Conference proceedings. International Conference on Image Formation in X-Ray Computed Tomography最新文献

Interest in spectral CT for diagnostics and therapy evaluation has been growing. Acquisitions of data from distinct energy spectra provide, among other advantages, quantitative density estimations for multiple materials. We introduce a novel spectral CT concept that includes a fine-pitch grating for prefiltration of the x-ray beam. The attenuation behavior of this grating changes significantly if x-rays are slightly angled in relation to the grating structures. To apply such an angle (i.e. switch between the different filtrations) we propose a fast, controllable, and precise solution by moving the focal spot of the x-ray tube. In this work, we performed preliminary evaluations with a grating prototype on a CT test bench. Our results include x-ray spectrometer measurements that reveal diverse and controllable spectral shaping between 4° and 6° for a specific grating design. Additional experiments with a contrast agent phantom illustrated the capability to decompose clinically relevant iodine concentrations (5, 10, 20, and 50mg/mL) - demonstrating the feasibility of the grating-based approach. Ongoing and future studies will investigate the potential of this novel concept as a relatively simple upgrade to standard energy-integrating CT.

Detector lag and gantry motion during x-ray exposure and integration both result in azimuthal blurring in CT reconstructions. These effects can degrade image quality both for high-resolution features as well as low-contrast details. In this work we consider a forward model for model-based iterative reconstruction (MBIR) that is sufficiently general to accommodate both of these physical effects. We integrate this forward model in a penalized, weighted, nonlinear least-square style objective function for joint reconstruction and correction of these blur effects. We show that modeling detector lag can reduce/remove the characteristic lag artifacts in head imaging in both a simulation study and physical experiments. Similarly, we show that azimuthal blur ordinarily introduced by gantry motion can be mitigated with proper reconstruction models. In particular, we find the largest image quality improvement at the periphery of the field-of-view where gantry motion artifacts are most pronounced. These experiments illustrate the generality of the underlying forward model, suggesting the potential application in modeling a number of physical effects that are traditionally ignored or mitigated through pre-corrections to measurement data.

This work reports experimental results of dynamic fluence field modulation (FFM) using a dual multiple aperture devices (MAD) system. MAD filters use Moiré patterns produced by relative motions between two sets of thin, highly attenuating tungsten bars of varying widths and spacings. Each MAD was affixed to a linear actuator and installed on an experimental cone-beam CT bench. Phantom-specific FFM profiles were designed based on a flatness and minimum mean variance objectives and realized through a combination of MAD translations and pulse width modulation at a constant tube current. To properly correct for gains associated with the MAD filters, a correction algorithm was designed to account for focal spot shifts during scanning, as well as spectral effects from incomplete blockage of x-rays by the tungsten bars. The FFM designs were demonstrated in an elliptical phantom (25.8×14.1 cm). Variance and noise power spectrum (NPS) analysis was performed on the resulting reconstructions. While conventionalgain correction produced reconstructions with high frequency ring artifacts in axial slices, the proposed correction algorithm effectively removed such artifacts while preserving phantom details. Fluence field designs for the elliptical phantom were achievedusing relative MAD motions over a 0.44 mm range, and measured beam profiles closely approximated the theoretically computed target profiles. The noise properties of the resulting reconstructions behave as expected: a flat detected fluence criterion yields nearly isotropic NPS and more homogeneous variance across the reconstruction as compared to an unmodulated scan; the minimum mean variance FFM results in lower mean variance compared to both the unmodulated and flat-field patterns at approximately matched total bare-beam fluence. These results suggest that a dual-MAD CT is an effective approach to provide fluence and image quality control and that can potentially accommodate a wide range of phantoms and design objectives.

Interior tomography is promising approach for retaining high quality CT images within a volume-of-interest (VOI) while reducing the total patient dose. A static collimating filter can only image a centered symmetric VOI, which requires careful patient positioning and may be suboptimal for many clinical applications. Multiple aperture devices (MADs) are an emerging technology based on sequential binary filters that can provide a wide range of fluence patterns that may be adjusted dynamically with relatively small motions. In this work, we introduce a general approach for VOI imaging using MAD-based fluence field modulation (FFM). Physical experiments using a CT test bench are conducted illustrating off-center x-ray beam control for imaging the spine in an abdominal phantom. Image quality and dose metrics are computed for both standard full-field CT and VOI CT. We find that the image quality within the VOI can be preserved for VOI CT with a significant drop in integral dose as compared with a standard full-field protocol.

Spectral CT with multiple contrast agents has been enabled by energy-discriminating detectors with multiple spectral channels. We propose a new approach that uses spatial-spectral filters to provide multiple beamlets with different incident spectra for spectral channels based on "source-side" control. Since these spatial-spectral filters yield spectral channels that are sparse, we adopt model-based material decomposition to directly reconstruct material densities from projection data. Simulation studies in three-and four-material decomposition experiments show the underlying feasibility of the spatial-spectral filtering technique. This methodology has the potential to facilitate imaging of multiple contrast agents simultaneously with relatively simple hardware, or to improve spectral CT performance via combination with other established spectral CT methods for additional control and flexibility.

A Multiple Aperture Device (MAD) is a novel x-ray beam modulator that uses binary filtration on a fine scale to spatially modulate an x-ray beam. Using two MADs in series enables a large variety of fluence profiles by shifting the MADS relative to each other. This work details the design and control of dual MADs for a specific class of desired fluence patterns. Specifically, models of MAD operation are integrated into a best fit objective followed by CMA-ES optimization. To illustrate this framework we demonstrate the design process for an abdominal phantom with the goal of uniform detected signal. Achievable fluence profiles show good agreement with target fluence profiles, and the ability to flatten projections when a phantom is scanned is demonstrated. Simulated data reconstruction using traditional tube current modulation (TCM) and MAD filtering with TCM are investigated with the dual MAD system demonstrating more uniformity in noise and illustrating the potential for dose reduction under a maximum noise level constraint.

Prompt and reliable detection of acute intracranial hemorrhage (ICH) is critical to treatment of a number of neurological disorders. Cone-beam CT (CBCT) systems are potentially suitable for detecting ICH (contrast 40-80 HU, size down to 1 mm) at the point of care but face major challenges in image quality requirements. Statistical reconstruction demonstrates improved noise-resolution tradeoffs in CBCT head imaging, but its capability in improving image quality with respect to the task of ICH detection remains to be fully investigated. Moreover, statistical reconstruction typically exhibits nonuniform spatial resolution and noise characteristics, leading to spatially varying detectability of ICH for a conventional penalty. In this work, we propose a spatially varying penalty design that maximizes detectability of ICH at each location throughout the image. We leverage theoretical analysis of spatial resolution and noise for a penalized weighted least-squares (PWLS) estimator, and employ a task-based imaging performance descriptor in terms of detectability index using a nonprewhitening observer model. Performance prediction was validated using a 3D anthropomorphic head phantom. The proposed penalty achieved superior detectability throughout the head and improved detectability in regions adjacent to the skull base by ~10% compared to a conventional uniform penalty. PWLS reconstruction with the proposed penalty demonstrated excellent visualization of simulated ICH in different regions of the head and provides further support for development of dedicated CBCT head scanning at the point-of-care in the neuro ICU and OR.

Flat-panel cone-beam CT (CBCT) has been applied clinically in a number of high-resolution applications. Increasing geometric magnification can potentially improve resolution, but also increases blur due to an extended x-ray focal-spot. We present a shift-variant focal-spot blur model and incorporate it into a model-based iterative-reconstruction algorithm. We apply this algorithm to simulation and CBCT test-bench data. In a trabecular bone simulation study, we find traditional reconstruction approaches without a blur model exhibit shift-variant resolution properties that depend greatly on the acquisition protocol (e.g. short vs. full scans) and the anode angles of the rays used to reconstruct a particular region. For physical CBCT experiments focal spot blur was characterized and a spatial resolution phantom was scanned and reconstructed. In both experiments image quality using the shift-variant model was significantly improved over approaches that modeled no blur or only a shift-invariant blur, suggesting a potential means to overcome traditional CBCT spatial resolution and system design limitations.

3D volumetric CT images hold the potential to become a rich source of information for 3D organ segmentation and far exceed that made available through 2D radiograph images. Acquiring and generating 3D volumetric images for scan preparation purposes, i.e. 3D scout, while delivering radiation dose equivalent to conventional 2D radiograph is challenging. We explore various acquisition parameters and post-processing methods to reduce dose of a 3D scout while reducing the noise and maintaining the edge strength around the target organ. We demonstrated that similar edge strength and noise to the conventional dose CT scan can be achieved with 3D scout acquisition and post-processing while being dose neutral to a 2D scout acquisition.

Computed Tomography (CT) has been in clinical use for several decades. The number of CT scans has increased significantly worldwide, which results in increased radiation dose delivered to the general population. Many technologies have been developed to minimize the dose from CT scans, including scanner hardware improvements, task-specific protocol design and advanced reconstruction algorithms. In this study, we focused on selection of X-ray tube voltage and filtration to achieve optimal dose efficiency given required image quality, more specifically the contrast to noise ratio. Our approach differs from previous studies in two aspects. Typically, Monte-Carlo simulation is used to estimate dose in simulations, but this is computationally costly. We instead use a projection-domain dose estimation method. No image reconstruction is required for the projection-domain method, which further simplifies the analysis. This study also includes tantalum, a new contrast agent, in addition to soft tissue (water), bone and iodine contrast. Optimal tube voltages and filtration are identified as a function of phantom size. The simulation analysis is confirmed with a limited phantom study.