Innovation culture

Photon-counting CT: quick and non-invasive scanning

Photon-counting computed tomography (CT) scanners1 provide additional information to physicians for diagnosis and treatment in cardiology, pulmonology, and oncology.
Andrea Lutz
Published on December 1, 2021

Computed tomography will soon play an essential role in treatment decisions thanks to the precise information generated by photon-counting detectors. The NAEOTOM Alpha is the first clinical photon-counting CT system to be launched.

<p>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.</p>
<p>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.</p>

<p>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.<br>When it comes to postprocessing, different materials are then color-coded – for example, calcium, iodine, and the actual <a href="lumen" data-ste-link-id="2344656238.on/cp-text-image-028c1e73:0103333805.lumen:0103333805">lumen</a>&nbsp;are depicted in different tones, allowing physicians to evaluate the degree of narrowing or plaques in the coronary arteries based on the CT scan.</p>
In medicine, the term “lumen” refers to the inner cavity of an organ or tube-shaped structure, such as a blood vessel.
<p>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 <a href="interstitial">interstitial</a>&nbsp;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]<br></p>
A group of lung diseases affecting the walls of the pulmonary alveoli or the connective tissue. They lead to an accumulation of inflammatory cells in the lung tissue and cause coughing and shortness of breath.

These findings are not, however, specific to COVID-19 <a href="Pneumonia">pneumonia</a>&nbsp;and also appear in other pneumonia patients.[4] Experience shows that CT scans primarily enable physicians to distinguish between different severities of pneumonia associated with COVID-19.[3] Accordingly, the principal tasks for CT are to diagnose complications associated with the inflammation of lung tissue and, in serious cases, to monitor disease progression. Specifically, these lung examinations will benefit from the combination of high image definition, short exposure time, and sophisticated tissue characterization. “We’re able to visualize changes in the thin septa, the trachea, and the bronchial walls that were previously invisible to us,” says Schlemmer. As signaled in a statement from the German Radiological Society, computed tomography is not intended to be used for screening or as a first-line test for diagnosing COVID-19.
The term “pneumonia” refers to acute or chronic inflammation of the lung tissue.
<p>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 <a href="Metastases" data-ste-link-id="1569180325.on/cp-text-image-2af16891:3777586342.Metastases:1784121478">metastases</a>&nbsp;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.<br>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.”</p>

Metastases are accumulations of tumor cells in a distant part of the body from the original tumor.

<p>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 <a href="Radiation dose">radiation dose</a> 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.”</p>

If you measure the energy of every single photon as it strikes the detector, it’s possible to distinguish the real signal from the pre-existing electronic noise. For the first time, disruptive electronic noise can therefore be eliminated altogether.

Organs such as the liver, heart, gall bladder, and spleen all appear in similar shades of gray in CT scans because they cause the same degree of <a href="X-ray attenuation">X-ray attenuation</a>. To obtain more information from the scan, a contrast agent – usually iodine-based – is administered to the patient beforehand. A liver tumor, for example, will absorb slightly less or slightly more contrast agent than healthy liver tissue – allowing physicians to track down even very slight changes. When it comes to treating liver cancer, radioactive iodine is then introduced into the liver. Because this substance accumulates almost exclusively in the tumor and not in other organs, it only destroys tumor cells. In follow-up examinations, however, the radioactive iodine often overshadows the image of the liver. This is one situation where it would make sense to have access to a different contrast agent for the follow-up scan. A photon-counting detector can also be used in conjunction with contrast materials such as bismuth or gold, which is beneficial not only for oncological follow-up examinations but in principle also for patients with diseases of the thyroid or kidney, as these diseases do not tolerate iodine. Today, considerable research is underway into the development of new contrast agents that can be used in conjunction with the photon-counting detector and will open up a plethora of new clinical applications.
X-rays become weaker as they pass through tissue and other materials. When photons strike the detector, their remaining energy influences the image contrast.

By Andrea Lutz
Andrea Lutz is a journalist and business trainer specialized on medical topics, technology, and healthcare IT. She lives in Nuremberg, Germany.