[18F]fluorodeoxyglucose (FDG) Positron Emission Tomography (PET) has emerged as a crucial tool in identifying the epileptic focus, especially in cases where Magnetic Resonance Imaging (MRI) diagnosis yields indeterminate results. FDG PET can provide the metabolic information of glucose and help identify abnormal areas that are not easily found through MRI. However, the effectiveness of FDG PET-based assessment and diagnosis depends on the selection of a healthy control group. The healthy control group typically consists of healthy individuals similar to epilepsy patients in terms of age, gender, and other aspects for providing normal FDG PET data, which will be used as a reference for enhancing the accuracy and reliability of the epilepsy diagnosis. However, significant challenges arise when a healthy PET control group is unattainable. Yaakub et al. have previously introduced a Pix2PixGAN-based method for MRI to PET translation. This method used paired MRI and FDG PET scans from healthy individuals for training, and produced pseudo normal FDG PET images from patient MRIs that are subsequently used for lesion detection. However, this approach requires a large amount of high-quality, paired MRI and PET images from healthy control subjects, which may not always be available. In this study, we investigated unsupervised learning methods for unpaired MRI to PET translation for generating pseudo normal FDG PET for epileptic focus localization. Two deep learning methods, CycleGAN and SynDiff, were employed, and we found that diffusion-based method achieved improved performance in accurately localizing the epileptic focus.
KEYWORDS: Medical imaging, Positron emission tomography, Signal to noise ratio, Image quality, Magnetic resonance imaging, Signal detection, Image processing, Data modeling
Image-to-image translation is a common task in computer vision and has been rapidly increasing the impact on the field of medical imaging. Deep learning-based methods that employ conditional generative adversarial networks (cGANs), such as Pix2PixGAN, have been extensively explored to perform image-to-image translation tasks. However, when noisy medical image data are considered, such methods cannot be directly applied to produce clean images. Recently, an augmented GAN architecture named AmbientGAN has been proposed that can be trained on noisy measurement data to synthesize high-quality clean medical images. Inspired by AmbientGAN, in this work, we propose a new cGAN architecture, Ambient-Pix2PixGAN, for performing medical image-to-image translation tasks by use of noisy measurement data. Numerical studies that consider MRI-to-PET translation are conducted. Both traditional image quality metrics and task-based image quality metrics are employed to assess the proposed Ambient-Pix2PixGAN. It is demonstrated that our proposed Ambient-Pix2PixGAN can be successfully trained on noisy measurement data to produce high-quality translated images in target imaging modality.
Medical imaging systems that are designed for producing diagnostically informative images should be objectively assessed via task-based measures of image quality (IQ). Ideally, computation of task-based measures of IQ needs to account for all sources of randomness in the measurement data, including the variability in the ensemble of objects to be imaged. To address this need, stochastic object models (SOMs) that can generate an ensemble of synthesized objects or phantoms can be employed. Various mathematical SOMs or phantoms were developed that can interpretably synthesize objects, such as lumpy object models and parameterized torso phantoms. However, such SOMs that are purely mathematically defined may not be able to comprehensively capture realistic object variations. To establish realistic SOMs, it is desirable to use experimental data. An augmented generative adversarial network (GAN), AmbientGAN, was recently proposed for establishing SOMs from medical imaging measurements. However, it remains unclear to which extent the AmbientGAN-produced objects can be interpretably controlled. This work introduces a novel approach called AmbientCycleGAN that translates mathematical SOMs to realistic SOMs by use of noisy measurement data. Numerical studies that consider clustered lumpy background (CLB) models and real mammograms are conducted. It is demonstrated that our proposed method can stably establish SOMs based on mathematical models and noisy measurement data. Moreover, the ability of the proposed AmbientCycleGAN to interpretably control image features in the synthesized objects is investigated.
KEYWORDS: Education and training, Signal detection, Imaging systems, Signal attenuation, Breast, Binary data, Information operations, Image restoration, Image processing, Signal processing
PurposeThe objective assessment of image quality (IQ) has been advocated for the analysis and optimization of medical imaging systems. One method of computing such IQ metrics is through a numerical observer. The Hotelling observer (HO) is the optimal linear observer, but conventional methods for obtaining the HO can become intractable due to large image sizes or insufficient data. Channelized methods are sometimes employed in such circumstances to approximate the HO. The performance of such channelized methods varies, with different methods obtaining superior performance to others depending on the imaging conditions and detection task. A channelized HO method using an AE is presented and implemented across several tasks to characterize its performance.ApproachThe process for training an AE is demonstrated to be equivalent to developing a set of channels for approximating the HO. The efficiency of the learned AE-channels is increased by modifying the conventional AE loss function to incorporate task-relevant information. Multiple binary detection tasks involving lumpy and breast phantom backgrounds across varying dataset sizes are considered to evaluate the performance of the proposed method and compare to current state-of-the-art channelized methods. Additionally, the ability of the channelized methods to generalize to images outside of the training dataset is investigated.ResultsAE-learned channels are demonstrated to have comparable performance with other state-of-the-art channel methods in the detection studies and superior performance in the generalization studies. Incorporating a cleaner estimate of the signal for the detection task is also demonstrated to significantly improve the performance of the proposed method, particularly in datasets with fewer images.ConclusionsAEs are demonstrated to be capable of learning efficient channels for the HO. The resulting significant increase in detection performance for small dataset sizes when incorporating a signal prior holds promising implications for future assessments of imaging technologies.
Humans process the visual world with varying resolution across the visual field and sample information using eye movements to point the high-resolution central vision (fovea) to regions of interest. What eye movements optimize decisions in tasks such as target detection and localization? In 2005, Najemnik & Geisler proposed the Bayesian Ideal searcher (IS) that takes into account the foveated properties of human visual and employs the optimal fixation selection strategy to maximize visual search performance. In addition, they proposed a computationally simpler model, entropy limit minimization (ELM), that approximates the IS searcher. One limitation of these models is that they were developed with the assumption that the visual system’s internal responses across fixations are statistically independent. This assumption will not hold for search tasks such as in 2D medical images for which the external noise and anatomical noise results in correlations in internal responses across saccadic fixations (inter-saccade response correlations). In this work, we present image-computable foveated IS and ELM models that accommodate inter-saccade response correlations. We demonstrate that for static images, the optimal searchers that account for the inter-saccade correlations (i.e., IS-COR and ELM-COR) significantly outperform the traditional methods that ignore such correlations. Moreover, the novel ELM-COR achieves a similar performance as the IS-COR but runs about 18 times faster. Together, the IS-COR and ELM-COR extend the optimal searcher framework to evaulate human fixations for more realistic search tasks with inter-saccadic correlations.
Medical imaging systems are commonly assessed and optimized by use of objective measures of image quality (IQ). The performance of the Ideal Observer (IO) acting on imaging measurements has long been advocated as a figure-of-merit to guide the optimization of imaging systems. For computed imaging systems, the performance of the IO acting on imaging measurements also sets an upper bound on task-performance that no image reconstruction method can transcend. As such, estimation of IO performance can provide valuable guidance when designing data-acquisition techniques by enabling the identification of designs that will not permit the reconstruction of diagnostically useful images for a specified task-no matter how advanced the reconstruction method is or plausible the reconstructed images appear. The need for such analyses is urgent because of the ubiquitous development of deep learning-based image reconstruction methods and the fact that they are often not assessed by use of objective image quality measures. However, until recently, estimation of the IO was generally intractable when clinically relevant objects and imaging conditions were assumed. In this work, for the first time, estimates of the IO acting on tomographic imaging measurements were computed with consideration of realistic object variability to establish task-based performance bounds for image reconstruction methods. This was accomplished by use of a recently developed learning-based procedure. Numerical studies that were inspired by breast x-ray computed tomography were conducted to validate and demonstrate the approach. The effectiveness of the approximation method was validated on raw measurements for a signal-known-exactly and background-known-exactly (SKE/BKE) binary signal detection task. A signal-known-statistically and background-known-statistically (SKS/BKS) binary signal detection task was also addressed and the impact of the number of views on IO performance was assessed
It has been suggested that medical imaging systems should be evaluated and optimized by use of task-based measures of image quality (IQ). Task-based measures of IQ summarize the performance of an observer at a specific task (e.g., tumor detection). The Hotelling observer (HO) is a commonly employed numerical observer for evaluating and optimizing medical imaging systems. However, the computation of the HO can be intractable when huge covariance matrices of the image data need to be inverted. One way to address this issue is to apply a set of channels to the image data and subsequently compute the HO on the channelized data. When the channels are efficient, the HO performance can be approximated by the performance of the channelized Hotelling observer (CHO). However, it remains unclear how efficient channels can be learned and subsequently employed when performing image processing tasks. In this work, I propose a task-aware method for training denoising autoencoders (DAEs) for establishing efficient channels that can be employed for image denoising. It is demonstrated that the HO performance can be closely approximated by use of the proposed task-aware DAE-learned channels. In addition, the images produced by the proposed task-aware DAEs can achieve improved signal detectability evaluated by a foveated CHO, which was developed for modeling human visual systems.
In the last 5 to 10 years, there has been an enormous increase in the interest and use of network models in imaging. These are being considered for numerous imaging applications, including denoising, decision support, learned-feature selection, and many others. Network models “learn” solutions to imaging problems from labelled training data and an elaborate training regime. When a successful model is developed, it represents a computational algorithm for performing some task of interest. But it also encodes a solution to an imaging problem that may be intractable by conventional analytical means. Network models are therefore of interest for how they formulate a solution to a problem of interest. This work focuses on that process. We present two case studies in the analysis of neural networks. The first consists of a denoising network for digital breast tomosynthesis (DBT) images developed using a complex anatomical simulation of breast tissues and realistic x-ray transport physics. The second looks at a lesion detection network, also for DBT images, based on the same anatomical simulation model. For the denoising network, we find that it is very well represented by a linear operation that is effectively a Gaussian convolution kernel. The detection filter appears to be locally linear, but the filter profile appears to depend on what stimulus is used to probe the network. There does not appear to be any clear structure in quadratic components from reverse correlation. Overall, this study shows how regression and reverse-correlation techniques can be used to analyze network models.
Humans process visual information with varying resolution (foveated visual system) and explore images by orienting through eye movements the high-resolution fovea to points of interest. The Bayesian ideal searcher (IS) that employs complete knowledge of task-relevant information optimizes eye movement strategy and achieves the optimal search performance. The IS can be employed as an important tool to evaluate the optimality of human eye movements, and potentially provide guidance to improve human observer visual search strategies. Najemnik and Geisler (2005) derived an IS for backgrounds of spatial 1/f noise. The corresponding template responses follow Gaussian distributions and the optimal search strategy can be analytically determined. However, the computation of the IS can be intractable when considering more realistic and complex backgrounds such as medical images. Modern reinforcement learning methods, successfully applied to obtain optimal policy for a variety of tasks, do not require complete knowledge of the background generating functions and can be potentially applied to anatomical backgrounds. An important first step is to validate the optimality of the reinforcement learning method. In this study, we investigate the ability of a reinforcement learning method that employs Q-network to approximate the IS. We demonstrate that the search strategy corresponding to the Q-network is consistent with the IS search strategy. The findings show the potential of the reinforcement learning with Q-network approach to estimate optimal eye movement planning with real anatomical backgrounds.
KEYWORDS: Imaging systems, Medical imaging, Gallium nitride, Stochastic processes, Magnetic resonance imaging, Signal to noise ratio, Statistical modeling, Data acquisition, Systems modeling, Visualization
Purpose: To objectively assess new medical imaging technologies via computer-simulations, it is important to account for the variability in the ensemble of objects to be imaged. This source of variability can be described by stochastic object models (SOMs). It is generally desirable to establish SOMs from experimental imaging measurements acquired by use of a well-characterized imaging system, but this task has remained challenging.
Approach: A generative adversarial network (GAN)-based method that employs AmbientGANs with modern progressive or multiresolution training approaches is proposed. AmbientGANs established using the proposed training procedure are systematically validated in a controlled way using computer-simulated magnetic resonance imaging (MRI) data corresponding to a stylized imaging system. Emulated single-coil experimental MRI data are also employed to demonstrate the methods under less stylized conditions.
Results: The proposed AmbientGAN method can generate clean images when the imaging measurements are contaminated by measurement noise. When the imaging measurement data are incomplete, the proposed AmbientGAN can reliably learn the distribution of the measurement components of the objects.
Conclusions: Both visual examinations and quantitative analyses, including task-specific validations using the Hotelling observer, demonstrated that the proposed AmbientGAN method holds promise to establish realistic SOMs from imaging measurements.
KEYWORDS: Gallium nitride, 3D image processing, Medical imaging, Magnetic resonance imaging, Super resolution, Stochastic processes, Neuroimaging, Medical image reconstruction, Imaging systems, Image restoration
Generative adversarial networks (GANs) have proven useful for several medical imaging tasks, including image reconstruction and stochastic object model generation. Thus far, most of the work with GANs has been constrained to twodimensional images. Considering that medical imaging data are often inherently three-dimensional (3D), a 3D GAN would be a more principled way to synthesize realistic volumes. Training a 3D GAN is both computationally and memory intensive. However, prior work has not considered the anisotropic nature of many medical imaging systems. In this paper, the SlabGAN is proposed to reduce the inefficiencies associated with training a 3D GAN. The SlabGAN uses the progressive GAN architecture extended to 3D, but removes the requirement of the three dimensions being equal sizes. This permits the generation of anisotropic 3D volumes with large x and y dimensions. The SlabGAN is trained on MRI brain images from the fastMRI dataset to generate images of dimension 256×256×16. The x and y dimensions of these images are comparable to previously published results while requiring significantly fewer computational resources to generate. The trained SlabGAN is applicable to tasks such as 3D medical image reconstruction and thin-slice MR super resolution.
In this work, we focus on developing a channelized Hotelling observer (CHO) that estimates ideal linear observer performance on signal detection in images resulting from non-linear image reconstruction in computed tomography. In particular, many options on specifying the channel functions are explored. A hybrid channel model is proposed where a set of traditional Laguerre-Gauss functions are concatenated with a set of central pixel functions. This expanded channel set allows the CHO to perform robustly over a wide range of image reconstruction and system parameters. The application of this model observer to determining of the total-variation constrained least-squares algorithm yields images that are seen to favor detection of small, subtle signals.
KEYWORDS: Systems modeling, Stochastic processes, Imaging systems, 3D image processing, Gallium nitride, 3D metrology, Stereoscopy, Network architectures, Medical imaging, Magnetic resonance imaging
Medical imaging systems are commonly assessed and optimized by use of objective-measures of image quality (IQ) that quantify the performance of an observer at specific tasks. Variation in the objects to-be-imaged is an important source of variability that can significantly limit observer performance. This object variability can be described by stochastic object models (SOMs). In order to establish SOMs that can accurately model realistic object variability, it is desirable to use experimental data. To achieve this, an augmented generative adversarial network (GAN) architecture called AmbientGAN has been developed and investigated. However, AmbientGANs cannot be immediately trained by use of advanced GAN training methods such as the progressive growing of GANs (ProGANs). Therefore, the ability of AmbientGANs to establish realistic object models is limited. To circumvent this, a progressively-growing AmbientGAN (ProAmGAN) has been proposed. However, ProAmGANs are designed for generating two-dimensional (2D) images while medical imaging modalities are commonly employed for imaging three-dimensional (3D) objects. Moreover, ProAmGANs that employ traditional generator architectures lack the ability to control specific image features such as fine-scale textures that are frequently considered when optimizing imaging systems. In this study, we address these limitations by proposing two advanced AmbientGAN architectures: 3D ProAmGANs and Style-AmbientGANs (StyAmGANs). Stylized numerical studies involving magnetic resonance (MR) imaging systems are conducted. The ability of 3D ProAmGANs to learn 3D SOMs from imaging measurements and the ability of StyAmGANs to control fine-scale textures of synthesized objects are demonstrated.
The ideal observer (IO) sets an upper performance limit among all observers and has been advocated for use in assessing and optimizing imaging systems. For joint detection-estimation tasks, the estimation ROC (EROC) curve has been proposed for evaluating the performance of observers. However, in practice, it is generally difficult to accurately approximate the IO that maximizes the area under the EROC curve (AEROC) for a general detection-estimation task. In this study, a hybrid method that employs machine learning is proposed to accomplish this. Specifically, a hybrid approach is developed that combines a multi-task convolutional neural network (CNN) and a Markov-Chain Monte Carlo (MCMC) method in order to approximate the IO for detectionestimation tasks. The multi-task CNN is designed to estimate the likelihood ratio and the parameter vector, while the MCMC method is employed to compute the utility-weighted posterior mean of the parameter vector. The IO test statistic is subsequently formed as the product of the likelihood ratio and the posterior mean of the parameter vector. Computer simulation studies were conducted to validate the proposed method, which include backgroundknown-exactly (BKE) and background-known-statistically (BKS) tasks. The proposed method provides a new approach for approximating the IO and may enable the application of EROC analysis for optimizing imaging systems.
KEYWORDS: Neural networks, Image denoising, Denoising, Signal detection, Surgery, Signal attenuation, Quality measurement, Medical imaging applications, Medical imaging, Information operations
Deep neural network (DNN)-based image denoising methods have been proposed for use with medical images. These methods are commonly optimized and evaluated by use of traditional physical measures of image quality (IQ). However, the objective evaluation of IQ for such methods remains largely lacking. In this study, task-based IQ measures are used to evaluate the performance of DNN-based denoising methods. Specifically, we consider signal detection tasks under background-known-statistically conditions. The performance of the ideal observer (IO) and the Hotelling observer (HO) are quantified and detection efficiencies are computed to investigate the impact of the denoising operation on task performance. The experimental results show that, in the cases considered, the application of a denoising network generally results in a loss of task-relevant information. The impact of the depth of the denoising networks on task performance is also assessed. While mean squared error improved as the network depths were increased, signal detection performance degraded. These results highlight the need for the objective evaluation of IQ for DNN-based denoising technologies and may suggest future avenues for improving their effectiveness in medical imaging applications.
Medical imaging systems are commonly assessed by use of objective image quality measures. Supervised deep learning methods have been investigated to implement numerical observers for task-based image quality assessment. However, labeling large amounts of experimental data to train deep neural networks is tedious, expensive, and prone to subjective errors. Computer-simulated image data can potentially be employed to circumvent these issues; however, it is often difficult to computationally model complicated anatomical structures, noise sources, and the response of real-world imaging systems. Hence, simulated image data will generally possess physical and statistical differences from the experimental image data they seek to emulate. Within the context of machine learning, these differences between the sets of two images is referred to as domain shift. In this study, we propose and investigate the use of an adversarial domain adaptation method to mitigate the deleterious effects of domain shift between simulated and experimental image data for deep learning-based numerical observers (DL-NOs) that are trained on simulated images but applied to experimental ones. In the proposed method, a DL-NO will initially be trained on computer-simulated image data and subsequently adapted for use with experimental image data, without the need for any labeled experimental images. As a proof of concept, a binary signal detection task is considered. The success of this strategy as a function of the degree of domain shift present between the simulated and experimental image data is investigated.
In medical imaging systems, task-based metrics have been advocated as a means of evaluating image quality. Mathematical observers are one method of computing such metrics. Although the Bayesian Ideal Observer (IO) is optimal by definition, it is frequently intractable and non-linear. Linear approximations to the IO are sometimes employed to obtain task-based statistics when computing the IO is infeasible. The optimal linear observer for maximizing the SNR of the test statistic is the Hotelling Observer (HO). However, the computational cost for computing the HO increases with image size and becomes intractable for larger images. Channelized methods of reducing the dimensionality of the data before computing the HO have become popular, with efficient channels capable of approximating the HO’s performance at significantly reduced computational cost. State-of-the-art channels have been learned by using an autoencoder (AE) to encode data by employing a known signal template as the desired reconstruction, but the method is dependant on a high-quality estimate of the signal. An alternative to channels is approximating the test statistic directly using a feed-forward neural network (FFNN). However, this approach can overfit when the amount of training data is limited. In this work, a generalized method for learning channels utilizing an AE with dual losses (AEDL) is proposed. The AEDL framework jointly minimizes both task-specific and reconstruction losses to learn a set of efficient channels, even when the number of training images is relatively small. Preliminary results indicate that the proposed network outperforms state-of-the-art methods on the selected imaging task. Additionally, the AEDL framework suffers from less overfitting than the FFNN.
The Ideal Observer (IO) performance has been advocated when optimizing medical imaging systems for signal detection tasks. However, analytical computation of the IO test statistic is generally intractable. To approximate the IO test statistic, sampling-based methods that employ Markov-Chain Monte Carlo (MCMC) techniques have been developed. However, current applications of MCMC techniques have been limited to several object models such as a lumpy object model and a binary texture model, and it remains unclear how MCMC methods can be implemented with other more sophisticated object models. Deep learning methods that employ generative adversarial networks (GANs) hold great promise to learn stochastic object models (SOMs) from image data. In this study, we described a method to approximate the IO by applying MCMC techniques to SOMs learned by use of GANs. The proposed method can be employed with arbitrary object models that can be learned by use of GANs, thereby the domain of applicability of MCMC techniques for approximating the IO performance is extended. In this study, both signal-known-exactly (SKE) and signal-known-statistically (SKS) binary signal detection tasks are considered. The IO performance computed by the proposed method is compared to that computed by the conventional MCMC method. The advantages of the proposed method are discussed.
KEYWORDS: Magnetic resonance imaging, Data modeling, Inverse problems, Medical image reconstruction, Reconstruction algorithms, Image restoration, Medical diagnostics, Imaging systems
Medical image reconstruction is often an ill-posed inverse problem. In order to address such ill-posed inverse problems, prior knowledge of the sought after object property is usually incorporated by means of regularization. For example, sparsity-promoting regularization in a suitable transform domain is widely used to reconstruct images with diagnostic quality from noisy and/or incomplete medical data. However, sparsity-promoting regularization may not be able to comprehensively describe the actual prior information of the objects being imaged. Deep generative models, such as generative adversarial networks (GANs) have shown great promise in learning the underlying distribution of images. Prior distributions for images estimated using GANs have been employed as a means of regularization with impressive results in several linear inverse problems in computer vision that are also relevant to medical imaging. However, in practice, it can be difficult for a GAN to comprehensively describe prior distributions, which can potentially lead to a lack of fidelity between the reconstructed image and the observed data. Recently, an image-adaptive GAN-based reconstruction method (IAGAN) was proposed to guarantee stronger data consistency by adapting the trained generative model parameters to the observed measurements. In this work, for the first time, we apply the IAGAN method to reconstruct images from undersampled magnetic resonance imaging (MRI) measurements. A state-of-the-art GAN model called Progressive Growing of GANs (ProGAN) was trained on a large number of ground truth images from the NYU fastMRI dataset, and the learned generator was subsequently employed in the IAGAN framework to reconstruct high fidelity images from retrospectively undersampled experimental k-space data in the validation dataset. It is demonstrated that by use of the GAN-based reconstruction method with noisy and/or incomplete measurements, we can potentially recover fine structures in the object that are relevant for medical diagnosis that may be difficult to achieve using traditional reconstruction methods relying on sparsity-promoting penalties.
The objective optimization of medical imaging systems requires full characterization of all sources of randomness in the measured data, which includes the variability within the ensemble of objects to-be-imaged. This can be accomplished by establishing a stochastic object model (SOM) that describes the variability in the class of objects to-be-imaged. Generative adversarial networks (GANs) can be potentially useful to establish SOMs because they hold great promise to learn generative models that describe the variability within an ensemble of training data. However, because medical imaging systems record imaging measurements that are noisy and indirect representations of object properties, GANs cannot be directly applied to establish stochastic models of objects to-be-imaged. To address this issue, an augmented GAN architecture named AmbientGAN was developed to establish SOMs from noisy and indirect measurement data. However, because the adversarial training can be unstable, the applicability of the AmbientGAN can be potentially limited. In this work, we propose a novel training strategy|Progressive Growing of AmbientGANs (ProAGAN)|to stabilize the training of AmbientGANs for establishing SOMs from noisy and indirect imaging measurements. An idealized magnetic resonance (MR) imaging system and clinical MR brain images are considered. The proposed methodology is evaluated by comparing signal detection performance computed by use of ProAGAN-generated synthetic images and images that depict the true object properties.
Task-based measures of image quality (IQ) quantify the ability of an observer to perform a specific task. Such measures are employed for assessing and optimizing medical imaging systems. Although the Bayesian ideal observer is optimal by definition, it is frequently both non-linear and intractable. In such cases, linear observers are commonly employed. However, the optimal linear observer, the Hotelling observer (HO), becomes intractable when considering large images. Channelized methods have become popular for reducing the dimensionality of image data. In this work, we propose a novel method for determining efficient channels by learning them with autoencoders (AEs). Autoencoders are neural networks that can be employed to learn concise representations of data, frequently for the purposes of reducing dimensionality. We trained several AEs to encode task-specific information by modifying the standard loss function and examined the effect of hidden layer size and the use of tied/untied weights on the resulting representation accuracy. Subsequently, HOs were applied to both the original images and the dimensionality-reduced versions of them produced by the AEs. It was demonstrated that, for a suitable specification of the AE, the performance of the HO was relatively unaffected by the encoding of the image. However, the computational cost of inverting the covariance matrix was greatly reduced when the HO was applied with the encoded data due to its reduced dimensionality. Our findings suggest that AEs may represent an attractive alternative to the use of heuristic channels for reducing the dimensionality of image data when seeking to accurately approximate the performance of the HO on signal detection tasks.
In medical imaging, task-based measures of image quality (IQ) have been commonly employed to assess and optimize imaging systems. To evaluate task-based measures of IQ, the performance of an observer on a relevant task is quantified. For a binary signal detection task, the Bayesian Ideal Observer sets an upper performance limit in a sense that it maximizes the area under the receiver operating characteristic (ROC) curve (AUC). When a joint signal detection and localization (detection-localization) task is considered, the modified generalized likelihood ratio test (MGLRT) has been advocated as an optimal decision strategy to maximize the area under the localization ROC (LROC) curve (ALROC). However, analytical computation of likelihood ratios employed in the MGLRT is generally intractable. In this work, a supervised learning-based method that employs convolutional neural networks (CNNs) is developed and implemented for approximating the Ideal Observer that maximizes the area under the LROC curve for signal detection-localization tasks. A background-known-exactly (BKE) case was considered. The resulting LROC curve and ALROC value are compared to those produced by an analytical calculation.
The objective optimization of image-derived statistics, including the test statistic of an observer for specific decision tasks, requires a characterization of all sources of variability in the measured data. To accomplish this, it is necessary to establish a stochastic object model (SOM) that describes the variability within a group of objects to-be imaged. In order for the SOM to be realistic, it is desirable to establish it by use of experimental image data, as opposed to establishing it in a non-data-driven manner. Deep learning methods that employ generative adversarial networks (GANs) hold promise for learning SOMs that can generate images that match distributions of training image data. However, because experimental data recorded by an imaging system represent noisy and indirect measurements of the object, conventional GANs cannot be directly employed for this task. Recently, an augmented GAN architecture named AmbientGAN was proposed that can characterize a distribution of images from noisy and indirect measurements of them and knowledge of the measurement operator. In this work, for the first time, we investigate AmbientGANs for establishing SOMs by use of noisy imaging measurements. A canonical tomographic imaging system that is described by a two-dimensional Radon transform model is investigated. The AmbientGAN is evaluated by performing binary signal detection tasks that employ the generated images and true images.
KEYWORDS: Signal detection, Statistical analysis, Binary data, Machine learning, Signal to noise ratio, Information operations, Matrices, Quality measurement, Image quality, Medical imaging
Task-based measures of image quality (IQ) quantify the ability of an observer to perform a specific task. Such measures are commonly employed for assessing and optimizing medical imaging systems. In binary signal detection tasks, the Bayesian ideal observer (IO) sets an upper performance limit. However, the IO test statistic is generally intractable to compute when the log-likelihood ratio depends non-linearly on the measurement data. In such cases, the Hotelling observer (HO), which is the optimal linear observer, can be employed. However, traditional implementations of the HO require estimation and inversion of covariance matrices; for large images this can be computationally burdensome or even intractable. In this work, we describe a novel supervised learning- based method that employs artificial neural networks (ANNs) for estimating the HO test statistic and does not require estimation or inversion of covariance matrices. A signal-known-exactly and background-known-exactly (SKE/BKE) signal detection task is considered. The receiver operating characteristic (ROC) curve and Hotelling template corresponding to the proposed method are compared to the corresponding analytical solutions.
X-ray phase-contrast imaging methods exploit variations in an object’s 3D refractive index distribution to form projection or volumetric images of weakly absorbing objects. Such techniques can resolve subtle tissue structures by employing coherent imaging principles, but retain the ability of traditional (incoherent) X-ray methods to image deep into tissue. In this talk, we describe recent advancements in image formation methods for benchtop applications of X-ray phase-contrast imaging and tomography and present applications of pre-clinical in vivo imaging.
Edge illumination X-ray phase-contrast tomography (EIXPCT) is an emerging imaging technology capable of estimating the complex-valued refractive index distribution with laboratory-based X-ray sources. Conventional image reconstruction approaches for EIXPCT require multiple images to be acquired at each tomographic view angle. This contributes to prolonged data-acquisition times and potentially elevated radiation doses, which can hinder in-vivo applications. A new “single-shot” method has been proposed for joint reconstruction (JR) of the real and imaginary-valued components of the refractive index distribution from a tomographic data set that contains only a single image acquired at each view angle. The JR method does not place restrictions on the types of measurement data that it can be applied to and therefore enables the exploration of innovation single-shot data-acquisition designs. However, there remains an important need to identify data-acquisition designs that will permit accurate JR. In this study, innovative, JR-enabled, single-shot data-acquisition designs for EIXPCT are proposed and characterized quantitatively in simulation studies.
KEYWORDS: Statistical analysis, Data modeling, Binary data, Signal detection, Convolutional neural networks, Machine learning, Monte Carlo methods, Medical imaging, Image processing
It has been advocated that task-based measures of image quality (IQ) should be employed to evaluate and optimize imaging systems. Task-based measures of IQ quantify the performance of an observer on a medically relevant task. The Bayesian Ideal Observer (IO), which employs complete statistical information of the object and noise, achieves the upper limit of the performance for a binary signal classification task. However, computing the IO performance is generally analytically intractable and can be computationally burdensome when Markov-chain Monte Carlo (MCMC) techniques are employed. In this paper, supervised learning with convolutional neural networks (CNNs) is employed to approximate the IO test statistics for a signal-known-exactly and background-known-exactly (SKE/BKE) binary detection task. The receiver operating characteristic (ROC) curve and the area under the ROC curve (AUC) are compared to those produced by the analytically computed IO. The advantages of the proposed supervised learning approach for approximating the IO are demonstrated.
Photoacoustic computed tomography (PACT) and ultrasound computed tomography (USCT) are emerging modalities for breast imaging. As in all emerging imaging technologies, computer-simulation studies play a critically important role in developing and optimizing the designs of hardware and image reconstruction methods for PACT and USCT. Using computer-simulations, the parameters of an imaging system can be systematically and comprehensively explored in a way that is generally not possible through experimentation. When conducting such studies, numerical phantoms are employed to represent the physical properties of the patient or object to-be-imaged that influence the measured image data. It is highly desirable to utilize numerical phantoms that are realistic, especially when task-based measures of image quality are to be utilized to guide system design. However, most reported computer-simulation studies of PACT and USCT breast imaging employ simple numerical phantoms that oversimplify the complex anatomical structures in the human female breast. We develop and implement a methodology for generating anatomically realistic numerical breast phantoms from clinical contrast-enhanced magnetic resonance imaging data. The phantoms will depict vascular structures and the volumetric distribution of different tissue types in the breast. By assigning optical and acoustic parameters to different tissue structures, both optical and acoustic breast phantoms will be established for use in PACT and USCT studies.
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