Neural network-based image reconstruction for positron emission tomography

Positron emission tomography (PET) is one of the key molecular imaging modalities in medicine and biology. Penalized iterative image reconstruction algorithms frequently used in PET are based on maximum-likelihood (ML) and maximum a posterior (MAP) estimation techniques. The ML algorithm produces noisy artifacts whereas the MAP algorithm eliminates noisy artifacts by utilizing availableprior information in the reconstruction process. The MAP-based algorithms fail to determine the density class in the reconstructed image and hence penalize the pixels irrespective of the density class and irrespective of the strength of interaction between the nearest neighbors. A Hebbian neural learning scheme is proposed to model the nature of interpixel interaction to reconstruct artifact-free edge preserving reconstruction. A key motivation of the proposed approach is to avoid oversmoothing across edges that is often the case with MAP algorithms. It is assumed that local correlation plays a significant role in PET image reconstruction, and proper modeling of correlation weight (which defines the strength of interpixel interaction) is essential to generate artifact-free reconstruction. The Hebbian learning-based approach modifies the interaction weight by adding a small correction that is proportional to the product of the input signal (neighborhood pixels) and output signal. Quantitative analysis shows that the Hebbian learning-based adaptive weight adjustment approach is capable of producing better reconstructed images compared with those reconstructed by conventional ML and MAP-based algorithms in PET image reconstruction.     

Graphics processing unit (GPU)-accelerated particle filter framework for positron emission tomography image reconstruction

As a consequence of the random nature of photon emissions and detections, the data collected by a positron emission tomography (PET) imaging system can be shown to be Poisson distributed. Meanwhile, there have been considerable efforts within the tracer kinetic modeling communities aimed at establishing the relationship between the PET data and physiological parameters that affect the uptake and metabolism of the tracer. Both statistical and physiological models are important to PET reconstruction. The majority of previous efforts are based on simplified, nonphysical mathematical expression, such as Poisson modeling of the measured data, which is, on the whole, completed without consideration of the underlying physiology. In this paper, we proposed a graphics processing unit (GPU)-accelerated reconstruction strategy that can take both statistical model and physiological model into consideration with the aid of state-space evolution equations. The proposed strategy formulates the organ activity distribution through tracer kinetics models and the photon-counting measurements through observation equations, thus making it possible to unify these two constraints into a general framework. In order to accelerate reconstruction, GPU-based parallel computing is introduced. Experiments of Zubal-thorax-phantom data, Monte Carlo simulated phantom data, and real phantom data show the power of the method. Furthermore, thanks to the computing power of the GPU, the reconstruction time is practical for clinical application.     

一种基于模型的光学层析图像重建方法

光学层析图像重建是个病态问题,测量误差会在重建过程中被放大,对此,提出一种以广义高斯马尔可夫随机场模型为先验信息的光学层析图像重建方法.重建过程是对目标函数的优化过程,目标函数关于光学参数的梯度计算是算法中的难点,因此,提出一种基于梯度树的梯度计算方法.文中分别给出了吸收系数和散射系数的重建结果,并引入三个指标因子衡量重建图像的质量,进而列出不同重建算法下,重建图像的指标值.最后通过对重建结果和指标因子取值的比较,分析基于模型的重建算法的有效性.     

Volume image reconstruction for diffuse optical tomography

Optical tomography is a potential diagnostic method for visualizing optical properties of tissues in vivo. We present an optical tomography method that has been designed for imaging of the human testes, particularly for spectroscopic tumor differentiation. In this application we need to compute three-dimensional distributions of the optical contrast (absorption coefficient) in the tissue in real time. Thus we have given special care to elaborate an efficient inverse algorithm that takes the limitations of spatial resolution and data space point density into account. Our inverse solution is based on a linearization approach and a dedicated object space discretization. Furthermore, we introduce the concept of fuzzy voxels, which enables a reconstruction-inherent image smoothing.     

A BaF2-MWPC gamma camera for positron emission tomography

The construction and performance of a position sensitive detector for 511 keV annihilation radiation are described. The detector consists of a barium fluoride crystal sandwiched between two wire chambers which are operated with 45 and 50°C TMAE vapour. Energy, time and position resolution are presented. The advantages and limitations of the detector are discussed. We also present some preliminary data on other UV scintillators.     

Characterization of a 82Rb generator for positron emission tomography

Characteristics of the developed ⁸²Rb medical generator intended for the production of Rubidium Chloride, ⁸²Rb from Generator radiopharmaceutical for positron emission tomography (PET) are examined. The results of laboratory tests and clinical trials are presented. The influence of thermal sterilization on the ionexchange characteristics of the sorbent based on hydrous tin(IV) oxide is studied. The possibility of radiation “self-sterilization” of the generator column and of the maintenance of aseptic conditions in it throughout the generator operation life is assessed. In the course of operation of one generator, it is possible to obtain up to 19 L of a sterile and apyrogenic solution of the radiopharmaceutical with permissible content of radioactive and stable impurities. The efficiency of the ⁸²Rb elution from the sorbent at different eluent flow rates is determined. The efficiency of using the generator in PET investigations in cardiology and oncology is demonstrated.     

Developments in high-resolution positron emission tomography at MGH

The development of high resolution PET systems is important for the wider application of this techniques. The resolution of PET is limited by a number of physical factors such as positron range, small angle deviation, and sampling frequency. The design of the detector array and its sensitivity remain critical factors; designs incorporating analog coding have proven to be useful. PCR-I, a single plane PET system, has demonstrated the concept and has produced useful images in animal studies. PCR-II will extend the concept to a two dimensional detector array resulting in a system with high resolution and high sensitivity.     

Constrained image reconstruction for magnetic detection electrical impedance tomography

Magnetic detection electrical impedance tomography (MD‐EIT) produces images of conductivity from magnetic field measurements taken around the body. The ill‐conditioned nature of the MD‐EIT inverse problem is improved by limiting the number of unknowns to be solved. In this article, a method of iterative grid refinement for MD‐EIT, which produces images significantly better than unconstrained solutions, is described. © 2008 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 17, 379–382, 2007     

Multicriteria maximum likelihood neural network approach to positron emission tomography

The emerging technology of positron emission image reconstruction is introduced in this paper as a multicriteria optimization problem. We show how selected families of objective functions may be used to reconstruct positron emission images. We develop a novel neural network approach to positron emission imaging problems. We also studied the most frequently used image reconstruction methods, namely, maximum likelihood under the framework of single performance criterion optimization. Finally, we introduced some of the results obtained by various reconstruction algorithms using computer‐generated noisy projection data from a chest phantom and real positron emission tomography (PET) scanner data. Comparison of the reconstructed images indicated that the multicriteria optimization method gave the best in error, smoothness (suppression of noise), gray value resolution, and ghost‐free images. © 2001 John Wiley & Sons, Inc. Int J Imaging Syst Technol 11, 361–364, 2000     

Local cone-beam tomography image reconstruction on chords

We develop reconstruction algorithms for local cone-beam tomography for use with generalized scanning trajectories. The algorithms are grounded theoretically in a recently developed chord-based theory for exact image reconstruction and principles of lambda tomography. Being chord based, they are distinct mathematically and conceptually from conventional local tomography reconstruction algorithms. The salient feature of our algorithms is that they permit reconstruction of discontinuities in the profiles of the object function along chords. By consideration of all possible chords, a 3D image that describes the locations of object discontinuities can be reconstructed. Results from microlocal analysis are applied for understanding the object features that can be reconstructed stably by use of the algorithms. A computer-simulation study is conducted to demonstrate the algorithms and compare their performance with an existing algorithm.     

Denoising multicriterion iterative reconstruction in emission spectral tomography

In the study of optical testing, the computed tomogaphy technique has been widely adopted to reconstruct three-dimensional distributions of physical parameters of various kinds of fluid fields, such as flame, plasma, etc. In most cases, projection data are often stained by noise due to environmental disturbance, instrumental inaccuracy, and other random interruptions. To improve the reconstruction performance in noisy cases, an algorithm that combines a self-adaptive prefiltering denoising approach (SPDA) with a multicriterion iterative reconstruction (MCIR) is proposed and studied. First, the level of noise is approximately estimated with a frequency domain statistical method. Then the cutoff frequency of a Butterworth low-pass filter was established based on the evaluated noise energy. After the SPDA processing, the MCIR algorithm was adopted for limited-view optical computed tomography reconstruction. Simulated reconstruction of two test phantoms and a flame emission spectral tomography experiment were employed to evaluate the performance of SPDA-MCIR in noisy cases. Comparison with some traditional methods and experiment results showed that the SPDA-MCIR combination had obvious improvement in the case of noisy data reconstructions.     

Dynamically adaptive mesh refinement technique for image reconstruction in optical tomography

A novel adaptive mesh technique is introduced for problems of image reconstruction in luminescence optical tomography. A dynamical adaptation of the three-dimensional scheme based on the finite-volume formulation reduces computational time and balances the ill-posed nature of the inverse problem. The arbitrary shape of the bounding surface is handled by an additional refinement of computational cells on the boundary. Dynamical shrinking of the search volume is introduced to improve computational performance and accuracy while locating the luminescence target. Light propagation in the medium is modeled by the telegraph equation, and the image-reconstruction algorithm is derived from the Fredholm integral equation of the first kind. Stability and computational efficiency of the introduced method are demonstrated for image reconstruction of one and two spherical luminescent objects embedded within a breastlike tissue phantom. Experimental measurements are simulated by the solution of the forward problem on a grid of 5x5 light guides attached to the surface of the phantom.     

A new projection-based iterative image reconstruction algorithm for dual-energy computed tomography

Dual-energy CT can be represented as the dual-energy equations by decomposing the linear attenuation coefficient of the X-ray scanned object into two material basis functions of photoelectric absorption and Compton scatter. To solve the dual-energy equations, in this paper, we apply the mean-value theorem for integrals and propose a new projection-based iterative algorithm. We discuss the convergence of the proposed algorithm and carry out various numerical simulations for demonstrating its feasibility.     

A method to modify coordinates of detectors in positron emission tomography systems

A new method to modify coordinates of detectors in any positron emission tomography (PET) system using a radioactive point source is developed. This method is based on selecting a centered detector in each detector block of PET and measuring coincidence counts between the two centered detectors in opposite detector blocks to find the coordinates of their LOR (Line of Response). Due to slight misalignment of detector positions, measured LORs may not intersect at a single point. Based on the proposed method, the coordinates of detectors can be measured with very high accuracy and the coordinate of the center of the gantry (which is normally the same as the center of field of view) can be defined correctly. The results of the application of our method to a small animal PET system (FinePET), which was recently developed at Tohoku University, Japan, are shown here. The method is expected to contribute to the design and development of PET systems which can realize a very high spatial resolution of less than 1 mm FWHM.     

On the use of tapered bismuth germanate crystals in positron emission tomography

An analytical model of gamma ray transport within the detectors of a positron camera, based on exponential absorption, has been developed and verified experimentally. The model has been used to study the intrinsic resolution obtained with scintillation detectors that have had their front corners removed. It is concluded that for crystals greater than 0.8 cm wide, tapering the face of the detector results in improved uniformity of resolution. Thus it is useful for medium and low resolution scanners. In these cases, it has been found that the loss in sensitivity resulting from the use of tapered crystals is less than that which occurs when septa are placed between crystals to achieve comparable uniformity of resolution. It has also been established that, in some instances, reducing the length of the detectors results in a more uniform resolution than that obtained with tapered crystals, although this leads to a loss in ring sensitivity and an increase in the detected scatter fraction.     

Assessment of testicular uptake in flourine‐18 fluorodeoxyglucose positron emission tomography/computed tomography

The testis shows physiologic uptake in flourine‐18 fluorodeoxyglucose (F‐18 FDG) positron emission tomography (PET). The purpose of this study was to examine the degree of FDG uptake of normal testis on PET/computed tomography (CT), especially in relationship to various clinical parameters. We obtained the maximum standardized uptake value (SUVmax) and the mean standardized uptake value (SUVmean) of each testis from PET/CT of 166 healthy men. We also calculated the testis‐to‐muscle (T/M) ratio, defined as the SUVmean of the testis divided by the SUVmean of the right thigh muscle. Subjects completed a questionnaire on their medical history and sexual habits, including frequency of sexual intercourse, latest date of sexual intercourse, and vasectomy history. We then evaluated the association between imaging parameters and sexual habits. The mean age of subjects was 48 ± 10 years (range, 21–74), and the mean volume of each testis was 17.1 ± 4.5 cm³ (range, 7.4–32.4). The mean SUVmax was 3.6 ± 0.5 (range, 2.2–5.5), the mean SUVmean was 2.5 ± 0.4 (range, 1.6–3.8), and the mean T/M ratio was 4.2 ± 0.8 (range, 2.3–7.3). Of the 166 subjects, 51 reported having sexual intercourse zero times per month and 109 had sexual intercourse at least per month. The interval between the latest date of sexual intercourse and PET/CT was less than 3 days in 48 subjects; between 3 days and 1 month in 58; and greater than 1 month in 60. Twenty subjects were currently vasectomized. The T/M ratio was significantly correlated with age (P = 0.040) but not with sexual habits. On the PET/CT, normal testicular FDG uptake decreased as age increased. © 2012 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 22, 245–249, 2012     

45Ti, a candidate for positron emission tomography: Study of the cyclotron production

The nuclear-physical properties of ⁴⁵Ti (half-life 3.08 h, 85% β⁺-decay, 15% electron capture, daughter isotope ⁴⁵Sc) make it suitable for positron emission tomography. ⁴⁵Ti can be produced at a small cyclotron in a high yield, 450 MBq μA⁻¹ h⁻¹, using the ⁴⁵Sc(p,n)⁴⁵Ti nuclear reaction on a natural scandium target. The excitation functions for the ⁴⁵Sc(p,n)⁴⁵Ti reaction were calculated by ALICE/91, ALICE/ASH, and TALYS-1.2 codes and compared to the existing data. Deposition of Sc₂O₃ on a Cu substrate was carried out by two sedimentation methods for the production of ⁴⁵Ti, and a simple method for radiochemical separation of radiotitanium was suggested. The targets were examined by SEM and subjected to thermal shock test. The optimal thicknesses of target was substantiated, and the production yield for the ⁴⁵Sc(p,n)⁴⁵Ti reaction was calculated.     

Three-dimensional plasma field reconstruction with multiobjective optimization emission spectral tomography

A novel emission spectral tomography algorithm based on multiobjective optimization is proposed. Its reconstruction results for asymmetrical emission coefficient fields are studied with computer simulation. The results show that this algorithm provides a significant improvement in reconstruction precision and convergence over traditional algorithms and is suitable for real-time reconstruction of an emission-coefficient field with incomplete data. In an experiment of the argon-arc plasma diagnosis, we adopted this algorithm and the spectrum relative-intensity method to obtain the three-dimensional distributions of temperature, ionization coefficient, and electron (ion) and atom densities.