Fan-beam reconstruction algorithm for a spatially varying focal length collimator

Fan-beam collimators are used in single-photon-emission computed tomography (SPECT) to improve the sensitivity for imaging of small organs. The disadvantage of fan-beam collimation is the truncation of projection data surrounding the organ of interest or, in those cases of imaging large patients, of the organ itself, producing reconstruction artifacts. A spatially varying focal length fan-beam collimator has been proposed to eliminate the truncation problem and to maintain good sensitivity for the organ of interest. The collimator is constructed so that the shortest focal lengths are located at the center of the collimator and the longest focal length is located at the periphery. The focal length is assumed to increase monotonically toward the edge of the collimator. A reconstruction algorithm for this type of fan-beam collimation, expressed as an infinite series of convolutions followed by one backprojection, is presented. Simulations show that only a small number of N terms in the series is needed to obtain high-quality reconstructions. Computer simulations showed that if the focal length function is smooth, the reconstructions are free of artifacts.     

Correction for ambiguous solutions in factor analysis using a penalized least squares objective

Factor analysis is a powerful tool used for the analysis of dynamic studies. One of the major drawbacks of factor analysis of dynamic structures (FADS) is that the solution is not mathematically unique when only nonnegativity constraints are used to determine factors and factor coefficients. In this paper, a method to correct for ambiguous FADS solutions has been developed. A nonambiguous solution (to within certain scaling factors) is obtained by constructing and minimizing a new objective function. The most common objective function consists of a least squares term that when minimized with nonnegativity constraints, forces agreement between the applied factor model and the measured data. In our method, this objective function is modified by adding a term that penalizes multiple components in the images of the factor coefficients. Due to nonuniqueness effects, these factor coefficients consist of more than one physiological component. The technique was tested on computer simulations, an experimental canine cardiac study using 99mTc-teboroxime, and a patient planar 99mTc-MAG3 renal study. The results show that the technique works well in comparison to the truth in computer simulations and to region of interest (ROI) measurements in the experimental studies.     

Unmatched projector/backprojector pairs in an iterative reconstruction algorithm

Computational burden is a major concern when an iterative algorithm is used to reconstruct a three-dimensional (3-D) image with attenuation, detector response, and scatter corrections. Most of the computation time is spent executing the projector and backprojector of an iterative algorithm. Usually, the projector and the backprojector are transposed operators of each other. The projector should model the imaging geometry and physics as accurately as possible. Some researchers have used backprojectors that are computationally less expensive than the projectors to reduce computation time. This paper points out that valid backprojectors should satisfy a condition that the projector/backprojector matrix must not contain negative eigen-values. This paper also investigates the effects when unmatched projector/backprojector pairs are used.     

Direct least-squares estimation of spatiotemporal distributions from dynamic SPECT projections using a spatial segmentation and temporal B-splines

Artifacts can result when reconstructing a dynamic image sequence from inconsistent, as well as insufficient and truncated, cone beam single photon emission computed tomography (SPECT) projection data acquired by a slowly rotating gantry. The artifacts can lead to biases in kinetic model parameters estimated from time-activity curves generated by overlaying volumes of interest on the images. However, the biases in time-activity curve estimates and subsequent kinetic parameter estimates can be reduced significantly by first modeling the spatial and temporal distribution of the radiopharmaceutical throughout the projected field of view, and then estimating the time-activity curves directly from the projections. This approach is potentially useful for clinical SPECT studies involving slowly rotating gantries, particularly those using a single-detector system or body contouring orbits with a multidetector system. We have implemented computationally efficient methods for fully four-dimensional (4-D) direct estimation of spatiotemporal distributions from dynamic SPECT projection data. Temporal B-splines providing various orders of temporal continuity, as well as various time samplings, were used to model the time-activity curves for segmented blood pool and tissue volumes in simulated cone beam and parallel beam cardiac data acquisitions. Least-squares estimates of time-activity curves were obtained quickly using a workstation. Given faithful spatial modeling, accurate curve estimates were obtained using cubic, quadratic, or linear B-splines and a relatively rapid time sampling during initial tracer uptake. From these curves, kinetic parameters were estimated accurately for noiseless data and with some bias for noisy data. A preliminary study of spatial segmentation errors showed that spatial model mismatch adversely affected quantitative accuracy, but also resulted in structured errors (projected model versus raw data) that were easily detected in our simulations. This suggests iterative refinement of the spatial model to reduce structured errors as an area of future research.     

Lobbying friends and foes in climate policy: The case of business and environmental interest groups in the European Union

Drawing on two conflicting hypotheses from the theoretical literature on lobbying, I consider the strategies applied by interest groups lobbying to influence climate policy in the European Union (EU). The first hypothesis claims that interest groups lobby their ‘friends’, decision-makers with positions similar to their own. The second claims that interest groups lobby their ‘foes’, decision-makers with positions opposed to their own. Using interviews with lobbyists and decision-makers, I demonstrate that in the field of climate policy, interest groups in the EU lobby both friends and foes, but under different conditions. Moreover, I find that the interest groups’ motives are not always in line with the theoretical hypotheses. Interest groups lobby their friends on single policy decisions to exchange information, to further a common cause and to exert pressure, and their foes because a foe on one issue might prove to be a friend on another issue. Interest groups direct general lobbying towards both friends and foes. This paper provides a new empirical contribution to a literature that has so far been heavily dominated by studies focusing on lobbying in the US.     

A study of apparent apical defects in attenuation corrected cardiacSPECT

There are a number of factors that affect the intensity of the apical region in cardiac SPECT images, which sometime lead to apparent defects in this region. In some patient studies, images reconstructed with non-uniform attenuation correction appear to have a significant decrease of apical intensity, whereas images reconstructed without such correction have only a slight decrease. This phenomenon has been evaluated using a series of simulations, phantom experiments, and a patient study. The effects of a number of factors upon the severity of the apparent apical defects were studied, including: partial volume effect, non-uniform attenuation, and depth-dependent detector response. Simulation and phantom results indicate that (1) since the apex is generally less attenuated than the rest of the heart wall, attenuation correction boosts the intensity of non-apical regions more than that of the apex (by as much as 20%); and (2) partial volume effects due to discretization inadequacies and finite spatial resolution may cause as much as a 26% decrease in reconstructed intensity in the apical region. These two effects, in combination, may partially explain the presence of apparent apical defects when attenuation correction is performed. In addition, detector response compensation consistently reduced the severity of apparent apical defects. These results agree with and help to explain the occurrence of apparent apical defects observed in patient data     

An SVD study of truncated transmission data in SPECT

Even though a noiseless, band-limited function is uniquely determined by its values in a local region, single photon emission computed tomography (SPECT) projections are not band-limited, and unmeasured projections may not be possible to be exactly estimated from the measured data. Projections from all views should be considered simultaneously and are modeled as a set of linear equations. The singular value decomposition (SVD) method is used to analyze and solve the equations. It is shown that truncation does not always result in an underdetermined problem, yet the problem may be ill-conditioned. An inaccurate pixel model may cause reconstruction artifacts via mismatch between the measured data and the modeled projections     

High-throughput data analysis for detecting and identifying differences between samples in GC/MS-based metabolomic analyses

In metabolomics, the objective is to identify differences in metabolite profiles between samples. A widely used tool in metabolomics investigations is gas chromatography-mass spectrometry (GC/MS). More than 400 compounds can be detected in a single analysis, if overlapping GC/MS peaks are deconvoluted. However, the deconvolution process is time-consuming and difficult to automate, and additional processing is needed in order to compare samples. Therefore, there is a need to improve and automate the data processing strategy for data generated in GC/MS-based metabolomics; if not, the processing step will be a major bottleneck for high-throughput analyses. Here we describe a new semiautomated strategy using a hierarchical multivariate curve resolution approach that processes all samples simultaneously. The presented strategy generates (after appropriate treatment, e.g., multivariate analysis) tables of all the detected metabolites that differ in relative concentrations between samples. The processing of 70 samples took similar time to that of the GC/TOFMS analyses of the samples. The strategy has been validated using two different sets of samples: a complex mixture of standard compounds and Arabidopsis samples.     

Outcome of pregnancy after conization

The outcome of 635 pregnancies in 197 women before and after conization is reported. After conization a significant increase in premature deliveries occurred in young women (age 21-25), nulliparae representing the highest risk. The rate of premature deliveries in this age group was 4.4% before conization and 30.6% after conization. No such increase could be found in any other age group. Labor was notably short in non-parous women and in parous women it started with rupture of the membranes significantly more often after conization. In addition, a significant increase in the frequency of cervical stenosis was observed, but this was of clinical significance in only one case. Treatment in the form of prophylactic cerclage did not reduce the incidence of either premature delivery or of late spontaneous abortion.