What is the advantage of Photon-counting CT over conventional CT in cardiology?

Photon-counting computed tomography is a relatively new technology which has several advantages in cardiovascular imaging. Some of the disadvantages of conventional energy-integrating detector CT are taken care of by photon-counting CT. Role of ultrahigh-spatial-resolution photon-counting CT in coronary artery disease [1] and in imaging prior to transcatheter aortic valve replacement [2] have been mentioned earlier.

Conventional CT uses two step indirect conversion process of detection. Photons in the X-ray beam are transformed by a scintillator to visible light. These secondary photons are absorbed by an array of photodiodes and converted to electrical signals. Thus by integrating the energy of all X-ray photons over a specific timeframe, the conventional CT detector loses the ability to retain the energy information of individual X-ray photons. The individual conventional CT detector cells are divided by optically opaque layers called septa. These are inactive areas on the detector surface causing loss of efficiency [3].

Instead of the two step conversion in conventional CT, photon-counting CT uses direct conversion using a layer of semiconductor. Semiconductor layer is made of cadmium telluride, cadmium zinc telluride, or silicon and has a large area cathode on the upper side. Lower side has pixelated anode electrodes. A high voltage in the range of 800 to 1000 V is applied between the cathode and anodes to create a strong electric field. When X-rays are absorbed by the semiconductor, charges in the form of electron-hole pairs are generated. Hole in semiconductor terminology is an atom from which an electron has moved away. Electrons with negative charge are free to move and are attracted to the positive anode, producing short current pulses. The amplitude of these pulses are proportional to the the energy of the X-ray photon received. This output signal is processed by multiple electronic comparators and counters, making it possible to determine the quantity of the interacting X-ray photons.

The U.S. Food and Drug Administration cleared the first photon-counting detector CT scanner in September 2021 [4]. Elimination of septa between detector elements significantly improves spatial resolution and radiation dose efficiency. Electronic noise is also eliminated to a large extent by photon-counting CT compared to conventional CT technology, leading to clearer images.

Still important is the blooming artifacts on CT coronary angiography in those with coronary stents or severe calcification. Blooming artifacts can lead to overestimation of stenosis and a false positive result. This can be overcome by the new photon counting detectors which enable ECG gated ultrahigh resolution CT coronary angiography. Ultrahigh resolution CT coronary angiography has excellent image quality and vessel sharpness with reduced calcium induced blooming in patients with high amount of coronary calcium. It is also useful in those with implanted coronary stents.

References

1. Halfmann MC, Bockius S, Emrich T, Hell M, Schoepf UJ, Laux GS, Kavermann L, Graafen D, Gori T, Yang Y, Klöckner R, Maurovich-Horvat P, Ricke J, Müller L, Varga-Szemes A, Fink N. Ultrahigh-Spatial-Resolution Photon-counting Detector CT Angiography of Coronary Artery Disease for Stenosis Assessment. Radiology. 2024 Feb;310(2):e231956. doi: 10.1148/radiol.231956. PMID: 38376407.
2. Hagar MT, Soschynski M, Saffar R, Rau A, Taron J, Weiss J, Stein T, Faby S, von Zur Muehlen C, Ruile P, Schlett CL, Bamberg F, Krauss T. Accuracy of Ultrahigh-Resolution Photon-counting CT for Detecting Coronary Artery Disease in a High-Risk Population. Radiology. 2023 Jun;307(5):e223305. doi: 10.1148/radiol.223305. PMID: 37338354.
3. Meloni A, Cademartiri F, Positano V, Celi S, Berti S, Clemente A, La Grutta L, Saba L, Bossone E, Cavaliere C, Punzo B, Maffei E. Cardiovascular Applications of Photon-Counting CT Technology: A Revolutionary New Diagnostic Step. J Cardiovasc Dev Dis. 2023 Aug 25;10(9):363. doi: 10.3390/jcdd10090363. PMID: 37754792; PMCID: PMC10531582.
4. Einstein AJ. Decreasing Unnecessary Invasive Coronary Angiograms With Photon-Counting Detector Coronary Computed Tomography Angiography. J Am Coll Cardiol. 2025 Feb 4;85(4):349-351. doi: 10.1016/j.jacc.2024.10.084. Epub 2024 Oct 28. PMID: 39471881.