Monte Carlo calculation of point-spread functions of Comptonscatter cameras

A technique has been devised for independently determining the energy and spatial resolution components of the point-spread functions of electronically collimated (Compton scatter) imaging devices using a Monte Carlo method. Monte Carlo runs are performed for the device being examined, and detailed records of exact energy loss and spatial coordinates of each interaction for each particle in the simulation are made. Taking resolution parameters from the known performance of the detectors, measured data is simulated by sampling from Gaussian distributions (for both energy and position) about the exact interaction energy and position in both detectors, for each Monte Carlo history. By back-projecting cones on an event-by-event basis and calculating the closest approach of the cone to the known source point using the simulated measurement data for one of the energy and position variables and the exact interaction data for the rest of the variables, the effect of the variable in question on the point spread can be isolated. In this paper the authors present the calculations of the contribution to the point-spread function of the energy and spatial resolution in each detector for various source positions for a representative Compton camera configuration     

Value of transthoracic echocardiography in predicting embolic events in active infective endocarditis. Duke Endocarditis Service

The purpose of this study was twofold: (1) to determine interobserver variability of echocardiographic characteristics of vegetations in patients with infective endocarditis, and (2) to assess the value of these vegetation characteristics in predicting embolic events. Although echocardiography contributes to the diagnosis of patients with infective endocarditis, its prognostic role in predicting embolic events is controversial. The echocardiograms of 41 patients with infective endocarditis were independently reviewed by 4 echocardiographers blinded to the clinical data. If a vegetation was present, the following characteristics were analyzed: involved site, size, mobility, shape, and pedunculated or sessile attachment. Each echocardiographer also made a "gestalt" estimate of embolic risk based on these vegetation characteristics. Interobserver agreement on vegetation characteristics and their relation to embolic events was then determined using kappa statistics and logistic regression analysis. Interobserver agreement was 98% with regard to echocardiographic vegetation presence and 97% with regard to the involved site. Of the 30 patients in whom vegetations were observed, complete observer agreement was achieved with regard to size in 22 (73%), mobility in 17 (57%), shape in 11 (37%), and attachment in 12 (40%). Vegetations with a maximal diameter of > 10 mm were associated with a 50% incidence of embolic events, compared with a 42% incidence of emboli in patients with vegetations measuring < or = 10 mm. Interobserver variability was great with respect to vegetation shape, mobility, and attachment characteristics. Echocardiographic vegetation characteristics were not helpful in defining the risk of embolic complications in patients with endocarditis.     

Application of adjoint sensitivity analysis to nuclear reactor fuel rod performance

Adjoint sensitivity analysis in nuclear fuel behavior modeling is extended to operate on the entire power history for both Zircaloy and stainless steel cladding via the computer codes FCODE-ALPHA/SS and SCODE/SS. The sensitivities of key variables to input parameters are found to be highly non-intuitive and strongly dependent on the fuel-clad gap status and the history of the fuel during the cycle. The sensitivities of five key variables, clad circumferential stress and strain, fission gas release, fuel centerline temperature and fuel-clad gap, to eleven input parameters are studied. The most important input parameters (yielding significances between 1 and 100) are fabricated clad inner and outer radii and fuel radius. The least important significances (less than 0.01) are the time since reactor start-up and fuel burnup densification rate. Intermediate to these are fabricated fuel porosity, linear heat generation rate, the power history scale factor, clad outer temperature, fill gas pressure and coolant pressure. Stainless steel and Zircaloy have similar sensitivities at start-up but these diverge as burnup proceeds due to the effect of the higher creep rate of Zircaloy which causes the system to be more responsive to changes in input parameters. The value of adjoint sensitivity analysis lies in its capability of uncovering dependencies of fuel variables on input parameters that cannot be determined by a sequential thought process.     

3D image reconstruction for a Compton SPECT camera model

Proposes a 3D image reconstruction algorithm for a 3D Compton camera being developed at the University of Michigan. The authors present a mathematical model of the transition matrix of the camera which exploits symmetries by using an adapted spatial sampling pattern in the object domain. For each projection angle, the sampling pattern is uniform over a set of equispaced nested hemispheres. By using this sampling pattern the system matrix is reduced to a product of a (approximately) block circulant matrix and a sparse interpolation matrix. This representation reduces the very high storage and computation requirement inherent to 3D reconstruction using transition matrix inversion methods. The authors geometrically optimize their hemispherical sampling and propose a 3D volumetric interpolation. Finally, the authors present a 3D image reconstruction method which uses the Gauss-Seidel algorithm to minimize a penalized least square objective