in the bones. When this happens, patients complain of severe pain and, in the worst-case scenario, may suffer bone fractures or damage to their spinal cord. Radiation oncologist Eckhard Wehrse and his colleagues at DKFZ carried out a study in which they compared images of these metastases from a photon-counting CT scanner with conventional CT scans. They found that physicians consistently rated images generated by the innovative detector technology as “sharp” and “more detailed.”

In particular, changes in very fine structures were said to be easier to identify.[1] Although further studies are planned, it’s already clear that photon-counting CT is an ideal tool for breast cancer follow-up – not least because the innovative detector technology means that the radiation dose can be kept low. As Schlemmer observed: “The high sensitivity of the photon-counting detector is one reason why the scanner can achieve a dose reduction of up to 40 percent when operated like a standard CT scanner rather than at full power.”