Maria Iatrou

CatSim: a new computer assisted tomography simulation environment

We present a new simulation environment for X-ray computed tomography, called CatSim. CatSim provides a research platform for GE researchers and collaborators to explore new reconstruction algorithms, CT architectures, and X-ray source or detector technologies. The main requirements for this simulator are accurate physics modeling, low computation times, and geometrical flexibility. CatSim allows simulating complex analytic phantoms, such as the FORBILD phantoms, including boxes, ellipsoids, elliptical cylinders, cones, and cut planes. CatSim incorporates polychromaticity, realistic quantum and electronic noise models, finite focal spot size and shape, finite detector cell size, detector cross-talk, detector lag or afterglow, bowtie filtration, finite detector efficiency, non-linear partial volume, scatter (variance-reduced Monte Carlo), and absorbed dose. We present an overview of CatSim along with a number of validation experiments.

Multi-source inverse geometry CT: a new system concept for x-ray computed tomography

Third-generation CT architectures are approaching fundamental limits. Spatial resolution is limited by the focal spot size and the detector cell size. Temporal resolution is limited by mechanical constraints on gantry rotation speed, and alternative geometries such as electron-beam CT and two-tube-two-detector CT come with severe tradeoffs in terms of image quality, dose-efficiency and complexity. Image noise is fundamentally linked to patient dose, and dose-efficiency is limited by finite detector efficiency and by limited spatio-temporal control over the X-ray flux. Finally, volumetric coverage is limited by detector size, scattered radiation, conebeam artifacts, Heel effect, and helical over-scan. We propose a new concept, multi-source inverse geometry CT, which allows CT to break through several of the above limitations. The proposed architecture has several advantages compared to third-generation CT: the detector is small and can have a high detection efficiency, the optical spot size is more consistent throughout the field-of-view, scatter is minimized even when eliminating the anti-scatter grid, the X-ray flux from each source can be modulated independently to achieve an optimal noise-dose tradeoff, and the geometry offers unlimited coverage without cone-beam artifacts. In this work we demonstrate the advantages of multi-source inverse geometry CT using computer simulations.

3D implementation of Scatter Estimation in 3D PET

Successful 3D imaging requires accurate and robust methods for scatter estimation and correction. We developed a computationally efficient fully 3D approach modeling both the axial and trans-axial scatter components. Simulation results showed good agreement with the Monte Carlo scatter and improved image quality (IQ). We tested the proposed algorithm on clinical data with similar IQ improvements.

Fully 3D PET Iterative Reconstruction Using Distance-Driven Projectors and Native Scanner Geometry

Incorporating all data corrections into the system model optimizes image quality in statistical iterative PET image reconstruction. We have previously shown that including attenuation, randoms and scatter in the forward 3D iterative model results in faster convergence and improved image quality for ML-OSEM. This paper extends this work to allow the accurate modeling of crystal efficiency, detector deadtime, and the native block-based detector geometry. In order to model these effects, it is necessary to perform forward and back-projections directly from image space to the projection geometry of the PET scanner, rather than to an idealized, equally spaced projection space. We have modified the distance-driven projectors to accurately model both the uneven spacing of the sinogram due to the ring curvature as well as the gaps resulting from the block structure of the scanner. This results in a reconstruction method, which can incorporate the crystal efficiency and block deadtime effects into the forward system model while maintaining the fast reconstruction times enabled by the distance driven projector design. Results on the GE Discovery STEtrade scanner show improvements in image resolution consistent with removing the interpolative smoothing of the data into the equally spaced projection space.

Coronary Artery Angiography Using Multislice Computed Tomography Images

Multislice CT scanners are the newest class of CT scanners and they have not one but many detectors. These scanners can acquire up to 4 slices of data from the body in the same time it takes a single-slice CT scanner to acquire one. Multislice CT allows for rapid cardiac imaging during a single breath-hold. A multislice scanner operated in helical mode provides information that can be used to reconstruct 3D cardiac images in arbitrary phases of the cardiac cycle. A 71-year-old man with hypertension, hypercholesterolemia, and known aortic and peripheral vascular disease was imaged with a LightSpeed 4-slice, multislice CT scanner (GE Medical Systems). Ten minutes before the cardiac scan, the patient received intravenous contrast material (150 mL of 300 mgI/mL) for a CT study of his abdomen. The cardiac …

Inverse geometry CT: The next-generation CT architecture?

We present a new system architecture for X-ray computed tomography (CT). A multi-source inverse-geometry CT scanner is composed of a large distributed X-ray source with an array of discrete electron emitters and focal spots, and a high frame-rate flat-panel X-ray detector. In this work we study the advantages and the challenges of this new architecture. We predict potential breakthroughs in volumetric coverage, dose efficiency, and spatial resolution. We also present experimental results obtained with a universal benchtop system.

Out-of-field scatter estimation in 3D whole body PET

Many implementations of model based scatter correction (MBSC) are based on the single scatter simulation (SSS) formulation within the scan field-of-view (FOV). A fully 3D approach that models both the axial and trans-axial scatter components can accurately model scatter from hot regions in neighboring slices and outside the scan FOV resulting in greater quantitative accuracy. Herein we discuss how to incorporate the estimation of out-of-field scatter in fully 3D MBSC.

A 3D study comparing filtered backprojection, weighted least squares, and penalized weighted least squares for CT reconstruction

We have extended 2D weighted least squares and penalized weighted least squares transmission reconstruction methods (WLSTR and PWLSTR respectively) to 3D to explore their impact on image noise and spatial resolution. 3D sinograms of circular and elliptical water cylinders were simulated using a conventional third generation CT system geometry. Data were generated using the realistic CT simulation software CATSIM. Resolution-Noise curves demonstrated that for high contrast wires the in-plane resolution can be improved using PWLS with a Huber-like prior compared to the resolution of the respective FBP image, maintaining the noise, or the noise can be reduced matching the resolution. The achieved noise reduction is greater for the elliptical cylinder than for the circular cylinder. For both phantoms we observed modest degradation in the xz-resolution compared to the in-plane resolution (for equal noise levels). Although in the case of the circular cylinder, improved resolution in the xz-plane is still achievable when compared to the respective resolution in the xz-plane observed in the FBP images (for equal noise levels), the resolution in the xz-plane for the elliptical cylinder is worse than the respective resolution of the FBP images. We present a method and results for resolution recovery in the latter case. Similarly to the previous 2D results, the Huber- like prior resulted in better resolution-noise curves than the quadratic. Furthermore, the use of priors reduces image noise significantly compared to the results generated without a prior.

Comparison of two 3D implementations of TOF scatter estimation in 3D PET

Time-of-Flight (TOF) PET requires the development of TOF scatter estimation methods. Two different approaches for modeling TOF 3D scatter are presented in this paper. We performed Monte Carlo simulations to compare the two methods. Both methods resulted in good agreement with Monte Carlo simulations. In addition, the second method is independent of the number of timing bins used and therefore, computationally less intensive.

Iterative reconstruction for multi-source inverse geometry CT: a feasibility study

In a 3rd generation CT system, a single source projects the entire field of view (FOV) onto a large detector opposite to the source. In multi-source inverse geometry CT imaging, a multitude of sources sequentially project complementary parts of the FOV on a much smaller detector. These sources may be distributed in both the trans-axial and axial directions and jointly cover the entire FOV. Multi-source CT has several important advantages, including large axial coverage, improved dose-efficiency, and improved spatial resolution. One of the challenges of this concept is to ensure that no artifacts emerge in the reconstructed images where the sampling switches from one source to the next. This work studies iterative reconstruction for multi-source imaging and focuses on the appearance of such artifacts. For that purpose, phantom data are simulated using a realistic multi-source CT geometry, iteratively reconstructed and inspected for artifact content. More realistic experiments using rebinned clinical datasets (emulating a multi-source CT system) have also been performed. The results confirm the feasibility of artifact-free multi-source CT imaging in both full-scan and half-scan situations.