Photon Counting Applications
Hollie Kipps
This book is an attempt to bridge the gap between the instrumental principles of multi-dimensional time-correlated single photon counting (TCSPC) and typical applications of the technique. Written by an originator of the technique and by sucessful users, it covers the basic principles of the technique, its interaction with optical imaging methods and its application to a wide range of experimental tasks in life sciences and clinical research.
The book is recommended for all users of time-resolved detection techniques in biology, bio-chemistry, spectroscopy of live systems, live cell microscopy, clinical imaging, spectroscopy of single molecules, and other applications that require the detection of low-level light signals at single-photon sensitivity and picosecond time resolution.
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The use of a photon counting detector in CT (PCD CT) is currently the subject of intense investigation and development. In this review article, we will describe potential clinical applications of this technology with a particular focus on the experience of our own institution with a prototype PCD CT scanner. PCDs have three primary advantages over conventional, energy integrating detectors (EIDs): they provide spectral information without need for a dedicated dual energy protocol; they are immune to electronic noise; and they can be made very high resolution without significant compromises to quantum efficiency. These advantages translate into several clinical applications. Metal artifacts, beam hardening artifacts, and noise streaks from photon starvation can be better mitigated using PCD CT. Certain incidental findings can be better characterized using the spectral information from PCD CT. High-contrast, high-resolution structures such as the temporal bone can be better visualized using PCD CT and at greatly reduced dose. We also discuss new possibilities on the horizon, including new contrast agents, and how anticipated improvements in PCD CT will translate to performance in these applications.
CT systems equipped with photon-counting detectors (PCDs), referred to as photon-counting CT (PCCT), are beginning to change imaging in several subspecialties, such as cardiac, vascular, thoracic, and musculoskeletal radiology. Evidence has been building in the literature underpinning the many advantages of PCCT for different clinical applications. These benefits derive from the distinct features of PCDs, which are made of semiconductor materials capable of converting photons directly into electric signal. PCCT advancements include, among the most important, improved spatial resolution, noise reduction, and spectral properties. PCCT spatial resolution on the order of 0.25 mm allows for the improved visualization of small structures (eg, small vessels, arterial walls, distal bronchi, and bone trabeculations) and their pathologies, as well as the identification of previously undetectable anomalies. In addition, blooming artifacts from calcifications, stents, and other dense structures are reduced. The benefits of the spectral capabilities of PCCT are broad and include reducing radiation and contrast material dose for patients. In addition, multiple types of information can be extracted from a single data set (ie, multiparametric imaging), including quantitative data often regarded as surrogates of functional information (eg, lung perfusion). PCCT also allows for a novel type of CT imaging, K-edge imaging. This technique, combined with new contrast materials specifically designed for this modality, opens the door to new applications for imaging in the future.
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Feature papers represent the most advanced research with significant potential for high impact in the field. A Feature Paper should be a substantial original Article that involves several techniques or approaches, provides an outlook for future research directions and describes possible research applications.
Abstract: Photon-counting computed tomography (PCCT) is an emerging technology that can potentially transform clinical CT imaging. After a brief description of the PCCT technology, this review summarizes its main advantages over conventional CT: improved spatial resolution, improved signal and contrast behavior, reduced electronic noise and artifacts, decreased radiation dose, and multi-energy capability with improved material discrimination. Moreover, by providing an overview of the existing literature, this review highlights how the PCCT benefits have been harnessed to enhance and broaden the diagnostic capabilities of CT for cardiovascular applications, including the detection of coronary artery calcifications, evaluation of coronary plaque extent and composition, evaluation of coronary stents, and assessment of myocardial tissue characteristics and perfusion. Keywords: photon-counting detectors; computed tomography angiography; heart; coronary arteries
Meloni, Antonella, Filippo Cademartiri, Vicenzo Positano, Simona Celi, Sergio Berti, Alberto Clemente, Ludovico La Grutta, Luca Saba, Eduardo Bossone, Carlo Cavaliere, and et al. 2023. "Cardiovascular Applications of Photon-Counting CT Technology: A Revolutionary New Diagnostic Step" Journal of Cardiovascular Development and Disease 10, no. 9: 363.
Representation of photon flux and detector output in case of high light intensity (a) and of low light intensity (b) and representation of the time-correlated single photon counting (TCSPC) technique (c) for the reconstruction of low light level periodic light signals.
Representation of two photon absorption cases in the depleted region and in the neutral region beneath it (a) and examples of the relative timing histograms (b). At λ = 400 nm, all the photons are absorbed in the depleted region, whereas at 850 nm, they are absorbed mostly in the neutral region, creating the tail.
Example of 32 32 SPAD array, where each pixel contains a SPAD, the quenching circuitry, counters, and the time-to-digital converter (TDC) [27] (a). Example of possible SPAD CMOS implementation with read-out electronics isolated inside the deep p-well (b) [28]. Example of analog time-gated SPAD pixel, with a reduced electronics-complexity to obtain a higher FF (c) [5].
Example of silicon photomultiplier chip (SiPM), with back contact and common top PAD (a). Example of SiPM signal, acquired with oscilloscope in persistence mode (b). Typical circuit for the readout of a SiPM, with trans-impedance amplifier (c) with the SiPM equivalent circuit, composed by quenching resistor (RQ), quenching capacitance (CQ), i.e., parasitic capacitance of the quenching resistor through the SPAD, and the metal grid equivalent capacitance (CGRID). Example of digital SiPM with TDC per each subpixel [38] (d) and schematic of dSiPM with one global TDC (e).
Picture of a test SiPM with several scintillator crystals (to be mounted on the top of it) (a). Example of setup for the measurement of coincidence resolving time (CRT), with two SiPMs with crystals detecting two coincident gamma rays (b). Example of a SiPM TILE with 6 6 element of 4 4 mm2 SiPMs (c).
SEM image of 10-μm cell ultra-high density SIPM (a), showing active areas, metal and polysilicon resistors. Nominal FF of UHD, HD and non-HD technologies from FBK (b). Typical single-cell signals of UHD SiPMs (c).
With a timing resolution of only 40 ps and a dead time of 45 ns, this module outperforms existing commercial detectors in all applications requiring single photon detection with high timing accuracy. The ID100 has excellent timing stability up to count rates of 20 MHz.
The PDM series photon counting detector modules are all solid-state instruments. They have a photon detection efficiency of 49% at 550nm and generate a TTL output pulse per detected photon. With fast-timing option (additional circuit board installed) they provide better than 50ps FWHM photon timing resolution.
The excellent photon detection efficiency and superior timing resolution is obtained through the use of epitaxial silicon Single Photon Avalanche Diodes (SPAD) and Active Quenching Circuits (AQC), specifically designed and optimized for photon counting applications.
6. Photon Counting CT: Clinical Applications and Future Developments
ISCT: Just how great is photon-counting CT?
Photon-counting CT: Scouting for Quantitative Imaging Biomarkers
MDPI: Image-Quality Assessment of Polyenergetic and Virtual Monoenergetic Reconstructions of Unenhanced CT Scans of the Head
8. Prof Philippe Douek, MD, PhD, Professor at Hospices Civils de Lyon (PDF) Spectral Photon Counting CT European Project
Photon Counting Clinical Images courtesy of Lyon, Hospices Civils de Lyon
A time and frequency analyzer is a versatile instrument that can accurately measure time intervals between events. These events are typically time-varying voltage signals or pulses, and the instrument starts or stops recording an event when the input voltage reaches a given threshold.
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