Photon-Counting CT, Siemens Healthineers

Photon-counting detector computed tomography (PCD-CT) is an emerging technology and represents the next milestone in CT developments. Compared to conventional energy-integrating detectors in which an indirect conversion technology is used to detect incident photons, PCD technology uses semiconductors that directly convert X-ray photons to an electrical signal. This generated signal includes the energy information of every individually detected photon enabling intrinsic spectral imaging in every CT scan.

The clinical benefits of PCD-CTs are the elimination of classical electronic noise, a higher spatial resolution, reduction of metal artifacts, improved iodine contrast- to-noise ratio (CNR), and improved radiation dose efficiency, Simultaneous multi-energy acquisition at a single X-ray tube potential also permits new ways of advanced data processing. The energy of every transmitted photon is allocated between multiple energy thresholds, called bins, leading to energy- based attenuation profiles of tissue. This allows, for example, for the simultaneous detection of one or more k-edge contrast agents. The specific visualization of exogenous contrast agent enables single-scan multiphase imaging demonstrating a new way of functional imaging.

Technical Specifications

Current medical CT systems use energy- integrating detectors (EID) in which the incident photons are converted into an electrical signal during a two-step detection process. The EID consists of a scintillation crystal attached to a photodiode made of semiconducting material. During the detection process, the incident X-rays first strike the scintillation crystal and generate secondary visible light photons. These are absorbed by the photodiode and are converted into an electrical signal. The intensity of the generated electrical signal depends on the amount of incident photons and is proportional to the total energy deposited during a measurement interval. Based on their detection principle, these detectors are called “energy-integrating detectors” and do not provide energy-resolved signals. The EIDs are separated by thin, optically intransparent collimator blades to prevent optical cross-talk.

The underlying principle in PCD-CT is the use of a semiconductor diode capable of directly converting the incident photons into an electrical signal. Research focused on cadmium telluride (CdTe), cadmium zinc telluride (CdZnTe), and silicon (Si) as semiconductor material. Between the cathode at the top of the thick layer of semiconductor material (1.4–30?mm depending on material) and the pixelated anode at the bottom a strong electric field is applied. During imaging the incident photon is absorbed in the photoconductor and creates an electron-hole pair. The electrons are immediately attracted by the anode and induce short currents of a few nanoseconds (10?9?s). The height of the voltage pulse is directly proportional to the amount of absorbed charge and is counted when exceeding a defined energy threshold level. The strong electrical field between the cathode and the pixelated anode obviates the need for collimator blades and increases geometrical dose efficiency. Moreover, current pulses are counted only when exceeding a preset energy threshold level, set above the electronic noise level, but lower than pulses generated by striking photons reducing electronic noise in the generated images. By defining several threshold levels, PCD can assign the incoming photons to precise energy bins, thus generating energy-based attenuation profiles of tissue.

The NAEOTOM Alpha is the world’s first dual-source PCD-CT system for full clinical use and was installed in April 2021 at the University Hospital Zurich, Switzerland. This system uses cadmium telluride (CdTe) as semiconductor material. The X-ray tubes can be operated at voltages up to 140 kVp, the tube current can be set to values between 10 and 1300?mA and the shortest rotation time of the system is 0.25?s.

In conclusion, PCD-CT will push the boundaries of future CT imaging and opens new, currently unpredictable areas of application

References

Spectral Imaging Dual-Energy, Multi-Energy and Photon-Counting CT

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