Photon-counting Imaging Research Articles

Bicolor K-edge spectral photon-counting CT imaging for the diagnosis of thoracic endoleaks: A dynamic phantom study

PurposeThe purpose of this study was to investigate the feasibility of identifying and characterizing the three most common types of endoleaks within a thoracic aorta aneurysm model using bicolor K-edge imaging with a spectral photon-counting computing tomography (SPCCT) system in combination with a biphasic contrast agent injection. Materials and methodsThree types of thoracic endoleaks (type 1, 2 and 3) were created in a dynamic anthropomorphic thoracic aorta phantom. Protocol consisted in an injection of an iodinated contrast material followed 80 seconds after an injection of a gadolinium-based contrast agent (GBCA). The phantom was scanned using a clinical prototype SPCCT during bicolor phase imaging consisting in an early distribution of GBCA and a late distribution of iodine. Conventional and spectral images were reconstructed for differentiating between the contrast agents and measuring their respective attenuation values and concentrations inside and outside the stent graft. ResultsConventional images failed to provide specific dynamic imaging contrast agents in the aneurysmal sac and outside the stent graft while spectral images differentiated their specific distribution. In type 1 and 3 thoracic endoleaks, GBCA concentration was measured outside the stent graft at 6.1 ± 3.7 (standard deviation [SD]) mg/mL and 6.0 ± 4.0 (SD) mg/mL, respectively, in favor of an early blood flow. In type 2 thoracic endoleak, iodine was measured outside the stent graft at 24.3 ± 5.5 (SD) mg/mL in favor of a late blood flow in the aneurysmal sac. ConclusionBicolor K-edge imaging enabled SPCCT allows a bicolor characterization of thoracic aorta endoleaks in a single acquisition in combination with a biphasic contrast agent injection.

Spectrally Selective Time-Resolved Emission through Fourier-Filtering (STEF).

We demonstrate a method for separating and resolving the dynamics of multiple emitters without the use of conventional filters. By directing the photon emission through a fixed path-length imbalanced Mach-Zehnder interferometer, we interferometrically cancel (or enhance) certain spectral signatures corresponding to one emissive species. Our approach, Spectrally selective Time-resolved Emission through Fourier-filtering (STEF), leverages the detection and subtraction of both outputs of a tuned Mach-Zehnder interferometer, which can be combined with time-correlated single photon counting (TCSPC) or confocal imaging to demix multiple emitter signatures. We develop a procedure to calibrate out imperfections in Mach-Zehnder interferometry schemes. Additionally, we demonstrate the range and utility of STEF by performing the following procedures with one measurement: (1) filtering out laser scatter from a sample, (2) separating and measuring a fluorescence lifetime from a binary chromophore mixture with overlapped emission spectra, (3) confocally imaging and separately resolving the standard fluorescent stains in bovine pulmonary endothelial cells and nearly overlapping fluorescent stains on RAW 264.7 cells. This form of spectral balancing can allow for robust and tunable signal sorting.

Characterization of 3.2 Gbps readout in 65 nm CMOS technology

A new class of photon-counting pixel detectors allows for capturing of an image in several photon energy bins in one shot. A decreased pixel pitch and an increased number of energy bins are needed to enhance the spatial and spectral resolution of the detector. This led to new requirements for the readout systems and their bandwidths, as more data is generated for the same detection area. Fast differential serial communication enables high-speed data rates, thus providing an ideal solution to transfer large amounts of data generated by the detector’s front-end electronics. However, its implementation provides extra challenges. This work introduces a novel high-speed serial readout designed in a 65 nm CMOS technology that will be used in the future photon-counting X-ray imaging detectors. The design of the serial transmitter is presented together with the characterization of jitter and channel performance.

LHRH conjugated gold nanoparticles assisted efficient ovarian cancer targeting evaluated via spectral photon-counting CT imaging: a proof-of-concept research.

Emerging multifunctional nanoparticulate formulations take advantage of nano-meter scale size and surface chemistry to work as a therapeutic delivery agent and a diagnostic tool for non-invasive real-time monitoring using imaging technologies. Here, we evaluate the selective uptake of 18 nm and 80 nm sized gold nanoparticles (AuNPs) by SKOV3 (4 times higher) ovarian cancer (OC) cells (compared to OVCAR5) in vitro, quantified by inductively coupled plasma (ICP) and MARS spectral photon-counting CT imaging (MARS SPCCT). Based on in vitro analysis, pristine AuNPs (18 nm) and surface modified AuNPs (18 nm) were chosen as a contrast agent for MARS SPCCT. The chemical analysis by FTIR spectroscopy confirmed the luteinizing hormone-releasing hormone (LHRH) conjugation to the AuNPs surface. For the first time, LHRH conjugated AuNPs were used for in vitro and selective in vivo OC targeting. The ICP-MS analysis confirmed preferential uptake of LHRH modified AuNPs by organs residing in the abdominal cavity with OC nodules (pancreas: 0.46 ng mg-1, mesentery: 0.89 ng mg-1, ovary: 1.43 ng mg-1, and abdominal wall: 2.12 ng mg-1) whereas the MARS SPCCT analysis suggested scattered accumulation of metal around the abdominal cavity. Therefore, the study showed the exciting potential of LHRH conjugated AuNPs to target ovarian cancer and also as a potential contrast agent for novel SPCCT imaging technology.

Time-space united coding spread spectrum single photon counting imaging method

In this paper, we demonstrate a new imaging architecture called time-space united coding spread spectrum single photon counting imaging technique by combining the space coding based single-pixel imaging technology and spread spectrum time coding based scanning imaging technology. This method has the advantages of range ambiguity-free and large time-bandwidth product. Under the interference of noise, this method can accurately restore depth images. In this work, the time-space united correlation nonlinear detection model based on single photon detection, forward imaging model and signal-to-noise ratio model is derived, and the depth image is restored by convex optimization inversion algorithm. The theoretical model and simulation experiments show that compared with the traditional single pixel imaging method based on spatial coding, this method improves the quality of scene reconstruction. Using m-sequence as time coding, imaging has higher noise robustness. In addition, compared with the traditional space coding single pixel imaging technology, the imaging mean square error of the proposed method is reduced by 4/5 and the imaging mean squared error is reduced by 9/10 after introducing the second correlated method. The proposed imaging architecture in this paper may provide a new path for non-scanning lidar imaging methods.

Position linearity analysis of circular arc terminated resistive anode using finite element method for photon-counting imaging detectors.

This study proposes a comprehensive model of the circular arc terminated (CAT) resistive anode based on the finite element method to explore the dynamic process of charge diffusion on this anode and its position linearity performance. The waveforms of charges of the electrodes on the anode are calculated for different electrical parameters and their influence on positional linearity is investigated. The influence of the signal development time and the non-uniformity of the resistance per square of the anode on positional linearity is also analyzed. The results of simulations show that the non-linearity of the image varies monotonically with the termination resistance and the non-uniformity of the resistance per square of the anode, but has a non-linear relationship with the signal development time and the ratio of the resistance per square. A CAT resistive anode with capacitance c and a resistance per square of the sensitive area of R▱ can be used to recover an image with a root mean-squared non-linearity of 2%, when the charge signals of the electrode are collected for at least 0.6R▱c s. The reliability of the results of the simulations was verified with experimental measurements.

Multi-voltage threshold digitizer using a time-varied threshold for photon-counting X-ray security inspection imaging

In this paper, we present a multi-voltage threshold (MVT) digitizer using a time-varied threshold and investigate the feasibility of using this digitizer to develop the data acquisition system for photon-counting X-ray security inspection imaging. This digitizer compares the scintillation pulse with a preset time-varied voltage signal and records the time information of the intersections. As the amplitude of the threshold varies as a function of time, the voltage values of the intersections could be calculated using the recorded time, and the energy information could be extracted via the acquired time–voltage samples. Unlike the constant voltage thresholds MVT digitizer requiring multiple voltage comparators, this method needs only one voltage comparator, which notably reduces the complexity in configuration. We demonstrate the development of such an MVT digitizer using a sine signal threshold and evaluate its energy performance with a yttrium orthosilicate (YSO) scintillator and Silicon Photomultiplier (SiPM) detector (designed for X-ray detection). The digitizer obtains energy resolutions of 41.4%, 26.6%, and 25.5% at 30, 59.5, and 80.9 keV, respectively. For comparison, the YSO/SiPM detector is also evaluated with a high-speed oscilloscope, and measured energy resolutions are 40.6%, 26.5%, and 25.1% at 30, 59.5, and 80.9 keV, respectively. These results indicate that the proposed MVT digitizer using a time-varied threshold is a promising method for developing a YSO/SiPM-based photon-counting X-ray detector.

Development of 64-channel LSI with ultrafast analog and digital signal processing dedicated for photon-counting computed tomography with multi-pixel photon counter

X-ray computed tomography (CT) is non destructive visualization inside the human body. Recently, X-ray photon-counting CT (PC-CT) has drawn attention for reducing the high doses used for patients and acquiring spectral information to identify materials. To enable photon-counting imaging with a wide region (∼60 mm long) of a small animal, we developed a photon-counting system using a 64-channel multi-pixel photon counter (MPPC) array with a fast temporal response (a few nanoseconds) and a very large signal gain (∼106), combined with a 64-channel yttrium–gadolinium–aluminum–gallium garnet scintillator array. In particular, to realize ultra-fast analog and digital signal processing (>10 MHz/channel), we developed a 64-channel large-scale integrated circuit (LSI) named MPPC-CT64. We had previously developed a 16-channel PC-CT system with a 16-channel LSI (MPPC-CT16). Although the MPPC-CT16 realized photon-counting imaging for a ∼16-mm-long phantom, there were some energy uncertainties in the LSI, which degraded the obtained CT image quality. The MPPC-CT64 implements a function for correcting the threshold energies and also increases the number of energy thresholds from four to six, which provides more precise measurements of CT values dependent on X-ray energy. In this paper, we briefly present the electric architecture and performance evaluation of the LSI combined with MPPCs.

Development of an MPPC-based gamma-ray detector onboard a radiation source imager under high-dose environments and initial performance results

Since the accident at the Fukushima Daiichi Nuclear Power Plant ∼10 years ago, various studies have been conducted focusing on the decontamination of radioactive materials and the decommissioning of the remains of the reactor. It is crucial to identify the locations of radioactive materials for the decontamination of a building that is exposed to high radiation doses. One of the most widely used methods is the visualization of gamma rays with energy information using a photon-counting imager. However, the existing imagers have limited functionality, making it difficult to implement them in such an environment. Therefore, we have developed a novel pinhole gamma imager containing multi-pixel photon counters (MPPCs) combined with fast scintillators, which are processed using fast signal-processing analog and digital large integrated circuits under high-dose environments. The two-dimensional sensor array developed in this study can obtain incident gamma-ray photon energies with a counting rate of a few MHz/mm2. Furthermore, we were able to perform two-dimensional gamma-ray imaging of an extremely strong radiation source of 60Co with ∼45 TBq, by combining the sensor array with the dedicated tungsten collimator and housing.

Efficient and accurate conversion-gain estimation of a photon-counting image sensor based on the maximum likelihood estimation.

We establish a method for estimating conversion gains of image sensors on the basis of a maximum likelihood estimation, one of the most common and well-established statistical approaches. A numerical simulation indicates the proposed method can evaluate the conversion gain more accurately with less data accumulation than known approaches. We also applied this method to experimental images accumulated under a photon-counting-regime illumination condition by a CMOS image sensor that can distinguish how many photoelectrons are generated in each pixel. Resultantly, the conversion gains were determined with an accuracy of three digits from 1000 observed images, whose number is at most 10 times smaller than that required for achieving a similar accuracy by known gain-estimation methods.

Physical design of photon-counting mode γ-ray large object radiation imaging system.

The detectors of existing large object radiation imaging systems generally work under current-integration mode and cannot distinguish effective signals of unreacted photons from interfering signals of electronic noise and scattered photons, therefore, resulting in image quality deterioration. This study aims to design a new photon-counting mode γ-ray large object radiation imaging system. Therefore, interfering signals with lower energy than effective signals can be eliminated by energy analysis. In addition, the system enables to work properly even under 30∼300Ci Co-60 intensity. Based on the physical analysis of the system, the design requirements are listed. Following the requirements, the best-performing photon-counting detector based on LYSO and SiPM is used in the system. ZP-SK and (ZP)2-SK filter circuits are designed for Co-60 radiation imaging system with the highest intensity of 100Ci and 300Ci, respectively. Then, a voltage comparator and an FPGA are followed to realize the function of energy analysis and photon counting. The proposed technical solution can improve the Steel Penetration (SP) by at least 60∼70 mmFe compared with the existing current-integration system, which is equivalent to the improvement obtained by increasing the intensity of the radioactive source more than 13 to 20 times. This study demonstrates the advantages of applying the new photon-counting mode γ-ray large object radiation imaging system to improve the radiation image quality and the penetration ability, which will have enormous potential for future applications.

Enhancement of three-dimensional image visualization under photon-starved conditions.

In this paper, we propose enhancement of three-dimensional (3D) image visualization under photon-starved conditions using preprocessing such as contrast-limited adaptive histogram equalization (CLAHE) and histogram matching. In conventional imaging techniques, photon-counting integral imaging can be utilized for 3D visualization. However, due to a lack of photons, it is challenging to enhance the visual quality of 3D images under severely photon-starved conditions. To improve the visual quality and accuracy of 3D images under these conditions, in this paper, we apply CLAHE and histogram matching to a scene before photon-counting integral imaging is used. To prove the feasibility of our proposed method, we implement the optical experiment and show the performance metric such as peak sidelobe ratio.

Significance of the spectral correction of photon counting detector response in material classification from spectral x-ray CT.

Purpose: Photon counting imaging detectors (PCD) has paved the way for spectral x-ray computed tomography (spectral CT), which simultaneously measures a sample's linear attenuation coefficient (LAC) at multiple energies. However, cadmium telluride (CdTe)-based PCDs working under high flux suffer from detector effects, such as charge sharing and photon pileup. These effects result in the severe spectral distortions of the measured spectra and significant deviation of the extracted LACs from the reference attenuation curve. We analyze the influence of the spectral distortion correction on material classification performance. Approach: We employ a spectral correction algorithm to reduce the primary spectral distortions. We use a method for material classification that measures system-independent material properties, such as electron density, , and effective atomic number, . These parameters are extracted from the LACs using attenuation decomposition and are independent of the scanner specification. The classification performance with the raw and corrected data is tested on different numbers of energy bins and projections and different radiation dose levels. We use experimental data with a broad range of materials in the range of , acquired with a custom laboratory instrument for spectral CT. Results: We show that using the spectral correction leads to an accuracy increase of 1.6 and 3.8 times in estimating and , respectively, when the image reconstruction is performed from only 12 projections and the 15 energy bins approach is used. Conclusions: The correction algorithm accurately reconstructs the measured attenuation curve and thus gives better classification performance.

Photon-Counting Detector CT Virtual Monoengergetic Images for Cochlear Implant Visualization-A Head to Head Comparison to Energy-Integrating Detector CT.

Cochlear implants (CIs) are the primary treatment method in patients with profound sensorineural hearing loss. Interpretation of postoperative imaging with conventional energy-integrating detector computed tomography (EID-CT) following CI surgery remains challenging due to metal artifacts. Still, the photon-counting detector (PCD-CT) is a new emerging technology with the potential to eliminate these problems. This study evaluated the performance of virtual monoenergetic (VME) EID-CT images versus PCD-CT in CI imaging. In this cadaveric study, two temporal bone specimens with implanted CIs were scanned with EID-CT and PCD-CT. The images were assessed according to the visibility of interelectrode wire, size of electrode contact, and diameter of halo artifacts. The visibility of interelectrode wire sections was significantly higher when reviewing PCD-CT images. The difference in diameter measurements for electrode contacts between the two CT scanner modalities showed that the PCD-CT technology generally led to significantly larger diameter readings. The larger measurements were closer to the manufacturer’s specifications for the CI electrode. The size of halo artifacts surrounding the electrode contacts did not differ significantly between the two imaging modalities. PCT-CT imaging is a promising technology for CI imaging with improved spatial resolution and better visibility of small structures than conventional EID-CT.

Crystalline Selenium Layer Stacked Image Sensors With Pixel-Parallel A/D Converters Toward Photon Counting

We report crystalline selenium (c-Se) photo-conversion layer stacked image sensors with pixel-parallel A/D converters (ADCs) toward photon counting. The sensor separates photo-conversion and multiplication function from complementary metal–oxide–semiconductor (CMOS) circuits, which has advantages in terms of high sensitivity and high spatial resolution of the sensors. The sensor also has a wide dynamic range for input illuminance because the pixel-parallel 1-bit ADCs prevent saturation of the floating diffusion (FD). The circuits require no quenching operation, which is suitable for high-speed operation. The c-Se layer is directly stacked onto the CMOS sensor surface, which has been flattened by chemical mechanical polishing, and connected to each pixel with Au electrodes. A photo-generated electron charge can be multiplied in the c-Se layers, and introduced to 1-bit ADC, where the charges are converted into pulse signals to be counted by in-pixel counters. Linear output characteristics are confirmed with a wide dynamic range for illuminance of 96 dB. Video images of 128 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times96$ </tex-math></inline-formula> pixels are successfully demonstrated including avalanche multiplication mode in the supply voltage of more than 10 V, which exhibits the feasibility of high spatial resolution and high-speed image sensing applicable to photon counting and wide dynamic range imaging.

Review of Quanta Image Sensors for Ultralow-Light Imaging

The quanta image sensor (QIS) is a photon-counting image sensor that has been implemented using different electron devices, including impact ionization-gain devices, such as the single-photon avalanche detectors (SPADs), and low-capacitance, high conversion-gain devices, such as modified CMOS image sensors (CIS) with deep subelectron read noise and/or low noise readout signal chains. This article primarily focuses on CIS QIS, but recent progress of both types is addressed. Signal processing progress, such as denoising, critical to improving apparent signal-to-noise ratio, is also reviewed as an enabling coinnovation.

2-D Photon-Detection-Probability Simulation and a Novel Guard-Ring Design for Small CMOS Single-Photon Avalanche Diodes

CMOS single-photon avalanche diode (SPAD) array is a high-timing-resolution photon-counting image sensor. Pixel size shrinkage for high image resolution is highly desirable but hindered by the lowered photon-detection-probability (PDP) as the edge area becomes significant. We propose and demonstrate a parameter-free method for simulating PDP with the edge effect. Considering the doping profile, electric field, photon generation, and trigger probability distributions in two dimensions, the PDP reduction at the device edge is analyzed in a quantitative way. Besides, a novel guard-ring design for enhancing PDP of small SPAD has been proposed and simulated by our method. A threefold enhancement was obtained with the newly designed guard-ring structure compared with the original one. Our work is useful for developing small-pitch SPAD array for high-resolution imaging applications.

A New Frontier in Temporal Bone Imaging: Photon-Counting Detector CT Demonstrates Superior Visualization of Critical Anatomic Structures at Reduced Radiation Dose.

Photon-counting detector CT is a new technology with a limiting spatial resolution of ≤150 μm. In vivo comparisons between photon-counting detector CT and conventional energy-integrating detector CT are needed to determine the clinical impact of photon counting-detector CT in temporal bone imaging. Prospectively recruited patients underwent temporal bone CT examinations on an investigational photon-counting detector CT system after clinically indicated temporal bone energy-integrating detector CT. Photon-counting detector CT images were obtained at an average 31% lower dose compared with those obtained on the energy-integrating detector CT scanner. Reconstructed images were evaluated in axial, coronal, and Pöschl planes using the smallest available section thickness on each system (0.4 mm on energy-integrating detector CT; 0.2 mm on photon-counting detector CT). Two blinded neuroradiologists compared images side-by-side and scored them using a 5-point Likert scale. A post hoc reassignment of readers' scores was performed so that the scores reflected photon-counting detector CT performance relative to energy-integrating detector CT. Thirteen patients were enrolled, resulting in 26 image sets (left and right sides). The average patient age was 63.6 [SD, 13.4] years; 7 were women. Images from the photon-counting detector CT scanner were significantly preferred by the readers in all reconstructed planes (P < .001). Photon-counting detector CT was rated superior for the evaluation of all individual anatomic structures, with the oval window (4.79) and incudostapedial joint (4.75) receiving the highest scores on a Likert scale of 1-5. Temporal bone CT images obtained on a photon-counting detector CT scanner were rated as having superior spatial resolution and better critical structure visualization than those obtained on a conventional energy-integrating detector scanner, even with a substantial dose reduction.

Spatially modulated scene illumination for intensity-compensated two-dimensional array photon-counting LiDAR imaging

Photon-counting LiDAR using a two-dimensional (2D) array detector has the advantages of high lateral resolution and fast acquisition speed. The non-uniform intensity profile of the illumination beam and non-uniform quantum efficiency of the detectors in the 2D array deteriorate the imaging quality. Herein, we propose a photon-counting LiDAR system that uses a spatial light modulator to control the spatial intensity to compensate for both the non-uniform intensity profile of the illumination beam, and the variation in the quantum efficiency of the detectors in the 2D array. By using a 635 nm peak wavelength and 4 mW average power semiconductor laser, lab-based experiments at a 4.27 m stand-off distance are performed to verify the effectiveness of the proposed method. Compared with the unmodulated method, the standard deviation of the intensity image of the proposed method is reduced from 0.109 to 0.089 for a whiteboard target, with an average signal photon number of 0.006 per pixel.

Multi-channel pseudo-random coding single-photon ranging and imaging

We demonstrate a multi-channel pseudo-random coding single-photon ranging system. A pseudo-random multiplexing technique is proposed, which realizes multi-channel pseudo-random ranging only by using one single-photon detector and processing circuit. Compared with the time division multiplexing technique, it will not reduce the maximum unambiguous range while increasing the number of the ranging channel. Eight-channel pseudo-random coding single-photon ranging was realized with the ranging accuracy better than 2 cm. Moreover, photon counting imaging was realized through scanning the laser beams of the eight-channel pseudo-random ranging system. There is no crosstalk between channels, which is suitable for multi-beam long-distance single-photon Lidar.