Search

Background: Few studies have examined variation in the associations between heat waves and mortality in an international context. Objectives: We aimed to systematically examine the impacts of heat waves on mortality with lag effects internationally. Methods: We collected daily data of temperature and mortality from 400 communities in 18 countries/regions and defined 12 types of heat waves by combining community-specific daily mean temperature [greater than or equal to] 90th, 92.5th, 95th, and 97.5th percentiles of temperature with duration [greater than or equal to] 2, 3, and 4 d. We used time-series analyses to estimate the community-specific heat wave-mortality relation over lags of 0-10 d. Then, we applied meta-analysis to pool heat wave effects at the country level for cumulative and lag effects for each type of heat wave definition. Results: Heat waves of all definitions had significant cumulative associations with mortality in all countries, but varied by community. The higher the temperature threshold used to define heat waves, the higher heat wave associations on mortality. However, heat wave duration did not modify the impacts. The association between heat waves and mortality appeared acutely and lasted for 3 and 4 d. Heat waves had higher associations with mortality in moderate cold and moderate hot areas than cold and hot areas. There were no added effects of heat waves on mortality in all countries/regions, except for Brazil, Moldova, and Taiwan. Heat waves defined by daily mean and maximum temperatures produced similar heat wave-mortality associations, but not daily minimum temperature. Conclusions: Results indicate that high temperatures create a substantial health burden, and effects of high temperatures over consecutive days are similar to what would be experienced if high temperature days occurred independently. People living in moderate cold and moderate hot areas are more sensitive to heat waves than those living in cold and hot areas. Daily mean and maximum temperatures had similar ability to define heat waves rather than minimum temperature. https://doi.org/10.1289/EHP1026, Introduction Globally, there is increasing interest in assessing heat wave-related health effects after major heat waves in the United States and Europe, for example, Chicago in 1995 and Europe in [...]

Recently, there has been much progress in algorithm development for image reconstruction in cone-beam computed tomography (CT). Current algorithms, including the chord-based algorithms, now accept minimal data sets for obtaining images on volume regions-of-interest (ROIs) thereby potentially allowing for reduction of X-ray dose in diagnostic CT. As these developments are relatively new, little effort has been directed at investigating the response of the resulting algorithm implementations to physical factors such as data noise. In this paper, we perform an investigation on the noise properties of ROI images reconstructed by using chord-based algorithms for different scanning configurations. We find that, for the cases under study, the chord-based algorithms yield images with comparable quality. Additionally, it is observed that, in many situations, large data sets contain extraneous data that may not reduce the ROI-image variances. Index Terms--Chord, computed tomography (CT), cone-beam CT, noise, reconstruction.

BACKGROUND: The evidence and method are limited for the associations between mortality and temperature variability (TV) within or between days. OBJECTIVES: We developed a novel method to calculate TV and investigated TV-mortality associations using a large multicountry data set. METHODS: We collected daily data for temperature and mortality from 372 locations in 12 countries/regions (Australia, Brazil, Canada, China, Japan, Moldova, South Korea, Spain, Taiwan, Thailand, the United Kingdom, and the United States). We calculated TV from the standard deviation of the minimum and maximum temperatures during the exposure days. Two-stage analyses were used to assess the relationship between TV and mortality. In the first stage, a Poisson regression model allowing over-dispersion was used to estimate the community-specific TV-mortality relationship, after controlling for potential confounders. In the second stage, a meta-analysis was used to pool the effect estimates within each country. RESULTS: There was a significant association between TV and mortality in all countries, even after controlling for the effects of daily mean temperature. In stratified analyses, TV was still significantly associated with mortality in cold, hot, and moderate seasons. Mortality risks related to TV were higher in hot areas than in cold areas when using short TV exposures (0-1 days), whereas TV-related mortality risks were higher in moderate areas than in cold and hot areas when using longer TV exposures (0-7 days). CONCLUSIONS: The results indicate that more attention should be paid to unstable weather conditions in order to protect health. These findings may have implications for developing public health policies to manage health risks of climate change. http://dx.doi.org/10.1289/EHP149, Introduction Time series data on daily air pollution concentrations, weather conditions, and daily measures of health outcomes (e.g., mortality, hospital admissions), have been used to assess how environmental factors may [...]

The circular scanning trajectory is one of the most widely adopted data-acquisition configurations in computed tomography (CT). The Feldkamp, Davis, Kress (FDK) algorithm and its various modifications have been developed for reconstructing approximately three-dimensional images from circular cone-beam data. When data contain transverse truncations, however, these algorithms may reconstruct images with significant truncation artifacts. It is of practical significance to develop algorithms that can reconstruct region-of-interest (ROI) images from truncated circular cone-beam data that are free of truncation artifacts and that have an accuracy comparable to that obtained from nontruncated cone-beam data. In this work, we have investigated and developed a backprojection-filtration (BPF)-based algorithm for ROI-image reconstruction from circular cone-beam data containing transverse truncations. Furthermore, we have developed a weighted BPF algorithm to exploit 'redundant' information in data for improving image quality. In an effort to validate and evaluate the proposed BPF algorithms for circular cone-beam CT, we have performed numerical studies by using both computer-simulation data and experimental data acquired with a radiotherapy cone-beam CT system. Quantitative results in these studies demonstrate that the proposed BPF algorithms for circular cone-beam CT can reconstruct ROI images free of truncation artifacts. Index Terms--Computed tomography (CT), cone-beam CT, image reconstruction, region of interest (ROI), truncated projection.

Reflectivity tomography is an imaging technique that seeks to reconstruct certain acoustic properties of a weakly scattering object. Besides being applicable to pure ultrasound imaging techniques, the reconstruction theory of reflectivity tomography is also pertinent to hybrid imaging techniques such as thermoacoustic tomography. In this work, assuming spherical scanning apertures, redundancies in the three-dimensional (3-D) reflectivity tomography data function are identified and formulated mathematically. These data redundancies are used to demonstrate that knowledge of the measured data function over half of its domain uniquely specifies the 3-D object function. This indicates that, in principle, exact image reconstruction can be performed using a 'half-scan' data function, which corresponds to temporally untruncated measurements acquired on a hemi-spherical aperture, or using a 'half-time' data function, which corresponds to temporally truncated measurements acquired on the entire spherical aperture. Both of these minimal scanning configurations have important biological imaging applications. An iterative reconstruction method is utilized for reconstruction of a simulated 3-D object from noiseless and noisy half-scan and half-time data functions. Index Terms--Image reconstruction, photoacoustic tomography, refectivity tomography, spherical Radon transform, thermoacoustic tomography.

Thermoacoustic tomography (TAT) is an emerging imaging technique with potential for a wide range of biomedical imaging applications. In this correspondence, we propose an infinite family of weighted expectation maximization (EM) algorithms for reconstruction of images from temporally truncated TAT measurement data. The weighted EM algorithms are equivalent mathematically to the conventional EM algorithm, but are shown to propagate data inconsistencies in different ways. Using simulated and experimental TAT measurement data, we demonstrate that suitable choices of weighted EM algorithms can effectively mitigate image artifacts that are attributable to temporal truncation of the TAT data function. Index Terms--Image reconstruction, optoacoustic tomography, photoacoustic tomography, thermoacoustic tomography.

Thermoacoustic tomography (TAT) is an emerging imaging technique with great potential for a wide range of biomedical imaging applications. In this paper, we propose and investigate reconstruction approaches for TAT that are based on the half-time reflectivity tomography paradigm. We reveal that half-time reconstruction approaches permit for the explicit control of statistically complementary information that can result in the optimal reduction of image variances. We also show that half-time reconstruction approaches can mitigate image artifacts due to heterogeneous acoustic properties of an object. Reconstructed images and numerical results produced from simulated and experimental TAT measurement data are employed to demonstrate these effects. Index Terms--Image reconstruction, photoacoustic tomography, reflectivity tomography, spherical Radon transform, thermoacoustic tomography.

We propose an approach that combines an asymmetric fan-beam configuration and a new reconstruction algorithm to enhancing spatial resolution in computed tomography (CT). The asymmetric configuration can be achieved by changing the center of rotation (COR) from the conventional symmetric configuration. It does not, however, require new detectors and X-ray source nor alter the relative geometry between the detector and the X-ray source. By effectively reducing the distance of the COR to the X-ray source, the asymmetric configuration can increase the effective sampling density in projection data without reducing the size of the field of view (FOV). The proposed algorithm, on the other hand, can reconstruct images directly from data acquired with this asymmetric configuration. We performed numerical studies to demonstrate and validate the proposed acquisition/reconstruction approach. Results in these studies confirm that the proposed approach can lead to enhanced spatial resolution in reconstructed images. The proposed acquisition/reconstruction approach may find applications in micro-CT and industrial CT in which the CORs may be changed. Index Terms--Computed tomography (CT), image reconstruction, image resolution.

In this paper, the sampling and aliasing consequences of employing a quarter-detector-offset (QDO) in helical computed tomography (CT) are analyzed. QDO is often used in conventional CT to reduce in-plane aliasing by eliminating data redundancies to improve radial sampling. In helical CT, these same redundancies are exploited to improve longitudinal sampling and so it might seem ill-advised to employ QDO. The relative merit of the two geometries for helical CT is studied by conducting a multidimensional sampling analysis of projection-space sampling as wall as a Fourier crosstalk analysis of crosstalk among the object's Fourier basis components. Both a standard fanbeam helical CT geometry and a hypothetical parallel-beam CT geometry, which helps illuminate the more complicated fanbeam results, are analyzed. Using the sampling analysis, it was found that the use of QDO leads to very different spectral filing than arise when not using QDO. However, due to the shape of the essential support of the projection data spectra that arises in practice, both configurations lead to very similar or identical amounts of spectral overlap. This perspective also predicts the spatially variant longitudinal aliasing that has been observed in helical CT. The crosstalk results were consistent with those of the multidimensional sampling analysis. Thus, from the standpoint of aliasing and crosstalk, no compelling difference is found between the two geometries. Index Terms--Aliasing, computed tomography, image reconstruction, sampling.

Data symmetries in reflectivity tomography are investigated and discussed.

Diffraction tomography (DT) is an inversion technique that reconstructs the refractive index distribution of a weakly scattering object. In this paper, a novel reconstruction algorithm for three-dimensional diffraction tomography employing spherical-wave sources is mathematically developed and numerically implemented. Our algorithm is numerically robust and is much more computationally efficient than the conventional filtered backpropagation algorithm. Our previously developed algorithm for DT using plane-wave sources is contained as a special case. Index Terms--Diffraction tomography, wavefield inversion, 3-D imaging.

Because the effects of physical factors such as photon attenuation and spatial resolution are distance-dependent in single-photon emission computed tomography (SPECT), it is generally considered that accurate image reconstruction requires knowledge of the data function over 2[pi]. In SPECT with only uniform attenuation, Noo and Wagner recently showed that an accurate image can be reconstructed from knowledge of the data function over the interval [0, [pi]). In this work, we consider image reconstruction in [pi]-scheme SPECT with nonuniform attenuation. In its most general form, [pi]-scheme SPECT entails data acquisition over disjoint angular intervals without conjugate views, totaling to [pi] radians. We develop an heuristic perspective for observing that the data function in full-scan SPECT contains redundant information and that the data function in [pi]-scheme SPECT contains complete information necessary for accurate image reconstruction. We conduct simulation studies in which the expectation maximization algorithm is used for image reconstruction from the data acquired in [pi]-scheme SPECT with nonuniform attenuation. The results in these simulation studies seem to corroborate hypothesis that accurate images can be reconstructed from [pi]-scheme data. [pi]-scheme SPECT allows data acquisition over different combinations of disjoint angular intervals, thereby providing a flexibility in choosing projection views at which the emitted gamma-rays may undergo the least attenuation and blurring. It should be stressed, however, that the proposed perspective on redundant information is intended only to heuristically reveal the data redundancy in SPECT with nonuniform attenuation and that it has not been demonstrated mathematically to guarantee the stability and exactness of image reconstruction from the [pi]-scheme data. Index Terms--Attenuation correction, consistency conditions, short-scan, SPECT.

In this study, we examine longitudinal aliasing properties in multislice helical computed tomography (CT) volumes reconstructed under the multiple parallel fanbeam approximation by use of a 180LI-type algorithm. We focus on the differences between the multislice case and the single-slice case, which has been studied previously. Specifically, we examine longitudinal aliasing properties in four-slice scanners for helical pitches 3 and 6, which are sometimes called 'preferred' in four-slice helical CT, because it is believed that the effective longitudinal sampling intervals at these pitches are equivalent to those in single-slice helical CT operating at pitches 1 and 2, respectively. While these equivalences have been supported by comparative studies of slice-sensitivity profiles in single- and multislice helical CT, artifacts have been observed in pitch-3 and pitch-6 multislice images that were not evident in their purported single-slice counterparts. We attribute these differences to aliasing arising in the multislice reconstructions that is not present in the single-slice counterparts. We find that the aliasing has two principal origins: sampling effects similar to those in the singie-slice case and cone-beam effects. The difference between the multislice, pitch-3 and single-slice, pitch-1 results is attributed to the small cone angle in multislice helical CT, which introduces inconsistencies among the measurements of different detector rows. The difference between multislice, pitch-6 and single-slice, pitch-2 results is attributed to a combination of the cone angle and genuine differences in sampling patterns. It is argued, however, that the lack of strict equivalence with single-slice counterparts does not necessarily undermine the claim that pitches 3 and 6 are 'preferred' relative to other pitches in multislice helical CT. Index Terms--Aliasing, computed tomography (CT), helical computed tomography, multislice helical computed tomography, sampling.

Spatially variant longitudinal aliasing plagues most volumes reconstructed from single-slice helical computed tomography data, and its presence can degrade resolution and distort image structures. We have recently developed a Fourier-based approach to longitudinal interpolation in helical computed tomography that can, for scans performed at pitch 1 or lower, essentially eliminate this longitudinal aliasing by exploiting a generalization of the Whittaker-Shannon sampling theorem whose conditions are satisfied by the interlaced pairs of direct and complementary longitudinal samples. However, the algorithm is computationally intensive and cannot be pipelined. In this paper, we address this short-coming by deriving two spatial-domain, projection-data weighting functions that approximate the application of the Fourier-based approach, and preserve its aliasing suppression properties to some degree, while allowing for a pipelined implementation. The first approach, which we call simply 180AA, for anti-aliasing, is a direct spatial-domain approximation of the 180FT approach. The second approach, which we call 180BSP, is based on an approximate generalized interpolation approach making use of B-splines. Studies of aliasing and resolution properties in reconstructions from simulated data indicate that while the 180AA and 180BSP approaches do not perfectly replicate the favorable aliasing suppression and resolution properties of the 180FT approach, they do represent an improvement over the clinically standard 180LI approach on these fronts. Index Terms--Helical computed tomography, image reconstruction, interpolation, sampling and aliasing.

When noniterative methods without adequate regularization are applied to correct for the effects of attenuation and distance-dependent spatial resolution in three-dimensional (3-D) single-photon emission computed tomography (SPECT) at clinical noise levels, they often yield images with conspicuous artifacts, thereby significantly limiting the usefulness of these reconstruction methods. In this work, we extend a regularized inverse-filtering approach that allows the incorporation of an a priori random image field of nonzero mean, as well as an estimated noise level of the SPECT data, for effective suppression of noise and artifacts without significantly compromising the resolution. Our results indicate that the proposed approach can substantially improve the visual quality and the quantitative accuracy of 3-D SPECT images. In combination with its speed advantage, the proposed method may be useful in 3-D SPECT clinical applications. Index Terms--Attenuation correction, distance-dependent spatial resolution (DDSR) correction, image reconstruction, single-photon emission computed tomography (SPECT).

Noise and artifacts may reduce lesion detectability in dual-isotope (Tc-99m/In-111) single photon emission computed tomography (SPECT) studies. The effects of attenuation can lead to artifacts near high-activity structures, e.g., the liver, spleen, kidneys, or urinary bladder. Without effective attenuation correction, these artifacts may reduce diagnostic utility. This study compares three processing methods: 1) zeroth-order Chang's attenuation correction (ZOCAC) with three-dimensional (3-D) Butterworth filtering; 2) quasi-optimal attenuation correction (QOAC) with 3-D Butterworth filtering; and 3) QOAC with a new adaptive filter. A physical phantom is used to determine effective attenuation coefficients to use with Tc-99m and with In-111. Computer simulation studies are used to optimize the parameters of the filters and to compare the three processing methods using bias versus variance plots. The results indicate that QOAC and the adaptive filter both improve the bias-variance tradeoff. Differences between the ZOCAC and the QOAC reconstructions are seen in clinical studies. QOAC noticeably reduces artifacts which have artificially low activity concentration near high-activity structures, e.g., near the urinary bladder and great vessels in Tc-99m labeled red blood cell images. QOAC, combined with the new adaptive filter, holds promise for improving Tc-99m/In-111 SPECT reconstructions. Index Terms--Attenuation correction, In-111, noise, single photon emission computed tomography (SPECT), Tc-99m.

Diffraction tomography (DT) is an inversion technique that reconstructs the refractive index distribution of a scattering object. We previously demonstrated that by exploiting the redundant information in the DT data, the scattering object could be exactly reconstructed using measurements taken over the angular range [0, [[PHI].sub.min]], where [pi] < [[PHI].sub.min] [less than or equal to] 3[pi]/2. In this paper, we reveal a relationship between the maximum scanning angle and image resolution when a filtered backpropagation (FBPP) reconstruction algorithm is employed for image reconstruction. Based on this observation, we develop short-scan FBPP algorithms that reconstruct a low-pass filtered scattering object from measurements acquired over the angular range [0, [[PHI].sup.c]], where [[PHI].sup.c] < [[PHI].sub.min]. Index Terms--Diffraction tomography, limited-view tomography, wavefield inversion techniques.

In this work, we studied a class of reconstruction algorithms, which are specified by a complex combination coefficient, in SPECT with uniform attenuation. We studied theoretically and evaluated numerically the impact of such a complex combination coefficient on the variances in reconstructed images. Specifically, we investigated whether and to what extent a complex combination coefficient can further reduce the global image variances in SPECT as compared to a real combination coefficient. We show that the algorithm specified by an optimal complex combination coefficient can provide theoretically a lower global image variance than does the quasi-optimal algorithm that is specified by a real combination coefficient. However, because such a reduction of global image variance is only a few percent, and because the optimal complex combination coefficient is generally unobtainable without prior knowledge of the object, it is concluded that the quasi-optimal algorithm probably should be used for image reconstruction in SPECT with uniform attenuation.

Successful applications of digital image processing techniques to medical ultrasound images have been limited in part because of the lack of an useful imaging model for clinical ultrasound B-scans. In this work, we derive a discrete linear imaging model appropriate for clinical ultrasound B-scans. based on the newly derived model, we developed a Bayesian restoration approach that is currently designed for the generation of correct edges of medical ultrasound images. Our results demonstrate that successful edge detection can indeed be achieved by the proposed method. The proposed Bayesian approach is very flexible and has the potential of being extended to perform speckle reduction, edge detection, and region segmentation at the same time.

In ultrasonic diffraction tomography, ultrasonic waves are used to probe the object of interest at various angles. The incident waves scatter when encountering inhomogeneities, unlike conventional X-ray CT. Theoretically, when the scattering inhomogeneities are considered weak, the scattering object can be reconstructed by algorithms developed from a generalized central slice theorem. We develop a hybrid algorithm for reconstruction of a scattering object by transforming the scattered data into a conventional X-ray-like sinogram thus allowing standard X-ray reconstruction algorithms, such as filtered back-projection, to be applied. We investigate the statistical properties of the filtered back-propagation, direct Fourier, and newly proposed hybrid reconstruction algorithms by performing analytical as well as numerical studies.