Computed Tomography Angiography (CTA) has been an important asset in cardiovascular diagnosis for decades. The continuous improvement of rapid data acquisition with often higher spatial resolution has established the method as indispensable for diagnosing and managing multiple cardiovascular entities such as aneurysms, aortic dissection, and pulmonary embolism.^, The use of CTA in heart and coronary imaging has also gained important territory and relevance.^, Computed Tomography (CT) techniques have been advancing significantly since its introduction in the medical field. Starting with single detectors, then helicoidal scanners, multidetector scanners with high pitch, improvement of reconstruction algorithms, the appearance of dual-energy technology, and lately the ability to acquire images of the whole heart in just one heartbeat with information from the entire cardiac cycle, are incredible milestones. Fractional Flow Reserve CT (FFR-CT) is also a very promising technique. Progression does not come without downsides: ionizing radiation is still concerning, and despite ways to reduce it such as dose modulation, some cardiovascular CTA exams still show relatively high levels of absorbed radiation, especially exams with multiple series, extremely thin slices, and retrospective reconstruction of larger parts of the cardiac cycle. Following this trend of improving image quality with less ionizing radiation as possible, a new complex technique emerged: Photon-counting CT (PCCT).

The main distinction between conventional CT and PCCT is in the detectors of X-rays. The conventional CT detector is based on receiving the X-ray photons that interact with the patient, converting them into light photons (it is an indirect system), and then converting this light into electrical signals that ultimately are going to be converted into digital signals. This system is called an Energy-Integrating Detector (EID). Basically, more light means more X-rays arriving and less interaction / more penetration in the patient’s tissues. It is not possible to differentiate exactly how many X-rays are arriving, nor the different energy levels of each X-ray photon. Moreover, the septa between each of the detectors also limit the reception of X-rays.^, On the other hand, PCCT detectors can detect separately each X-ray photon that is arriving and measure the energy of each one of the X-ray photons directly. This is possible because the PCCT detector has a high-voltage crystal layer that relocates electrons for each X-ray photon received, and the number of electrons relocated is proportional to the energy of each X-ray. The relocated electrons generate the electrical pulse that is going to be finally converted into the digital signal. And even smaller energy X-rays can be detected, improving the image contrast-to-noise ratio. It is a direct system, and there is no septa between the detectors. In the end, less X-rays are necessary (therefore less radiation) and it is possible to separate the X-rays according to the energy of each one, leading to applications of tissue recognition (different known energy X-rays are likely to have interacted with different known tissues) functioning as a dual energy CT.^,

[…]