“The photon-counting CT scanner is our workhorse”

In cardiac imaging, photon-counting computed tomography (CT) visualizes small coronary vessels, stents, and plaques in high resolution and helps physicians provide precise answers to guide treatment and therapy decisions.

Doris Pischitz
Published on November 15, 2022

Cardiologist and radiologist Professor Pál Maurovich-Horvat, MD1, reports on the experiences he made with the new technology. 

Pál Maurovich-Horvat is head of the Medical Imaging Centre and chairman of Radiology at Semmelweis University in Budapest, Hungary.

We installed the scanner right after the RSNA meeting in December 2021 and started scanning patients a couple of days later. Our expectations were obviously high. And the first images didn’t disappoint! One of our first cases on the NAEOTOM Alpha2 scanner was a post-TAVI [transcatheter aortic valve implantation] patient with suspected leaflet thrombosis. We were amazed by the crisp image quality, which was thanks to the new scanner’s low image noise and high spatial and temporal resolution.

The speed of image acquisition, spatial resolution, and soft-tissue resolution of traditional CT scanners are all limitations for cardiac imaging. The best scanners with conventional energy-integrating detectors have 0.5 to 0.6 millimeters of isotropic resolution. So even the best of these scanners can struggle with heavily calcified coronary arteries and stent imaging. In addition, differentiating between peri-coronary fat and low-attenuation noncalcified plaque can be difficult due to the limited gray scale resolution of conventional CT scanners. That said, temporal resolution is probably the most important factor for achieving robust image quality in cardiac CT imaging. Some traditional scanners provide great imaging speed, but the spatial and soft-tissue resolution are still limiting factors.
The NAEOTOM Alpha scanner’s standard spatial resolution is 0.4 millimeters, which visibly improves the image quality of cardiac scans. Better spatial resolution allows for better luminal stenosis quantification and improves coronary atherosclerotic plaque assessment. The inherent spectral capability is very helpful in reducing calcium blooming and improving soft-tissue resolution. Our scanner recently had a major software update and can now image the heart with 0.2 millimeters of spatial resolution, which is truly a game-changer [Figure 1].
Image of in-stent restenosis acquired with photon-counting CT.
The ultra-high resolution [UHR] coronary CT angiographies [CTA] are astonishing every time we look at the images. Seeing the stent struts and even minimal in-stent restenosis is a new experience for us. Currently, I think the UHR mode is the scanner’s most exciting cardiac imaging feature.

We generally start our cardiac exams with a low-dose calcium score. If we see increased calcium burden, we continue with a UHR coronary CTA. UHR imaging reduces calcium blooming, which has made us a lot more confident in ruling out obstructive disease in challenging cases. We also use UHR imaging for patients with stents. We are pretty sure that the traditional 3-millimeter stent diameter threshold for cardiac CT will be 2 to 2.5 millimeters with a photon-counting scanner. Importantly, the UHR mode is also very dose-efficient. We were surprised to see that the UHR cardiac scans have a dose length product [DLP] of less than 200 milligray-centimeters. Of course, the standard 0.4 millimeter resolution cardiac scans have very low DLPs (around 50 milligray-centimeters), but with UHR we gain information that was unachievable with traditional scanners.

The improved image contrast using spectral reconstructions is a helpful feature, especially in patients with heightened risk of kidney injury, since using low-energy virtual monoenergetic images [VMIs] increases the iodine signal.

The VMIs are helpful not only for reducing iodine load but also for improving diagnostic certainty in patients with heavily calcified plaques. The high-energy VMIs help reduce blooming artifacts. Using flash coronary CT image acquisition, we can routinely achieve sub-millisievert scans with spectral information.

Pál Maurovich-Horvat talks about photon-counting CT in Cardiology.

The photon-counting CT system is our main scanner. It’s our workhorse. Cardiac imaging makes up just a small fraction of the exams we perform on our NAEOTOM Alpha. The scanner provides high image quality in every field. Our colleagues from the ear-nose-throat department have been amazed by the beautiful, high-resolution images it produces of the inner ear and cochlea implants. The chest images had a similar effect on our pulmonologist colleagues. And our surgeons love the high-resolution cinematic renderings of the abdomen.
A patient with a full metal jacket—in this case, 15 stents in the left and right coronary arteries—is basically an impossible task for traditional scanners. The photon-counting scanner allowed us to rule out in-stent restenosis even in smaller stents, and to identify the problematic proximal left anterior descending artery stent with severe restenosis [Figure 2].
Image of severe stenosis acquired with photon-counting CT.
The increased depth of information provides new opportunities for precision diagnostics and disease phenotyping. More voxels equal more information. And the spectral data adds an extra dimension to the already vast datasets. The challenge is how to best utilize these data. We’re working on several projects that use radiomics and machine learning to extract information that’s essentially invisible to the human eye.

I truly believe that photon-counting technology will transform CT imaging in the coming decade. This is the next big thing. It’s easy to see that in a couple of years, photon-counting will be the standard CT technology. It provides more information with less radiation than conventional CT scanners. This is a good combination indeed! As I said, UHR is currently my favorite feature of the photon-counting scanner. My next favorite feature is probably the multicontrast imaging or k-edge imaging. This will open up totally new avenues in diagnostics.

Professor Pál Maurovich-Horvat, MD, is also one of the speakers in our Siemens Healthineers Shape 23 Keynote. Watch now to hear him and some of the world’s most respected healthcare leaders talk about how high-quality care can be brought to more patients, how to address the challenges posed by cancer, and how to address the workforce crisis.

By Doris Pischitz
Doris Pischitz is an editor in corporate communications at Siemens Healthineers. The team specializes in topics related to healthcare, medical technology, disease areas, and digitalization.