Spatial Resolution Of Imaging System Research Articles

Super-resolution image reconstruction of a neutron thick pinhole imaging system using image denoiser prior based on half quadratic splitting method

Thick pinhole imaging system is a powerful tool to diagnose intense pulsed radiation sources. However, high spatial resolution of a thick pinhole imaging system cannot be achieved due to the limitation of field of view (FOV) and detection efficiency. In this paper, we propose a novel super-resolution image reconstruction algorithm to improve the spatial resolution of a practical thick pinhole imaging system designed for 14.1 MeV neutrons with 100 mm FOV. Point spread function (PSF) of the imaging system is obtained through complete simulation methods, indicating that the spatial resolution of the imaging system is about 500 μ m. A 4 × super-resolution image reconstruction algorithm using image denoiser as implicit image prior, named image denoiser prior, based on half quadratic splitting (HQS) method is firstly applied to restore more details of neutron sources. The results show that the proposed algorithm performs better than the Richardson-Lucy (RL) algorithm with fewer artifacts and sharper boundaries in reconstructed images. Notably, at a low noise level, the proposed algorithm can surpass the Nyquist sampling limit of 200 μ m, which is determined by the equivalent camera pixel size, to achieve a spatial resolution of 150 μ m.

Narrow beam breast CT: proof-of-concept.

In breast CT, scattered photons form a large portion of the acquired signal, adversely impacting image quality throughout the frequency response of the imaging system. Prior studies provided evidence for a new image acquisition design, dubbed Narrow Beam Breast CT (NB-bCT), in preventing scatter acquisition. Here, we report the design, implementation, and initial characterization of the first NB-bCT prototype. The imaging system's apparatus is composed of two primary assemblies: a dynamic Fluence Modulator (collimator) and a photon-counting line detector. The design of the assemblies enables them to operate in lockstep during image acquisition, converting sourced x-rays into a moving narrow beam. During a projection, this narrow beam sweeps the entire fan angle coverage of the imaging system. The assemblies are each comprised of a metal housing, a sensory system, and a robotic system. A controller unit handles their relative movements. To study the impact of fluence modulation on the signal received in the detector, three physical breast phantoms, representative of small, average, and large size breasts, were developed and imaged, and acquired projections analyzed. The scatter acquisition in each projection as a function of breast phantom size was investigated. The imaging system's spatial resolution at the center and periphery of the field of view was measured. Minimal acquisition of scattered rays occurs during image acquisition with NB-bCT; results in minimal scatter to primary ratios in small, average, and large breast phantoms imaged were 0.05, 0.07, and 0.9, respectively. System spatial resolution of 5.2 lp/mm at 10% max MTF and 2.9 lp/mm at 50% max MTF at the center of the field of view was achieved, with minimal loss with the shift toward the corner (5.0 lp/mm at 10% max MTF and 2.5 lp/mm at 50% max MTF). The disclosed development, implementation, and characterization of a physical NB-bCT prototype system demonstrates a new method of CT-based image acquisition that yields high spatial resolution while minimizing scatter-components in acquired projections. This methodology holds promise for high-resolution CT-imaging applications in which reduction of scatter contamination is desirable.

Uncertainty analysis of tumour absorbed dose calculations in molecular radiotherapy

BackgroundInternal dosimetry evaluation consists of a multi-step process ranging from imaging acquisition to absorbed dose calculations. Assessment of uncertainty is complicated and, for that reason, it is commonly ignored in clinical routine. However, it is essential for adequate interpretation of the results. Recently, the EANM published a practical guidance on uncertainty analysis for molecular radiotherapy based on the application of the law of propagation of uncertainty. In this study, we investigated the overall uncertainty on a sample of a patient following the EANM guidelines. The aim of this study was to provide an indication of the typical uncertainties that may be expected from performing dosimetry, to determine parameters that have the greatest effect on the accuracy of calculations and to consider the potential improvements that could be made if these effects were reduced.ResultsAbsorbed doses and the relative uncertainties were calculated for a sample of 49 patients and a total of 154 tumours. A wide range of relative absorbed dose uncertainty values was observed (14–102%). Uncertainties associated with each quantity along the absorbed dose calculation chain (i.e. volume, recovery coefficient, calibration factor, activity, time-activity curve fitting, time-integrated activity and absorbed dose) were estimated. An equation was derived to describe the relationship between the uncertainty in the absorbed dose and the volume. The largest source of error was the VOI delineation. By postulating different values of FWHM, the impact of the imaging system spatial resolution on the uncertainties was investigated.DiscussionTo the best of our knowledge, this is the first analysis of uncertainty in molecular radiotherapy based on a cohort of clinical cases. Wide inter-lesion variability of absorbed dose uncertainty was observed. Hence, a proper assessment of the uncertainties associated with the calculations should be considered as a basic scientific standard. A model for a quick estimate of uncertainty without implementing the entire error propagation schema, which may be useful in clinical practice, was presented. Ameliorating spatial resolution may be in future the key factor for accurate absorbed dose assessment.

Miniaturization of a-Si guided mode resonance filter arrays for near-IR multi-spectral filtering

Sub-wavelength periodic arrays exhibit narrow near-unity reflection bands that arise from guided mode resonances. These resonances have extremely high quality factor (i.e., narrow band features) and are ideal for filtering applications. A high quality factor requires many periods, causing large lateral footprints that limit an imaging system's spatial resolution. We present a 1D ultra-thin (<100 nm) compact finite design of seven periods of amorphous Si slabs with subwavelength periodicity surrounded by Al mirrors, which allow the finite array to approximate an infinite array and enabling a small footprint (∼5 μm), for near-infrared applications (λ = 800–2000 nm). We demonstrate spectral tunability (amplitude, bandwidth, and peak location) via geometric parameter variation and demonstrate the performance of these filters both in experiment and in simulation. This work miniaturizes guided-mode resonance filters, previously limited by extremely large footprints, while being relatively cheap and simple to fabricate compared to many existing designs.

Paradigm shifts in super-resolution techniques for remote sensing applications

Super-resolution (SR) algorithms have now become a bottleneck for several remote sensing applications. SR is a technique that enhances minute details of the image by increasing spatial resolution of imaging systems. SR overcomes the problems of conventional resolution enhancement techniques such as introduction of noise, spectral distortion, and lack of clarity in the details of the image. In this paper, a survey has been conducted since the inception of SR algorithm till the latest state-of-the-art SR techniques to elucidate the importance of the SR algorithms that lead to paradigm shifts in the last two decades revolutionizing toward visually pleasing high-resolution image. Inspired from the natural images, the algorithms addressing the SR problems such as ill-posed, prior and regularization problem, inverse problem, multi-frame problem and illumination and shadow problem in remote sensing applications are analyzed. For an intuitive understanding of the paradigm shifts, publicly available images are tested with representative paradigm shift SR algorithms. The result of this paradigm shift analysis is done both qualitatively and quantitatively in terms of blurs in the image, pattern clarity, edge strength, and super-resolving capability. The convergence of the natural image to the remote sensed image is critically analyzed. The challenges with possible solutions for super-resolving the remote sensed image are recommended. On experimentation, it is found that deep learning-based SR algorithms produces visually pleasing images retaining sharp edges, enhanced spatial data, and clarity in feature representation while zooming at a certain level beyond interest.

Characterizing the Degrading Effects of Scattering by Atmospheric Fog on the Spatial Resolution of Imaging Systems

In this paper the effects of scattering, with four atmospheric fog aerosols models, on the overall point spread function (PSF) of an imaging system are investigated. The PSF and radiometric calculations are then used to estimate the mean numbers of direct path and scattered photons detected by the imaging system in the visible and near infra-red (NIR) spectral bands of the electro-magnetic spectrum (EMS), for a horizontal near-ground imaging scenario. The image spectrum signal-to-noise ratio (SNR) is then used to estimate the noise effective spatial resolution of the imaging system which emphasizes the degrading effect of fog aerosol models on the spatial resolution of the imaging systems.

Estimating the image spectrum signal-to-noise ratio for imaging through scattering media

The image spectrum signal-to-noise ratio (SNR) provides a means of estimating the noise effective spatial resolution of an imaging system and a means of estimating the highest spatial frequency which can be reconstructed with a postdetection image reconstruction algorithm. Previous work has addressed the effects of aerosol scattering on the overall point spread function (PSF). Here, we seek to extend these results to also account for the effects of measurement noise and to then estimate the noise effective resolution of the system, which accounts for scattering effects on the PSF and measurement noise in the detector. We use a previously published approach to estimating the effective PSF and radiometric calculations to estimate the mean numbers of direct and scattered photons detected by an imaging system due to reflected radiation in the visible and near-infrared, and emitted radiation in mid-infrared (MIR) band, for a horizontal near-ground imaging scenario. The analysis of the image spectrum SNR presented here shows a reduction in the value of noise effective cutoff spatial frequency for images taken through fog aerosol media, and hence emphasizes the degrading effect of fog aerosol models on the spatial resolution of imaging systems.

Dark-field X-ray imaging of unsaturated water transport in porous materials

We introduce in this Letter an approach to X-ray imaging of unsaturated water transport in porous materials based upon the intrinsic X-ray scattering produced by the material microstructural heterogeneity at a length scale below the imaging system spatial resolution. The basic principle for image contrast creation consists in a reduction of such scattering by permeation of the porosity by water. The implementation of the approach is based upon X-ray dark-field imaging via Talbot-Lau interferometry. The proof-of-concept is provided by performing laboratory-scale dark-field X-ray radiography of mortar samples during a water capillary uptake experiment. The results suggest that the proposed approach to visualizing unsaturated water transport in porous materials is complementary to neutron and magnetic resonance imaging and alternative to standard X-ray imaging, the latter requiring the use of contrast agents because based upon X-ray attenuation only.

Development of a PET/Cerenkov‐light hybrid imaging system

Cerenkov-light imaging is a new molecular imaging technology that detects visible photons from high-speed electrons using a high sensitivity optical camera. However, the merit of Cerenkov-light imaging remains unclear. If a PET/Cerenkov-light hybrid imaging system were developed, the merit of Cerenkov-light imaging would be clarified by directly comparing these two imaging modalities. The authors developed and tested a PET/Cerenkov-light hybrid imaging system that consists of a dual-head PET system, a reflection mirror located above the subject, and a high sensitivity charge coupled device (CCD) camera. The authors installed these systems inside a black box for imaging the Cerenkov-light. The dual-head PET system employed a 1.2×1.2×10 mm3 GSO arranged in a 33 × 33 matrix that was optically coupled to a position sensitive photomultiplier tube to form a GSO block detector. The authors arranged two GSO block detectors 10 cm apart and positioned the subject between them. The Cerenkov-light above the subject is reflected by the mirror and changes its direction to the side of the PET system and is imaged by the high sensitivity CCD camera. The dual-head PET system had a spatial resolution of ∼1.2 mm FWHM and sensitivity of ∼0.31% at the center of the FOV. The Cerenkov-light imaging system's spatial resolution was ∼275 μm for a 22Na point source. Using the combined PET/Cerenkov-light hybrid imaging system, the authors successfully obtained fused images from simultaneously acquired images. The image distributions are sometimes different due to the light transmission and absorption in the body of the subject in the Cerenkov-light images. In simultaneous imaging of rat, the authors found that 18F-FDG accumulation was observed mainly in the Harderian gland on the PET image, while the distribution of Cerenkov-light was observed in the eyes. The authors conclude that their developed PET/Cerenkov-light hybrid imaging system is useful to evaluate the merits and the limitations of Cerenkov-light imaging in molecular imaging research.

Optimized detection of phase amplified light for imaging resolution improvement

The use of quantum phase amplification has been proposed to enhance the spatial resolution of imaging systems. A key component of this approach is the production of phase-amplified light, which can be generated in a phase-sensitive three-wave nonlinear mixing process, such as an optical parametric amplifier. Here we present a refined model for detection of phase-amplified light produced by this method, which includes optimization of detector segmentation, detector noise, and detection in both the spatial and the spatial frequency domain. The implications of quantum phase amplification for resolution of optical imaging are considered in a canonical imaging resolution problem.

90Y-PET for the assessment of microsphere biodistribution after selective internal radiotherapy

To demonstrate the feasibility of 90Y-PET imaging for biodistribution assessment after selective internal radiotherapy treatments with 90Y-microspheres, comparing the results with 99mTc-macroaggregated albumin (MAA) images obtained with single-photon emission computed tomography. Preliminary studies were performed with the aim of evaluating the imaging system spatial resolution and scanner sensitivity for detecting annihilation photons. Subsequently, microsphere distribution was evaluated in 10 patients who underwent liver selective internal radiotherapy treatment. 99mTc-MAA and 90Y-microsphere were simultaneously injected for immediate monitoring after treatment. For each patient, the metastases detected with 90Y-PET and 99mTc-MAA were assessed and compared with 18F-fluorodeoxyglucose-PET (18F-FDG-PET) obtained before treatment and used as an imaging benchmark procedure. The correlation between these techniques was thus investigated in terms of matching lesions. Lesions were considered true positive in the case of matching with 18F-FDG-PET. The sensitivity of both techniques was evaluated as the true-positive fraction of detected spots in the treated liver sectors. With our experimental setup, a maximum scanner sensitivity of 0.577 and 0.077 cps/MBq was obtained for three-dimensional and two-dimensional acquisitions, respectively. A good correlation was obtained between images obtained before and after treatment, with 90Y-PET being by far the most accurate technique in detecting microsphere distribution and tumor nonhomogeneity areas. A sensitivity as high as 0.91 was obtained with 90Y-PET, whereas 99mTc-MAA imaging showed a SE of 0.75. 90Y-PET is a promising and reliable technique for microsphere biodistribution evaluation after liver selective internal radiotherapy treatment. Because of the better resolution and the possibility to perform computed tomography fusion, 90Y-PET images are more accurate than 99mTc-MAA single-photon emission computed tomography, which is now considered the gold standard for biodistribution assessment.

Fluorescent microscopy using localized excitation source with gold nanotriangles: A computational study

A new method for fluorescent microscopy has been proposed. Proposed method uses indirect excitation of fluorophores with nanometer localized illuminating source. Localized source is created at corners of gold nanotriangles which are deposited on glass substrate. Actually the combination of gold nanotriangle (deposited on glass) acts as active substrate (where species will be placed) for our proposed method. The structure will be scanned with a focused beam of laser (or combination of beams). Due to electric field enhancement in corners and edges of nanotringle (because of surface plasmons), third order nonlinear effect will be enhanced accordingly. Enhancement in third order nonlinearity will give the chance to third harmonic generation and four wave mixing at hot spots. Enhancement in nonlinear process will give the chance of localized source creation for excitation of fluorophores in near field region. In order to prove our claim, we have numerically analyzed interaction of light with gold nanotriangle deposited on glass substrate. Our method is based on solving Maxwell's equation in three dimensions, considering dispersion behaviour of permittivity and third order nonlinear susceptibility by nonlinear finite difference time domain method. Simulation results show that generated light is highly localized and its wavelength can be tuned by changing the excitation wavelength. As a result of omitting background noise and space dependency of excitation source, enhancement in spectral and spatial resolution of imaging system will result.

Precision analysis for standard deviation measurements of immobile single fluorescent molecule images.

Standard deviation measurements of intensity profiles of stationary single fluorescent molecules are useful for studying axial localization, molecular orientation, and a fluorescence imaging system's spatial resolution. Here we report on the analysis of the precision of standard deviation measurements of intensity profiles of single fluorescent molecules imaged using an EMCCD camera.We have developed an analytical expression for the standard deviation measurement error of a single image which is a function of the total number of detected photons, the background photon noise, and the camera pixel size. The theoretical results agree well with the experimental, simulation, and numerical integration results. Using this expression, we show that single-molecule standard deviation measurements offer nanometer precision for a large range of experimental parameters.

Measurement and detailed simulation of the Modulation Transfer Function of the Medipix2

The Modulation Transfer Function (MTF) is an important quality characteristic for the description of the spatial resolution of imaging systems. For X-ray detectors, like the Medipix2, it depends for instance on the pixel size and the incident spectrum. The detection of single photons in the Medipix2 is additionally influenced by the lower energy threshold. The dependency of the MTF on this threshold and on the incident spectrum has been evaluated by simulations and measurements. By choosing an adequate threshold, the so-called low frequency drop can be suppressed or the spatial resolution in general is influenced.

Causes of atmospheric blur: comment on Atmospheric scattering effect on spatial resolution of imaging systems

A paper by Ben Dor [J. Opt. Soc. Am. A14, 1329 (1997)] concludes that the blur we measured in our experiments was not atmospherically scattered light and that our theoretical model is incorrect because it violates the rules of linearity. Their work is based in part on “lack of raw data” in one of our experimental papers [J. Opt. Soc. Am A12, 970 (1995)]. We present here the raw data measured in the experiments in question, which show clearly the measured atmospherically scattered light. Similar raw data has also been published elsewhere regarding other experiments. We also clarify some rules of linear systems that justify our conceptual approach, which is shown to be similar to that of turbulence modulation transfer function. A review of several dozen experiments and analyses by other investigators all over the world that directly contradict the Ben Dor et al. results and conclusions is presented. The well-known significance of aerosol blur in imaging through the atmosphere from satellites is discussed, and pictorial examples of satellite imagery are shown for different atmospheric optical depths. It is noted that atmospheric point-spread-function analyses in the remote-sensing literature generally neglect turbulence blur altogether and deal with aerosol blur only, which is often called the adjacency effect, and that such phenomena are well supported by many different types of experiments and many different Monte Carlo simulations for many different aerosol and instrumentation parameter situations. The Monte Carlo simulation results of Ben Dor et al. are shown also to contradict everyday reality such as the solar aureole. This wealth of literature by others strongly contradicts the results of Ben Dor et al. and confirms our conclusion that forward scatter of light by aerosols is indeed a significant source of blur in imaging through the atmosphere, especially if atmospheric optical depth is on the order of unity or more. This can be confirmed, too, by any observer looking through binoculars at the moon and surrounding moonlight even on a clear night. A broad system engineering approach involving both aerosol and turbulence blur is called for.

Atmospheric scattering effect on spatial resolution of imaging systems

The issue that we examine was at the center of a recent scientific debate. A new model by Sadot Kopeika [J. Opt. Soc. Am. A10, 172 (1993)] suggested a practical approach to the imaging process through the atmosphere. A comment on that model by Bissonnette [J. Opt. Soc. Am. A11, 1175 (1994)] and the response to that comment by Kopeika Sadot [J. Opt. Soc. Am. A12, 1017 (1995)] only underlined the open questions in this field of atmospheric blurring effects. We suggest a physical model that describes the relationship between the optical properties of the atmosphere and the characteristics of an imaging system. The model describes how different components of the light that are reaching the imaging system, after passing through the atmosphere, are detected by it. The model includes the effects of the finite size of the detector elements of the imaging system and the dynamic range and the finite field-of-view limits of the imager. After the presentation of the model, theoretical predictions and their comparisons with the experimental data of Kopeika and of Bissonnette are given.

A method for comparative evaluation of EIT algorithms using a standard data set

The point spread function (PSF) is the most widely used tool for quantifying the spatial resolution of imaging systems. However, prerequisites for the proper use of this tool are linearity and space invariance. Because EIT is non-linear it is only possible to compare different reconstruction algorithms using a standard data set. In this study, the FEM is used to generate simulation data, which are used to investigate the non-linear behaviour of EIT, the space dependence of its PSF and its capability of resolving nearby objects. It is found that for the case of iterative backprojection (IterB), the full width half maximum (FWHM) values of single-object perturbations for central, intermediate and peripheral high-contrast objects are 27%, 18% and 14% of the imaging region diameter respectively. For the method based on singular value decomposition of the Geselowitz lead sensitivity matrix (GS-SVD), the FWHM is not space dependent and is 12% of the imaging region diameter. Conclusions obtained using single-object PSP studies must also be checked with double-object or more complex perturbations because EIT is non-linear. For example, the GS-SVD method fails to detect two widely separated objects unless the truncation level of SVD is carefully adjusted. With more truncation, however, the resolution of the method is worsened. Based on these and similar observations a set of simulation data, which is proposed for comparative evaluation of different EIT algorithms, is specified and explained in the conclusion section.

A contrast‐detail analysis of diagnostic ultrasound imaging

For diagnostic ultrasound imaging, as in computed tomography, a feature of prime importance is the detection of focal lesions of varying size and contrast (echo amplitude) from surrounding tissue. This study describes a new tissue-stimulating phantom which has been used to measure the threshold detection of varying contrast, simulated lesions. The phantom consists of a block of tissue-mimicking gelatin which contains a row of conical targets at a depth of 7 cm. Each cone contains a different tissue-mimicking material so that the echo amplitude of the cones relative to the background material covers a dynamic range of 20 dB. Cross-sectional B-scans, perpendicular to the lengths of the cones, result in images of disks of constant diameter but varying contrast. Parallel cross-sectional scans yield "lesions" varying in diameter from 20 to 1 mm. Relative contrast of the cones vs background tissue is obtained by varying scattering particle sizes from 90 to 300 microns. Ultrasound B-scans of the phantom were examined by medical physicist observers to determine threshold detection of lesions as a function of size and contrast. The results indicate that detection of high contrast targets is limited by the imaging system's spatial resolution. Detection of low contrast targets is limited by the image speckle, i.e., coherent noise.