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Computed Tomography (CT) has been widely used in the medical field. Sparse-view CT is an effective and feasible method to reduce the radiation dose. However, the conventional filtered back projection (FBP) algorithm will suffer from severe artifacts in sparse-view CT. Iterative reconstruction algorithms have been adopted to remove artifacts, but they are time-consuming due to repeated projection and back projection and may cause blocky effects. To overcome the difficulty in sparse-view CT, we proposed a dual-domain sparse-view CT algorithm CT Transformer (CTTR) and paid attention to sinogram information.CTTR treats sinograms as sentences and enhances reconstructed images with sinogram's characteristics. We qualitatively evaluate the CTTR, an iterative method TVM-POCS, a convolutional neural network based method FBPConvNet in terms of a reduction in artifacts and a preservation of details. Besides, we also quantitatively evaluate these methods in terms of RMSE, PSNR and SSIM.We evaluate our method on the Lung Image Database Consortium image collection with different numbers of projection views and noise levels. Experiment studies show that, compared with other methods, CTTR can reduce more artifacts and preserve more details on various scenarios. Specifically, CTTR improves the FBPConvNet performance of PSNR by 0.76dB with 30 projections.The performance of our proposed CTTR is better than the method based on CNN in the case of extremely sparse views both on visual results and quantitative evaluation. Our proposed method provides a new idea for the application of Transformers to CT image processing.

The Compton camera (CC) has great potential in nuclear medicine imaging due to the high detection efficiency and the ability to simultaneously detect multi-energy radioactive sources. However, the finite resolution of the detectors will degrade the images that the real-world CC can obtain. Besides, the CC sometimes can be limited by the detection efficiency, leading to difficulty in using sparse projection data to realize high-resolution reconstruction with short-time measurement, which limits its clinical application for real-time or rapid radiopharmaceutical imaging.To overcome the difficulty and promote the usage of the CC in radiopharmaceutical imaging, we present a deep learning (DL)-based CC reconstruction method to realize rapid and high-resolution imaging with short-time measurement.We developed a DL-based algorithm MCBP-CCnet via Monte Carlo sampling-based back projection and a dedicated convolutional neural network, called CC-Net, to realize the rapid and high-resolution reconstruction with sparse projection data. A CC prototype based on a single three-dimensional position-sensitive CdZnTe (3D-CZT) detector was used to demonstrate the feasibility of our proposed method. The simulations and experiments of radiopharmaceutical imaging used the 3D-CZT CC and [The simulation and experimental results showed that the proposed method could realize the images reconstruction within 5 s for list-mode projection data and realized a rapid reconstruction within 35 s for experimental radiopharmaceutical imaging based on the 3D-printing mouse phantom, as well as realized the high-resolution imaging with an accuracy of within 0.78 mm in terms of the sparse projection data that only contained hundreds of events. Besides, the deviations between the reconstructed radiative activities and the exact values were less than 1.51%.The results demonstrated that the proposed method could realize the rapid and high-resolution CC reconstruction with sparse projection data obtained by the 3D-CZT CC and realize the high-resolution radiopharmaceutical imaging. The study in this paper also demonstrated the potential and feasibility of future applications of a 3D-CZT CC for real-time high-resolution radiopharmaceutical imaging with short-time measurement.

Proton beam therapy has the advantages over other forms of radiation therapy because it does less damage to healthy tissue while treating cancerous tissue. These advantages arise from the high dose deposition at the end of the proton beam which is known as the Bragg peak. This paper proposes a new architecture for graphics processing unit (GPU)-accelerated stochastic origin ensemble (SOE) algorithm, eventually achieves a great acceleration and allows to reconstruct images of the gamma radiation produced by proton beam in real time. The object of this paper is to improve the run time of SOE algorithm for improving the accuracy of the proton therapy. In previous studies, it is difficult to reconstruct images of prompt gamma (PG) ray in real time in proton therapy. After analyzing the structure of the SOE algorithm, we correspond each gamma ray detected by Compton camera (CC) to each thread in GPU, initialize the origin cones in GPU, and use data structures which are suitable for parallelization, shared memory, and random number lists to speed up parallel program. To evaluate the proposed method, we use the parallel SOE algorithm to reconstruct the sources with the CC imaging system, which is simulated by Geant4. The parallel program on the GPU eventually achieves a $25\times $ speedup over serial CPU run, reconstructing images of gamma radiation in 0.4 s. In the end, we also accelerate the Monte Carlo-based back projection (MC-BP) algorithm, the parallel program achieves a $79\times $ speedup over serial CPU run.

Objective. Lutetium–yttrium orthosilicate (LYSO)-based Compton camera (CC) has been proposed for prompt gamma imaging due to its high detection efficiency and position resolution. However, very few LYSO CC prototypes have been built and used for practical evaluation. In this study, we built a lightweight dense-pixel silicon photomultiplier-based two-layer LYSO CC prototype for future prompt gamma imaging. Approach. We attempt the first-ever effort to use the double-encoding with the thick light guide and coding circuit structure for 46 × 46 dense-pixel LYSO detectors construction and use pixel segmentation based on centroid mapping to obtain 4232 spectral calibrations. We also present a framework for list-mode projection data acquisition based on the decoding of the time series data obtained by data acquisition card in this study. Finally, the standard source calibration, ring-like 22Na source with non-uniform intensity, and mixed point-like source with a wide energy spectrum experiments were implemented to evaluate the resolution metrics and imaging performance of the prototype. Main results. The lateral position resolution of the prototype was 1 mm, and the maximum measurement deviation is 2.5 mm and 5 mm in the depth direction for the scatterer and absorber, respectively. In the experiments, the measured energy resolution was 9.63% @ 1.33 MeV for the scatterer and 10.8% @ 1.33 MeV for the absorber. And the detection efficiency of the prototype for a spherical 60Co source with a diameter of 2.8 mm at 10 cm far was 5.7 × 10−3 @ 1.33 MeV and the full width at half maximum of the reconstruction was 5.5 mm. Besides, the spatial position offset within 2 mm of the radioactive source at 10 cm can be distinguished. Signification. The developed two-layer dense-pixel LYSO CC contributes to incorporating Compton imaging techniques for prompt gamma detection and multiple energy sources into nuclear medical imaging.

This paper provides the physical design of a low-energy proton irradiation facility for space radiation effect research, based on an existing linear accelerator (linac)-based compact neutron source . The beam requirement of the low-energy proton irradiation consists of a proton flux range of 1 0 8 – 1 0 9 p ∕ cm 2 ∕ s with a nonuniformity of less than 10% in the target diameter area of 2 cm, and optional proton energies of 1 MeV to 13 MeV. Based on the existing high-current proton linac, the flux range and nonuniformity of the proton beam at the target can be achieved by inserting beam apertures, adjusting quadrupole fields to expand the beam size, and decreasing the repetition rate of the beam pulse. To measure the energy distribution of the protons, one CdZnTe detector can be employed. The above approach is performed on the physical design of the low-energy proton facility for the Compact Pulsed Hadron Source at Tsinghua University. The proton flux at the target can be reduced from 6 . 2 × 1 0 14 p ∕ cm 2 ∕ s to 1 . 2 × 1 0 8 − 3 . 0 × 1 0 9 p ∕ cm 2 ∕ s , and proton energies of 0.56, 5.0, 8.2, and 10.5 MeV can be obtained separately with the energy degrader (aluminum) thicknesses of 75, 800, 550, and 300 μ m. Irradiation nonuniformity is less than 10%. Energy spread is less than 10% for the proton energy of 3.0, 5.0, 8.2, and 10.5 MeV, except for the low energy of 0.56 MeV.

In this paper, we firstly study the effect of Doppler broadening on deposited energy and Compton scattering angle for the 4.44 MeV gamma-rays. We measured and fitted the models of the Doppler broadening effect, as well as compared them with theoretical calculations. Then we presented two kinds of approaches (i.e. unidirectional and bidirectional) to correct the Doppler broadening effect via OE with different sampling distribution. Finally, we evaluated the performance of these correction methods for prompt gamma imaging. The corrections with Lorenz distribution of methods □ and □ (i.e. unidirectional and bidirectional) have the best performance on the Doppler broadening correction compared with Gaussian and uniform distribution. Both the calculated model obtained by theoretical formula and fitting model based on the measurement data can realize effective correction. After the correction, the signal-to-noise ratio of the reconstructed image is improved, and the peak prediction is closer to the true value. Moreover, the results of the Doppler broadening correction have a narrower full width at half maximum, which is corresponding to the expected theoretical prediction. The results show that the Doppler broadening correction can further reduce the inherent point spread function of the Compton reconstruction, and improve the accuracy of the reconstruction.

In this work, we present a deconvolution approach to predict depth dose distributions from prompt gamma images obtained by a Compton camera. Based on the assumption that the one-dimensional PG distribution can be derived from the convolution of the depth dose distribution and a filter kernel, an approximation function of Bragg curve and an evolutionary algorithm (EA) are introduced to achieve the inverse process and estimate the depth dose distribution from a measured PG distribution. Filter kernels used in the approach are obtained by the same evolutionary algorithm with predictions of the PG and depth dose distributions. The applicability of this approach is demonstrated for the monoenergetic proton irradiation of several homogeneous phantoms made of different tissue materials, using Monte-Carlo simulated data. The distal falloff of depth dose distribution is reconstructed within less than 1 mm deviation, showing its potential for real-time dose estimation in future.

The characteristics of the X-ray focal spot strongly influence XCT image sharpness and structural resolution. However, current international standards only yield a limited characterization of the focal spot as two perpendicular sizes. Therefore, this paper presents a method for full characterization of the focal spot in terms of both 2D shape and intensity distribution, by applying the inverse Radon transform on a radiograph of a Siemens star. The method is validated through simulations and applied experimentally to two XCT sources. Subsequently, an investigation on how the X-ray tube settings influence focal spot size, shape, intensity and stability is presented.

In this paper, we propose an approach for spatial interaction position correction with subdivision grid and three-dimensional transmission probability model in Compton camera. Then we present the optimized origin ensemble algorithm with the acceptance-rejection method based on the model to obtain the corrected images. We incorporate the different transmission in hypothetical differential detectors’ pixels of each crystal into the resolution recovery process. The preliminary simulation results of ideal point-like source with pixelated Cadmium Zinc Telluride Compton camera, demonstrate that the proposed method can accelerate the convergence by about 7% over OE with resolution recovery (OE-RR). The small double kidney-like phantom imaging shows that our optimized OE-RR algorithm can reproduce the sources’ spatial distribution more accurate and further improve the effect of resolution recovery. The deviation of the outline of phantom is less than 2.4 mm and the maximum relative error is about 5% at half maximum between the reconstruction and the exact values, showing its potential in future Compton-based medical imaging.

Prompt gamma ray (PG) imaging based on Compton camera (CC) is promising to realize in vivo verification during the proton therapy. However, the finite spatial and energy resolution of current CC, as well as the Doppler broaden effect, degrade the quality and resolution of PG images. In addition, due to the inherent geometrical complexity of Compton camera data, PG imaging can be time-consuming and difficult to reconstruct in real-time, while using standard techniques such as filtered back-projection or maximum likelihood-expectation maximization. In this paper, we propose three modifications of origin ensembles with resolution recovery (OE-RR) algorithm based on Markov chains to accelerate the convergence to equilibrium of OE-RR algorithm and improve the image quality. For evaluation, we performed a Monte Carlo simulation of a three-stage CZT Compton camera with resolution loss to detect the PG produced by a proton beam in a water phantom, and evaluate image quality of the gamma rays emitted during proton irradiation. The results show that our ordered OE-RR algorithm realized a good resolution recovery and accurate estimation of the position, including the peak and the distal falloff of the PG emission with remarkably faster reconstruction, thus demonstrating the feasibility of this new method in non-idealized PG-based proton range verification.

In this paper, we present an algebraic method to acquire the point on the event conical surface using dimension-reduction and canonical form, for direct reconstruction, pre-calculation of list-mode maximum likelihood expectation maximization(LM-MLEM) algorithm as a fast back-projection method, or an approach of spatial symmetrically sampling with resolution recovery in stochastic origin ensemble(SOE) algorithm. The 3D image reconstruction based on the dimension-reduction and canonical form sampling method is not affected by the choice of imaging space such as the size of voxels and orientation. Moreover, it can achieve nearly real-time imaging for changeable imaging space because of its fewer computations. Simulation and experiment results show that this method can achieve high resolution and speed in image reconstruction for both multiple point-like sources and extended sources. The method is also promising to weaken the effect of spatial orientation of imaging space, which can cause distortion in the direction perpendicular to the imaging plane.

In this paper, we proposed a method that uses Monte Carlo method to achieve resolution recovery and a changeable space based maximum likelihood expectation maximization (MLEM) algorithm to achieve image reconstruction. Our resolution recovery process will be directly incorporated into the reconstruction algorithm without the need for previous calculations. The Monte Carlo spatial sampling method realizes the resolution recovery as well as the pre-calculation of the back projection of the image space, which improve the convergence speed of the expectation maximization algorithm. Since the Monte Carlo spatial sampling is independent of the voxel size of the reconstruction spaces, the iteration can be performed in a changeable space. This kind of reconstruction achieves almost real-time reconstruction with resolution recovery. Preliminary experimental results demonstrate the feasibility and effectiveness of this method. With the usage of the proposed resolution recovery method, the reconstructed results are in agreement with the real data reconstruction results, and the FWHM ratio is only about 2mm large than the exact value. The position and relative intensity of the radioactive sources are well reconstructed.

In this paper, we present an algebraic method to acquire the point on the event conical surface using inverse and transfer matrix, which has simple form and high precision. The statistical reconstruction method based on origin ensembles (OE) was introduced, which adopt a probability density function (pdf) to determine ensemble expectation of numbers for detected event origins per voxel. An OE algorithm with multi-focus method (OE-MF) to improve resolution was proposed in this paper. Instead of increase the multi-views by using multiple detectors to combine image, the multi-focus method rely on the software. The calculation of the probability density of the sources in different orientation imaging planes at the same time, is promising to weaken even eliminate the effect of spatial orientation, as well as the distortion in the direction perpendicular to the imaging plane. Our numerical simulation and experiments results demonstrate the feasibility and effectiveness of the proposed OE-MF method, which works well in multi sources reconstruction and enhances the spatial resolution. Future work will focus on combining with multi-angle sampling to reconstruct, more efficient by parallel computation, higher precision by filtration et al.

Aero engine is the heart of aircraft, and engine blades are the most important parts. They usually work in bad conditions such as high temperature, high pressure, high rotation speed and alternating load. Some tiny defects may cause disastrous consequences. An effective measure to detect the defects is nondestructive detection method bases on industrial CT technology. However, this detection method has several disadvantages, such as complicated control system, high cost and long testing cycle. And some kinds of defects are not evident during the shutdown period. Based on researches about the insitu nondestructive testing methods for aero engine, we put forward a system using CT technique to detect the rotating parts of the aero engine, which gets projection datasets of the blades from different views through its self-rotation. In this paper, the industrial CT system based on synchronous triggering method is described, and the simulation and experiment are performed to verify the method.

A small angle CT system is unable to scan a big object, which is larger than the field of view. In order to make full use of this kind of systems, several methods have been proposed to solve this problem, such as Half-Covered Helical CT, but additional scan process is also needed. This paper describes a novel designed rotation system to apply the small angle system for large object testing, whose scan process is as same as the conventional CT system. In this newly designed overlay rotation CT scanning system, two turntables are used to make up a twodegree- of-freedom system. These two turntables rotate at constant rotation rates and projection data is collected at the same time. In this paper, new projection data rebinning method is adopted and the reconstructed algorithm is also described. Numerical experiments are taken to test a simple prototype and verify the feasibility of this CT system. In conclusion, this novel designed overlay rotation CT system is effective to get enough projection data for reconstruction.

“Stimulated reservoir volume” (SRV) makes it possible for commercial production of shale gas by means of multistage fracturing of horizontal wells. However, the formation mechanism of effective fracture network has not been well understood. The mechanism of fracture propagation in shale with hydraulic fracturing needs to be further explored, in order to realize the control on morphology of fracture network with SRV and increase the single well production of shale gas. In this article, the true triaxial test system was deployed for horizontal well hydraulic fracturing simulation experiments of shale outcrops for the first time. The effects of multiple factors on propagating rules of fractures of horizontal well in shale with SRV were studied, and the fracture morphology of post-fracturing rock cores was observed for the first time by high-energy CT scanning using the large-scale non-destructive testing system based on linear accelerator. The results show that the influence of flow rate (for SRV) on fracture complexity differs when its value falls in different intervals. When the horizontal in situ stress difference is less than 9 MPa, the hydraulic fracture easily propagates along the natural fractures, forming a fracture network. In this range, when the stress difference is increased, the appearance of the main hydraulic fracture contributes to interconnecting more natural fractures, forming a relatively more complex fracture system. Under the condition of the same horizontal stress difference, if the coefficient of stress difference K h > 0.25, there is an obvious trend to form single main fracture. The effects of viscosity of fracturing fluid and flow rate on the fracture propagation can be expressed by the parameter q ⋅ μ . When the order of magnitude of q ⋅ μ -value is 10 −9 N m, it is favorable to the generation of fracture network, but too small or too large values are both harmful. The development and cementing strength of depositional beddings seriously affect the propagating complexity of fracture network with hydraulic fracturing. Refracturing helps to produce some new fractures which are different from the initial ones, and better fracturing effect is achieved. The fracture initiation morphology in perforation fracturing is closely related to the distribution of natural fractures (depositional bedding) around the perforations. If the pressure curve fluctuates significantly, it is indicated that the shale core contains a large number of opened natural fractures or depositional beddings before fracturing, which causes the serious loss of fracturing fluid in the propagation process.

Airplane engines are vital aircraft components, so regular inspections of the engines are required to ensure their stable operation. A dynamic computed tomography (CT) system has been proposed by our group for in situ nondestructive testing of airplane engines, which takes advantage of the rotor's self-rotation. However, static parts of the engines cause blocked artifacts in the reconstructed image, leading to misinterpretations of the condition of engines. In this paper, in order to remove the artifacts produced by the projection of the static parts in CT reconstruction, two deep-learning-based methods are proposed, which use U-Net to perform correction in the projection domain. The projection of static parts can be estimated by a well-trained U-Net and subsequently can be subtracted from the projections of the engine. Finally, the rotor can be reconstructed from the corrected projections. The results shown in this paper indicate that the proposed methods are practical and effective for removing those blocked artifacts and recovering the details of rotating parts, which will, in turn, maximize the utilization of the dynamic CT system for in situ engine tests. ispartof: APPLIED OPTICS vol:58 issue:14 pages:3748-3753 ispartof: location:United States status: published

In recent years, the total variation (TV) minimization method has been widely used for compressed sensing (CS) based CT image reconstruction. In this paper, we propose a few-view reweighted sparsity hunting (FRESH) method for CT image reconstruction, and demonstrate the superior performance of this method. Specifically, the key of the purposed method is that a reweighted total variation (RwTV) measure is used to characterize image sparsity in the cost function, outperforming the conventional TV counterpart. To solve the RwTV minimization problem efficiently, the Split-Bregman method and other state-of-the-art L1 optimization methods are compared. Inspired by the fast iterative shrinkage/thresholding algorithm (FISTA), a predication step is incorporated for fast computation in the Split-Bregman framework. Extensive numerical experiments have shown that our FRESH approach performs significantly better than competing algorithms in terms of image quality and convergence speed for few-view CT. High-quality images were reconstructed by our FRESH method after 250 iterations using only 15 few-view projections of the Forbild head phantom while other competitors needed more than 800 iterations. Remarkable improvements in details in the experimental evaluation using actual sheep thorax data further indicate the potential real-world application of the FRESH method.

It is important and meaningful to measure exact geometry and position of the internal structure in the aero-engine under various working statuses to improve the performance and efficiency of aero-engine. In this work, we introduce the x-ray digital radiography system to monitor the aero-engine internal structure changes under different rotating speed. For synchronized acquired process of radiography system, a flexible universal trigger module is designed to control the image data acquisition process on the specified rotary position of the operating aero-engine with the image detector and accelerator based X-ray source. In the paper, the details about the universal trigger module design and the data calibration process will be introduced.

This article investigates methods in social network analysis to identify the most important or the most prominent actors in a social network by ranking them appropriately. Although much has been done since Seeley's (1949) seminal work, several questions remain: How does scaling an adjacency matrix of a social network affect its eigenvalues and their corresponding eigenvectors? How can the differences between the left and the right eigenvectors be reconciled? Do both depict the two sides of the same coin? Can they be “merged” objectively to yield a single eigenvector, and how? Under what conditions does an adjacency matrix characterize, as a first-order approximation, the dynamics of its corresponding social network? Most importantly, how can a general dynamic model be derived to make clear the interrelationships of existing models? In answering these questions, we derive a general model for the evolving rank (which we define as a general measure of prominence) of a (fixed) set of actors in a social networ...

A cold neutron radiography/tomography instrument was designed and will soon undergo construction at the Compact Pulsed Hadron Source (CPHS) of Tsinghua University, China. In this paper, we report the physical design of the instrument and propose a scheme to implement several techniques at a later phase to enhance the utilization of the flux on larger samples. This includes code-aperture, grating-based imaging, prompt gamma-ray analysis and 3D emission CT.

Since the launching of the Compact Pulsed Hadron Source (CPHS) project of Tsinghua University in 2009, works have begun on the design and engineering of an imaging/radiography instrument for the neutron source provided by CPHS. The instrument will perform basic tasks such as transmission imaging and computerized tomography. Additionally, we include in the design the utilization of coded-aperture and grating-based phase contrast methodology, as well as the options of prompt gamma-ray analysis and neutron-energy selective imaging. Previously, we had implemented the hardware and data-analysis software for grating-based X-ray phase contrast imaging. Here, we investigate Geant4-based Monte Carlo simulations of neutron refraction phenomena and then model the grating-based neutron phase contrast imaging system according to the classic-optics-based method. The simulated experimental results of the retrieving phase shift gradient information by five-step phase-stepping approach indicate the feasibility of grating-based neutron phase contrast imaging as an option for the cold neutron imaging instrument at the CPHS.

A cold neutron radiography/tomography instrument is under construction at the Compact Pulsed Hadron Source (CPHS) at Tsinghua University, China. The neutron flux is so low that an acceptable neutron radiographic image requires a long exposure time in the single-hole imaging mode. The coded-source-based imaging technique is helpful to increase the utilization of neutron flux to reduce the exposure time without loss in spatial resolution and provides high signal-to-noise ratio (SNR) images. Here we report a preliminary study on the feasibility of coded-source-based technique applied to the cold neutron imaging with a low-brilliance neutron source at the CPHS. A proper coded aperture is designed to be used in the beamline instead of the single-hole aperture. Two image retrieval algorithms, the Wiener filter algorithm and the Richardson–Lucy algorithm, are evaluated by using analytical and Monte Carlo simulations. The simulation results reveal that the coded source imaging technique is suitable for the CPHS to partially solve the problem of low neutron flux.

Gulland's [Gulland, J.A., 1965. Estimation of mortality rates. Annex to Arctic Fisheries Working Group Report (meeting in Hamburg, January 1965). ICES, C.M. 1965, Doc. No. 3 (mimeographed)] virtual population analysis (VPA) is commonly used for studying the dynamics of harvested fish populations. However, it necessitates the solving of a nonlinear equation for the instantaneous rate of fishing mortality of the fish in a population. Pope [Pope, J.G., 1972. An investigation of the accuracy of Virtual Population Analysis using cohort analysis. ICNAF Res. Bull. 9, 65-74. Also available in D.H. Cushing (ed.) (1983), Key Papers on Fish Populations, p. 291-301, IRL Press, Oxford, 405 p.] eliminated this necessity in his cohort analysis by approximating its underlying age- and time-dependent population model. His approximation has since become one of the most commonly used age- and time-dependent fish population models in fisheries science. However, some of its properties are not well understood. For example, many assert that it describes the dynamics of a fish population, from which the catch of fish is taken instantaneously in the middle of the year. Such an assertion has never been proven, nor has its implied instantaneous rate of fishing mortality of the fish of a particular age at a particular time been examined, nor has its implied catch equation been derived from a general catch equation. In this paper, we prove this assertion, examine its implied instantaneous rate of fishing mortality of the fish of a particular age at a particular time, derive its implied catch equation from a general catch equation, and comment on how to structure an age- and time-dependent population model to ensure its internal consistency. This work shows that Gulland's (1965) virtual population analysis and Pope's (1972) cohort analysis lie at the opposite end of a continuous spectrum as a general model for a seasonally occurring fishery; Pope's (1972) approximation implies an infinitely large instantaneous rate of fishing mortality of the fish of a particular age at a particular time in a fishing season of zero length; and its implied catch equation has an undefined instantaneous rate of fishing mortality of the fish in a population, but a well-defined cumulative instantaneous rate of fishing mortality of the fish in the population. This work also highlights a need for a more careful treatment of the times of start and end of a fishing season in fish population models.

Two of the most significant developments in fisheries science are the single species virtual population analysis (SSVPA) and multi-species virtual population analysis (MSVPA). Since its development, MSVPA and its variants have been used in fish population studies, fish stock assessment, and the management of the fisheries in many parts of the world, including the North Sea, the Baltic Sea, the Barents Sea, the Bering Sea, Georges Bank, and the Benguela Current, and have been accepted as routine and essential tools for making efficient use of the world's major fisheries resources for food production, and for conserving and managing those resources. However, serious questions arise from the fact that some of their equations are “borrowed” straight from SSVPA, except for adding a species index. In this paper, I derive the fundamental equations of MSVPA and its variants as a system of ordinary differential equations (if the ages of fish are discrete) or as a system of first order partial differential–integral equations (if the ages of fish are continuous), both subject to appropriate constraints and boundary conditions (biological observations and data). Their formulation as well known mathematical problems has not only revealed their mathematical nature and hence laid the foundation for their speedy solution by use of a great variety of known and readily available mathematical techniques, but also revealed the logical inconsistency in the currently used MSVPA and its variants. This inconsistency highlights an urgent need to revise all past work on MSVPA and its variants, by replacing the existing equations of MSVPA and its variants with the fundamental equations derived above. Finally, I discuss (1) the conversion of data on the stomach contents of fish of different ages and of different species into their coefficients of suitability, (2) two possible extensions of the present work, (3) its general ecological and biological significance as a much needed practical method or routine for parameterizing a system of age- and time-dependent or more general Lotka–Volterra equations to understand the underlying mechanisms for the rapid changes of the ecosystems and for us to maintain a harmonious co-existence within a biological system, and (4) its significance to several areas of fisheries science, including food production, effective use and management of fisheries resources, marine and freshwater conservation, and the maintenance of a healthy aquatic environment.

The finite focal spot size is one of the major physical limitations to the spatial resolution for CT reconstruction. In this paper, a projection restoration based method is proposed to improve the spatial resolution. First of all, the focal spot size is considered into the projection model. Then the equivalence relation between the measured projection and the ideal projection is deduced. Based on such relation, the equivalent ideal projection can be recovered from the measured data before reconstruction. Finally, a high-resolution image is reconstructed using the recovered projection. Furthermore, an improved Nonlocal Mean method is presented to suppress the noise in the highresolution image. Both numerical simulation and experiment results have shown that the proposed method with the outstanding performance and efficiency characteristics can significantly enhance the spatial resolutions.

In order to detect the deformations and monitor inner structures during operation more effectively, an innovative CT system making use of the objects' self-rotation is designed by us. However, some parts remain static and they contaminate the reconstruction image with ring artifacts. In this paper, we put forward a method taking advantage of limited-angle scan to get the prior image, then using interactive image registration and forward projection to eliminate these artifacts. Both numerical and experimental researches are presented to verify the feasibility of the method.

The finite focal spot is one of the major limitations of the high spatial resolution CT, especially to the high-energy industrial CT system with a macro-focus x-ray source. In this paper, we propose an efficient reconstruction framework through finite focal spot size based projection modeling to improve the spatial resolution of current industrial CT system, and demonstrate the superior performance of this method. First of all, the blurred projection produced by a finite size source is modeled as the integral ideal projection of a given point source over the finite focal spot support. Under the model discretization, the approximate linear equivalence relation between the actual finite focus model and the ideal point source model is established. Then a projection recovery method with this relationship is presented to recover the projection of the finer focal spot from the blurred projection. Finally, a high-spatial resolution image can be reconstructed from the recovered projections using the standard Filtered Back-Projection (FBP) algorithm. Furthermore the noise in the reconstructed image with different model parameters is studied and a difference image based fusion method is presented for the further suppression of the noise caused by the projection analysis processing. Both numerical simulations and real experiments have shown that the proposed reconstruction framework with the outstanding performance and efficiency characteristics can significantly enhance the spatial resolutions of current high-energy industrial CT systems.

The radiation dose reduction without sacrificing the image quality as an important issue has raised the attention of CT manufacturers and different automatic exposure control (AEC) strategies have been adopted in their products. In this paper, we focus on the strategy of tube current modulation. It is deduced based on the signal-to-noise (SNR) of the sinogram. The main idea behind the proposed modulation strategy is to keep the SNR of the sinogram proximately invariable using the few-view reconstruction as a good reference because it directly affects the noise level of the reconstructions. The numerical experiment results demonstrate that, compared with constant tube current, the noise distribution is more uniform and the SNR and CNR of the reconstruction are better when the proposed strategy is applied. Furthermore it has the potential to distinguish the low-contrast target and to reduce the radiation dose.

A general model is developed to examine the patterns of the regional movement of tagged and released fish from mark-recapture experiments. It is a stochastic model that incorporates fishing mortality, natural mortality, fish movement, tag-shedding, and different rates of reporting. A likelihood function is constructed for estimating its parameters. We used this model to analyze data on the Pacific halibut from mark-recapture experiments conducted by the International Pacific Halibut Commission (IPHC), with a total of 36,058 releases from 1982 to 1986 and 5,826 recoveries from 1982 to 2000. We estimated their rates of movement among IPHC management areas, along with their instantaneous rates of natural and fishing mortalities. Our analysis revealed that fish movement was not significant among areas, with a resident probability of > 0.92. This lends support to the IPHC catch-at-age stock assessment model (which has no built-in movement components). The estimated instantaneous rate of natural mortality (0.198 year−1) lies between that assumed in all IPHC stock assessments before 1998 (0.20 year−1) and that from 1999 onwards (0.15 year−1). The estimates of the instantaneous rates of fishing mortality were consistent with those from the IPHC stock assessment model.

In this note, I explain why, in the deterministic context, an age-dependent population model is equivalent to a size-dependent one, and discuss several other related issues that must be understood, in order to implement, correctly, a size-dependent population model. Several conclusions are in order. The age-based instantaneous rates of fishing and natural mortalities must be used in an age-dependent model, whereas size-based ones must be used in a size-dependent model. Their huge differences both in value and in dimension imply that any use of an age-based instantaneous rate of natural mortality in place of a size-based one in a size-dependent model for a fish population is equivalent to assuming that its population is far more dynamic than real, the size of its population is far smaller than real, its recruitment strength is far greater than real, and its population (if depleted) recovers much faster than real. Thus, such a mis-specified size-dependent model will produce a far more optimistic picture than real. Also, both size- and age-dependent population models apply to the populations of animals that can or cannot be aged. Any attempt at constructing, from a set of conventional tagging data, a growth function or a size-transition matrix that can account for the incremental change in the size of a crustacean as a function of its age is doomed to failure, unless data are available on the moult number and the duration of the stages of individual animals. Moreover, for logical consistency, the stochasticity in the quantities of interest in a population model must be introduced into the underlying differential equations to yield stochastic differential equations or by formulating new stochastic processes, but must not be introduced into an existing population model as the solution of some deterministic differential equations. It is high time to follow examples of the work before the 1990s on size- and age-dependent population models and move forward.