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Evaluation of the effect of image noise on CT perfusion measurements using digital perfusion phantoms
摘要： Objectives To assess the influence of image noise on computed tomography (CT) perfusion studies, CT perfusion software algorithms were evaluated for susceptibility to image noise and results applied to clinical perfusion studies. Methods Digital perfusion phantoms were generated using a published deconvolution model to create time-attenuation curves (TACs) for 16 different combinations of blood flow (BF; 30/60/90/120 ml/100 ml/min) and flow extraction product (FEP; 10/20/30/40 ml/100 ml/min) corresponding to values encountered in clinical studies. TACs were distorted with Gaussian noise at 50 different strengths to approximate image noise, performing 200 repetitions for each noise level. A total of 160,000 TACs were evaluated by measuring BF and FEP with CT perfusion software, comparing results for the maximum slope and Patlak models with those obtained with a deconvolution model. To translate results to clinical practice, data of 23 patients from a CT perfusion study were assessed for image noise, and the accuracy of reported CT perfusion measurements was estimated. Results Perfusion measurements depend on image noise as means and standard deviations of BF and FEP over repetitions increase with increasing image noise, especially for low BF and FEP values. BF measurements derived by deconvolution show larger standard deviations than those performed with the maximum slope model. Image noise in the evaluated CT perfusion study was 26.46 ± 3.52 HU, indicating possible overestimation of BF by up to 85% in a clinical setting. Conclusions Measurements of perfusion parameters depend heavily upon the magnitude of image noise, which has to be taken into account during selection of acquisition parameters and interpretation of results, e.g., as a quantitative imaging biomarker.
关键词： Tomography, x-ray computed,Phantoms, imaging,Perfusion imaging,Software,Artifacts
更新于2025-09-23 15:23:52
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Artifacts in Transient Absorption Measurements of Perovskite Films Induced by Transient Reflection from Morphological Microstructures
摘要： Organolead halide perovskites MAPbX3 (MA = CH3NH3+; X = Cl?, Br?, I?) have attracted broad tremendous interest in the past 10 years for applications in solar cells and light-emitting devices. In evaluating the quality of the perovskite materials, spectroscopic characterizations such as static and time-resolved absorption and photoluminescence measurements are essential to examine their photophysical properties. A recent report found that the correct measurement of static absorption spectra of MAPbX3 films is indeed difficult due to the strong light scattering caused by their poor surface coverage or complex microstructures. These morphological complexities seem to be inevitable in thin-film fabrication and should not only affect the steady-state spectroscopic measurements but also can significantly impact the time-resolved spectroscopic characterizations, whose results are crucial for understanding photoinduced carrier dynamics in the examined materials. Photoexcited states in semiconductor materials induce changes in the real and imaginary parts of the dielectric function. This leads to changes in absorption (imaginary part) and reflectivity (real part), which can be substantial for materials with significant values of refractive index such as lead halide perovskites. Transient absorption (TA) spectroscopy is a typical technique that has been broadly used to probe photoexcited state dynamics in perovskites and other semiconductor materials. In TA measurements, a pump laser pulse is used to excite the perovskite films, and the induced absorption changes (ΔA) are recorded as a function of both wavelength and time. With the transmitted light as the probe (Figure 1a), the TA signal (ΔA) is mainly decided by the ratio of the intensity of transmitted probe light with and without pump excitation (see eq S1 in the SI), assuming that the loss of transmitted probe light completely results from the sample absorption. On the basis of the same experimental setup, transient reflection (TR) measurements can also be carried out by using the reflected probe light as detection signal (Figure 1b). The TR signal (ΔR/R) can also be determined by the ratio of the intensity of reflected probe light with and without pump excitation (see eq S4 in the SI). Unlike the TA measurements that mainly probe the bulk property of samples, the TR signal mainly detects the photoinduced reflection variations due to the refractive index change at the sample surface. Therefore, the TR spectrum and kinetics can be significantly different from those of TA even in the same sample. For example, previous TA and TR measurements have found dramatically faster carrier recombination kinetics on the surface than in the bulk of MAPbX3 perovskite films or single crystals because of the presence of more surface defects. There is an abnormal case in the regular TA measurements particularly when performed on the films with large and heterogeneous microstructures (e.g., films with poor coverage, large grains, and pinholes) because the loss of transmitted probe light in their TA measurements likely results not only from the sample absorption but also from the reflection of the film surface or the boundary of microstructures in samples. In this case, the measured transient spectrum, though collected in the transmittance mode as in TA, can contain contributions from both TA and TR signals (see Figure 1c and eq S6 in the SI). This could lead to distorted TA spectra and thus inaccurate analysis of photoinduced kinetics. A solution-processed organic or inorganic halide perovskite thin film is a typical material whose morphological microstructures were found to have significant impact on device performance. Although the photoinduced carrier dynamics in perovskite films has been extensively studied using TA spectroscopy, the possible artifacts in TA results induced by TR signal originating from the photoinduced reflectivity variation of film surfaces and microstructures have been overlooked. Herein, in order to clarify the influence of TR signal in the regular TA measurements, we performed a careful transient spectroscopic analysis on a series of MAPbBr3 perovskite films with different microstructure morphology. Meanwhile, TR measurements on MAPbBr3 single crystals (SCs) were carried out for comparison. We confirmed that the TA spectra measured in MAPbBr3 perovskite films with large and heterogeneous microstructures do comprise non-negligible TR signals from the photoinduced reflection of microstructures, with the weight of contribution increased from ～20 to ～100% as the size of the microstructure increased from <200 nm to 1?2 μm. The presence of TR signal leads to an “artifact” feature in the TA spectra and faster observed kinetics owing to the faster surface carrier recombination, which will thus mislead the analysis of bulk carrier dynamics. We also provided a method to reduce the TR signal in actual TA measurements by adding solvent with its refractive index close to the samples, by which the TR distortion can be suppressed to some extent.
关键词： artifacts,transient reflection,transient absorption,carrier dynamics,perovskite films,microstructures
更新于2025-09-23 15:23:52
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Multiple limited-angles computed tomography reconstruction based on multi-direction total variation minimization
摘要： Accurate computed tomography (CT) reconstruction from incomplete projections is an important research topic. Sparse sampling and limited-angle sampling are two effective ways to reduce the X-ray radiation dose or the scanning time. However, it is technically complicated to realize sparse sampling in medical CT since the tube power or the pre-patient collimator is difficult to be switched frequently. Limited-angle sampling makes it difficult to reconstruct an accurate image. The developed multiple limited-angles (MLA) sampling scheme could well balance the technical implementation complexity and the CT reconstruction difficulty. It does not require frequent switching of the tube power or the pre-patient collimator. The data correlation of the acquired projections is lower than that in limited-angle sampling. Using the projections acquired by MLA sampling, CT images reconstructed by the total variation minimization (TVM) method suffer from shading artifacts. Because the artifacts are distributed in several fixed directions, the artifact-suppression reconstruction model based on multi-direction total variation was designed for MLA CT reconstruction in this work. The multi-direction total variation minimization (MDTVM) was utilized to solve the optimization model. Experiments on digital phantoms and real projections indicated that MDTVM can better suppress the shading artifacts than TVM.
关键词： image reconstruction,total variation minimization,computed tomography,limited-angle sampling,artifacts suppression
更新于2025-09-23 15:23:52
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[IEEE 2018 IEEE International Conference on Imaging Systems and Techniques (IST) - Krakow, Poland (2018.10.16-2018.10.18)] 2018 IEEE International Conference on Imaging Systems and Techniques (IST) - A Temporal Tone Mapping Adaptation: An Application using Logarithmic Camera
摘要： Noise, ?icker, temporal inconsistencies disturb the tone mapping creation for High Dynamic Range image (HDR). A method to alleviate these artifacts in tone mapped HDR video sequences is presented in this paper. A temporal ?ltering is developed in order to minimize undesirable artifacts in the low dynamic range image reconstruction process. In the experiments, tone mapping is applied on video sequences obtained by a logarithmic camera. This temporal process provides visual comfort without any kind of blink and prevents Low Dynamic Range (LDR) videos to be disturbed by some random brightness values of isolated pixels.
关键词： logarithmic sensor,temporal artifacts,high dynamic range video,tone mapping
更新于2025-09-23 15:22:29
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Comparison and Noise Suppression of the Transmitted and Reflected Photoplethysmography Signals
摘要： The photoplethysmography (PPG) is inevitably corrupted by many kinds of noise no matter whether its acquisition mode is transmittance or reflectance. To enhance the quality of PPG signals, many studies have made great progress in PPG denoising by adding extra sensors and developing complex algorithms. Considering the reasonable cost, compact size, and real-time and easy implementation, this study proposed a simple real-time denoising method based on double median filters which can be integrated in microcontroller of commercial or portable pulse oximeters without adding extra hardware. First, we used the boundary extension to preserve the signal boundary distortion and designed a first median filter with the time window at approximately 78 ms to eliminate the high-frequency components of the signal. Then, through the second median filter with a time window which was about 780 ms, we estimated the low-frequency components. Finally, we removed the estimated low-frequency components from the signal to obtain the denoised signal. Through comparing the multiple sets of signals under calmly sitting and slightly moving postures, the PPG signals contained noises no matter whether collected by the transmittance-mode or the reflectance-mode. To evaluate the proposed method, we conducted measured, simulated experiments and a strong noisy environment experiment. Through comparing the morphology distortions, frequency spectra, and the signal-to-noise ratios (SNRs), the results showed that the proposed method can suppress noise effectively and preserve the essential morphological features from PPG signals. As a result, the proposed method can enhance the quality of PPG signals and, thus, can contribute to the improvement of the calculation accuracy of the subsequent physiological parameters. In addition, the proposed method could be a good choice to address the real-time noise reduction of portable PPG measuring instruments.
关键词： median filters,photoplethysmography,PPG,signal-to-noise ratio,motion artifacts,denoising
更新于2025-09-23 15:21:01
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Automatic Flare Spot Artifact Detection and Removal in Photographs
摘要： Flare spot is one type of ?are artifact caused by a number of conditions, frequently provoked by one or more high-luminance sources within or close to the camera ?eld of view. When light rays coming from a high-luminance source reach the front element of a camera, it can produce intra-re?ections within camera elements that emerge at the ?lm plane forming non-image information or ?are on the captured image. Even though preventive mechanisms are used, artifacts can appear. In this paper, we propose a robust computational method to automatically detect and remove ?are spot artifacts. Our contribution is threefold: ?rstly, we propose a characterization which is based on intrinsic properties that a ?are spot is likely to satisfy; secondly, we de?ne a new con?dence measure able to select ?are spots among the candidates; and, ?nally, a method to accurately determine the ?are region is given. Then, the detected artifacts are removed by using exemplar-based inpainting. We show that our algorithm achieves top-tier quantitative and qualitative performance.
关键词： Mathematical morphology,Inpainting,Flare artifacts,Flare spot,Con?dence measure
更新于2025-09-23 15:21:01
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[IEEE 2019 IEEE 46th Photovoltaic Specialists Conference (PVSC) - Chicago, IL, USA (2019.6.16-2019.6.21)] 2019 IEEE 46th Photovoltaic Specialists Conference (PVSC) - Preliminary Study on Super Radiation-Resistant Mechanical-Stack Triple-Junction Space Solar Cell: PHOENIX
摘要： Stained glass windows are designed to reveal their powerful artistry under diverse and time-varying lighting conditions; virtual relighting of stained glass, therefore, represents an exceptional tool for the appreciation of this age old art form. However, as opposed to most other artifacts, stained glass windows are extremely difficult if not impossible to analyze using controlled illumination because of their size and position. In this paper, we present novel methods built upon image based priors to perform virtual relighting of stained glass artwork by acquiring the actual light transport properties of a given artifact. In a preprocessing step, we build a material-dependent dictionary for light transport by studying the scattering properties of glass samples in a laboratory setup. We can now use the dictionary to recover a light transport matrix in two ways: under controlled illuminations the dictionary constitutes a sparsifying basis for a compressive sensing acquisition, while in the case of uncontrolled illuminations the dictionary is used to perform sparse regularization. The proposed basis preserves volume impurities and we show that the retrieved light transport matrix is heterogeneous, as in the case of real world objects. We present the rendering results of several stained glass artifacts, including the Rose Window of the Cathedral of Lausanne, digitized using the presented methods.
关键词： light transport,recovery,dictionary learning,Banded matrices,stained glass,sparse cultural artifacts,computational relighting
更新于2025-09-23 15:19:57
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Effects of variation in background mixing ratios of N<sub>2</sub>, O<sub>2</sub>, and Ar on the measurement of <i>δ</i><sup>18</sup>O–H<sub>2</sub>O and <i>δ</i><sup>2</sup>H–H<sub>2</sub>O values by cavity ring-down spectroscopy
摘要： Cavity ring-down spectrometers have generally been designed to operate under conditions in which the background gas has a constant composition. However, there are a number of observational and experimental situations of interest in which the background gas has a variable composition. In this study, we examine the effect of background gas composition on a cavity ring-down spectrometer that measures δ18O–H2O and δ2H–H2O values based on the amplitude of water isotopologue absorption features around 7184 cm?1 (L2120-i, Picarro, Inc.). For background mixtures balanced with N2, the apparent δ18O values deviate from true values by ?0.50 ± 0.001 ‰ O2 %?1 and ?0.57 ± 0.001 ‰ Ar %?1, and apparent δ2H values deviate from true values by 0.26 ± 0.004 ‰ O2 %?1 and 0.42 ± 0.004 ‰ Ar %?1. The artifacts are the result of broadening, narrowing, and shifting of both the target absorption lines and strong neighboring lines. While the background-induced isotopic artifacts can largely be corrected with simple empirical or semi-mechanistic models, neither type of model is capable of completely correcting the isotopic artifacts to within the inherent instrument precision. The development of strategies for dynamically detecting and accommodating background variation in N2, O2, and/or Ar would facilitate the application of cavity ring-down spectrometers to a new class of observations and experiments.
关键词： background gas composition,correction models,Cavity ring-down spectroscopy,water isotopologues,isotopic artifacts
更新于2025-09-23 15:19:57
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[IEEE 2019 Photonics & Electromagnetics Research Symposium - Fall (PIERS - Fall) - Xiamen, China (2019.12.17-2019.12.20)] 2019 Photonics & Electromagnetics Research Symposium - Fall (PIERS - Fall) - Coordinate-free Formulation and Evaluation of Tensor Greena??s Functions for General Homogeneous Uniaxial Anisotropic Media
摘要： Stained glass windows are designed to reveal their powerful artistry under diverse and time-varying lighting conditions; virtual relighting of stained glass, therefore, represents an exceptional tool for the appreciation of this age old art form. However, as opposed to most other artifacts, stained glass windows are extremely difficult if not impossible to analyze using controlled illumination because of their size and position. In this paper, we present novel methods built upon image based priors to perform virtual relighting of stained glass artwork by acquiring the actual light transport properties of a given artifact. In a preprocessing step, we build a material-dependent dictionary for light transport by studying the scattering properties of glass samples in a laboratory setup. We can now use the dictionary to recover a light transport matrix in two ways: under controlled illuminations the dictionary constitutes a sparsifying basis for a compressive sensing acquisition, while in the case of uncontrolled illuminations the dictionary is used to perform sparse regularization. The proposed basis preserves volume impurities and we show that the retrieved light transport matrix is heterogeneous, as in the case of real world objects. We present the rendering results of several stained glass artifacts, including the Rose Window of the Cathedral of Lausanne, digitized using the presented methods.
关键词： light transport,recovery,dictionary learning,Banded matrices,stained glass,sparse cultural artifacts,computational relighting
更新于2025-09-19 17:13:59
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[IEEE 2019 IEEE 8th International Conference on Advanced Optoelectronics and Lasers (CAOL) - Sozopol, Bulgaria (2019.9.6-2019.9.8)] 2019 IEEE 8th International Conference on Advanced Optoelectronics and Lasers (CAOL) - Nanocomposites Polymer/Gold Nanoparticles/Chlorin e6 for Antitumor Laser Medicine
摘要： Stained glass windows are designed to reveal their powerful artistry under diverse and time-varying lighting conditions; virtual relighting of stained glass, therefore, represents an exceptional tool for the appreciation of this age old art form. However, as opposed to most other artifacts, stained glass windows are extremely difficult if not impossible to analyze using controlled illumination because of their size and position. In this paper, we present novel methods built upon image based priors to perform virtual relighting of stained glass artwork by acquiring the actual light transport properties of a given artifact. In a preprocessing step, we build a material-dependent dictionary for light transport by studying the scattering properties of glass samples in a laboratory setup. We can now use the dictionary to recover a light transport matrix in two ways: under controlled illuminations the dictionary constitutes a sparsifying basis for a compressive sensing acquisition, while in the case of uncontrolled illuminations the dictionary is used to perform sparse regularization. The proposed basis preserves volume impurities and we show that the retrieved light transport matrix is heterogeneous, as in the case of real world objects. We present the rendering results of several stained glass artifacts, including the Rose Window of the Cathedral of Lausanne, digitized using the presented methods.
关键词： computational relighting,stained glass,recovery,Banded matrices,light transport,dictionary learning,sparse cultural artifacts
更新于2025-09-19 17:13:59