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PubMed1995

An analytical approach for compensation of non-uniform attenuation in cardiac SPECT imaging.

Photon attenuation can reduce the diagnostic accuracy of cardiac SPECT imaging. Bellini et al have previously derived a mathematically exact method to compensate for attenuation in a uniform attenuator. Since the human thorax contains structures with differing attenuation properties, non-uniform attenuation compensation is required in cardiac SPECT. Given an estimate of the patient attenuation map, we show that the Bellini attenuation compensation method can be used in cardiac SPECT to provide a quantitatively accurate reconstruction of a central region in the image which includes the heart and surrounding soft tissue. Simulations using a mathematical cardiac-torso phantom were conducted to evaluate the Bellini method and to compare its performance to the ML-EM iterative algorithm, and to 180 degrees and 360 degrees filtered backprojection (FBP) with no attenuation compensation. 'Bulls-eye' polar maps and circumferential profiles showed that both the Bellini method and the ML-EM algorithm provided quantitatively accurate reconstructions of the myocardium, with a substantial reduction in attenuation-induced artifacts that were observed in the FBP images. The computational load required to implement the Bellini method is approximately equivalent to that required for one iteration of the ML-EM algorithm, thus it is suitable for routine clinical use.

Algorithms↗

PubMed1991

Quantitative SPECT reconstruction of iodine-123 data.

Many clinical and research studies in nuclear medicine require quantitation of iodine-123 (123I) distribution for the determination of kinetics or localization. The objective of this study was to implement several reconstruction methods designed for single-photon emission computed tomography (SPECT) using 123I and to evaluate their performance in terms of quantitative accuracy, image artifacts, and noise. The methods consisted of four attenuation and scatter compensation schemes incorporated into both the filtered backprojection/Chang (FBP) and maximum likelihood-expectation maximization (ML-EM) reconstruction algorithms. The methods were evaluated on data acquired of a phantom containing a hot sphere of 123I activity in a lower level background 123I distribution and nonuniform density media. For both reconstruction algorithms, nonuniform attenuation compensation combined with either scatter subtraction or Metz filtering produced images that were quantitatively accurate to within 15% of the true value. The ML-EM algorithm demonstrated quantitative accuracy comparable to FBP and smaller relative noise magnitude for all compensation schemes.

Humans↗

PubMed1989

Estimation of parameters and missing values under a regression model with non-normally distributed and non-randomly incomplete data.

We carried out a simulation study to compare the performance of three algorithms (complete cases, ALLVALUE, and expectation maximization, EM) in estimating regression parameters and missing values for situations that have varying amounts of missing data, distributions (normal, mixture of normals and lognormal), patterns of incomplete data (random, related and censored), and degrees of correlational structure among the dependent and independent variables. We found that the EM and complete cases algorithms performed equally well regardless of the correlational structure, when the percentage of incomplete data was only 5 per cent. When this percentage increased to 25 per cent, the EM algorithm was generally best for estimation, but the complete cases algorithm was safe and conservative. This finding may be attributed to the study design, which required that the slopes be the same in the population of all cases, and in the population of complete cases. In addition, the one-step imputing method (ALLVALUE) was competitive only for situations with weak correlational structure and/or little missing data. In that situation the bias caused with use of all available information was less than that caused with use of only complete cases. On the other hand, for imputation, the EM algorithm performed optimally, even in situations of censored or log-normally distributed data.

Algorithms↗

PubMed1996

A non-negative fast multiplicative algorithm in 3D scatter-compensated SPET reconstruction.

Single-photon emission tomographic (SPET) reconstruction can be improved, especially for noisy images, by using the iterative expectation-maximization of the maximum-likelihood (EM-ML) algorithm. Its application to clinical routine is, however, hampered by the high number of iterations necessary to achieve acceptable results. Therefore various methods have been developed to accelerate the EM-ML algorithm. In this paper a new accelerated EM-ML-like multiplicative algorithm is proposed for SPET reconstruction. Contrary to some other accelerating methods, it preserves two of the most important properties of the EM-ML, namely pixel positivity inside the patient body and null activity outside. The convergence speed is improved by a factor which can reach 100 in high spatial frequency or low count regions. Good estimates in the low count region are obtained without any smoothing, even at typical routine clinical count rates. The algorithm used in conjunction with the 3D effective one scatter path model provides high-quality SPET images and accurate quantitation.

Adult↗

PubMed1997

A focus-of-attention preprocessing scheme for EM-ML PET reconstruction.

The expectation-maximization maximum-likelihood (EM-ML) algorithm belongs to a family of algorithms that compute positron emission tomography (PET) reconstructions by iteratively solving a large linear system of equations. We describe a preprocessing scheme for automatically focusing the attention, and thus the computational resources, on a subset of the equations and unknowns. Experimental work with a CM-5 parallel computer implementation using a simulated phantom as well as real data obtained from an ECAT 921 PET scanner indicates that quite significant savings can be obtained with respect to both time and space requirements of the EM-ML algorithm without compromising the quality of the reconstructed images.

Abdomen↗

PubMed1995

A two-step iterative algorithm for estimation in nonlinear mixed-effect models with an evaluation in population pharmacokinetics.

This article proposes an EM-like algorithm for estimating, by maximum likelihood, the population parameters of a nonlinear mixed-effect model given sparse individual data. The first step involves Bayesian estimation of the individual parameters. During the second step, population parameters are estimated using a linearization about those Bayesian estimates. This algorithm (implemented in P-PHARM) is evaluated on simulated data, mimicking pharmacokinetic analyses and compared to the First-Order method and the First-Order Conditional Estimates method (both implemented in NONMEM). The accuracy of the results, within few iterations, shows the estimation capabilities of the proposed approach.

Algorithms↗

PubMed2026

A flexible framework for robust and efficient Mendelian randomization with debiasing.

Mendelian randomization (MR) has been widely used to infer causal relationships between exposures and outcomes in epidemiological studies. However, classical MR assumptions can be violated when genetic variants are associated with outcomes through pathways other than the exposure, leading to uncorrelated and/or correlated pleiotropy. Additionally, measurement error arising from the inherent uncertainty in summary statistics obtained from large-scale genome-wide association studies can introduce bias into the causal effect estimate. To address these issues, we develop a debiased mixture inverse variance weighting ($\mathsf{dmIVW}$) method with three major advantages. First, it is capable of simultaneously handling various types of pleiotropy and eliminating the bias caused by uncertainty. Second, it can guard against distortion caused by invalid genetic variants while effectively harnessing their information. Third, our unified framework facilitates a fair comparison and combination of a series of submodels, encompassing several popular MR methods as special cases. Through real data applications, the effectiveness and robustness of $\mathsf{dmIVW}$ in estimating the causal effects of risk factors on common diseases are demonstrated.

Mendelian Randomization Analysis↗

PubMedDate not supplied

Iterative algebraic reconstruction algorithms for emission computed tomography: a unified framework and its application to positron emission tomography.

In this paper, a unified framework of iterative algebraic reconstruction for emission computed tomography (ECT) and its application to positron emission tomography (PET) is presented. The unified framework is based on an algebraic image restoration model and contains conventional iterative algebraic reconstruction algorithms: ART, SIRT, Landweber iteration (LWB), the generalized Landweber iteration (GLWB), the steepest descent method (STP), as well as iterative filtered backprojection (IFBP) reconstruction algorithms: Chang's method, Walters' method, and a modified iterative MAP. The framework provides an effective tool to systematically study conventional iterative algebraic algorithms and IFBP algorithms. Based on this framework, conventional iterative algebraic algorithms and IFBP algorithms are generalized. It is shown from the algebraic point of view that IFBP algorithms are not only excellent methods for correction of attenuation (either uniform or nonuniform) but are also good general iterative reconstruction algorithms (they can be applied to either attenuated or attenuation-free projections and converge very fast). The convergence behavior of iterative algebraic algorithms is discussed and insight is drawn into the fast convergence property of IFBP algorithms. A simulated PET system is used to evaluate IFBP algorithms and LWB in comparison with the maximum likelihood estimation via expectation maximization algorithm (MLE-EM) and the filtered backprojection (FBP) algorithm. The simulation results indicate that for both attenuation-free projection and attenuated projection cases IFBP algorithms have a significant computational advantage over LWB and MLE-EM, and have performance advantages over FBP in terms of contrast recovery and/or noise-to-signal ratios (NSRs) in regions of interest.

Algorithms↗

PubMed1992

An evaluation of maximum likelihood-expectation maximization reconstruction for SPECT by ROC analysis.

A ROC study was performed in order to evaluate whether the maximum likelihood expectation maximization (ML-EM) reconstruction algorithm improves diagnostic performance compared to the conventional filtered backprojection method in SPECT. Several implementations of the algorithm were tested including 25 and 50 iteration stopping points, with and without nonuniform attenuation compensation, and with and without Metz filtering. Filtered backprojection was with Metz filter and without attenuation compensation. The test data were computer simulated to model cardiac 201Tl SPECT. The data incorporated the effects of nonuniform attenuation, distance-dependent collimator response, and scatter. Patient CT images provided realistic anatomy and attenuation information for the data simulation. Four observers each viewed 120 images for each of the reconstruction methods. Lesion detectability with ML-EM increased with Metz filtering and decreased with nonuniform attenuation compensation. The best MIL-EM implementation, 50 iterations with Metz filtering and without attenuation compensation, was not statistically better than filtered backprojection.

Algorithms↗

PubMed1993

Restricted maximum likelihood estimation of variance components from field data for number of pigs born alive.

Variance components for number of pigs born alive (NBA) were estimated from sow productivity field records collected by purebred breed associations. Data sets analyzed were as follows: Hampshire (n = 13,537), Landrace (n = 10,822), and Spotted (n = 3,949). Variance components for service sire, sire of sow, dam of sow, and residual effects on NBA (adjusted for parity) were estimated. The single-trait model included relationships between service sires, sires of sows, and dams of sows. The model was implemented using an expectation maximization (EM) REML algorithm. A sparse-matrix solver was also used. Heritability estimates for NBA were .13, .13, and .12 for Hampshire, Spotted, and Landrace, respectively. Estimates of maternal genetic (co)variances (m2) expressed as a proportion of the phenotypic variance were .05, .01, and .03 for Hampshire, Spotted, and Landrace, respectively. Results indicated that service sires account for 1 to 2% of the total variation for NBA. Genetic effects influencing NBA seem to be small in these data sets, but selection for increased NBA should be effective.

Analysis of Variance↗

PubMed1994

Genetic correlations in the feed conversion complex of primiparous cows at a recommended and a reduced plane of nutrition.

An experiment at the Agricultural University of Norway provided data to estimate genetic parameters of roughage intake (RI), fat-corrected milk yield (FCM), smoothed BW (SW), weight change (WC), and energy balance (EB). Weekly measurements were averaged in four consecutive 6-wk periods after parturition for each of 331 primiparous Norwegian cows of 20 sires. Amount of concentrate fed was adjusted according to stage of lactation and cows were randomly assigned to a normal or a low level. Animals were given ad libitum access to grass silage. Multiple-trait animal models with all additive genetic relationships incorporated were applied to the subsets of all averages of one trait and all averages in one period. Each model contained for every trait 165 observations, 24 mo-year seasons of calving, and 313 additive genetic effects. (Co)variance components were estimated by an EM-REML algorithm. The heritability of RI increased with negative energy balance from .25 to .86, whereas the estimates were approximately .2 for FCM, .6 to .7 for SW, and .03 to .4 for WC and EB. The correlations between RI and FCM were .37 to .58 phenotypically and -.11 to .88 genetically; between RI and SW they were .57 to .73 phenotypically and .7 to 1 genetically; between FCM and SW they were -.06 to .35 phenotypically and -.33 to .70 genetically; and between WC and EB they were .52 to .92 phenotypically and -.09 to .88 genetically. Correlations between WC and EB and other traits were inconsistent over periods and had very high SE.

Animal Nutritional Physiological Phenomena↗

PubMed1995

An empirical Bayes approach to smoothing in backcalculation of HIV infection rates.

Backcalculation is a methodology to reconstruct the past human immunodeficiency virus (HIV) infection rates from the AIDS incidence data and incubation distribution by deconvolution. Smoothing has proved important in backcalculation, and a key question is how to choose the amount of smoothing. This paper proposes an empirical Bayes approach in which the smoothing parameter is estimated from the data. We introduce a family of priors that reflect the notion of closeness of neighboring infection rates. The variance parameter in the prior family plays the role of the smoothing parameter and is estimated by a method similar to the residual maximum likelihood in linear random effects model through an efficient EM (expectation/maximization) algorithm. A number of penalized likelihood functions that have been used in backcalculation have an empirical Bayes formulation. A bootstrap confidence interval for the infection rates is proposed. The methodology is illustrated with United States AIDS incidence data.

Acquired Immunodeficiency Syndrome↗

PubMed1996

[Evaluation of simultaneous acquisition of transmission and emission data on thallium-201 myocardial SPECT].

This study evaluates the usefulness of attenuation correction on regional myocardial tracer distributions defined by Thallium-201 myocardial SPECT images obtained from cardiac phantoms and patients with or without coronary heart disease. A three-detector SPECT system equipped with a Technetium-99m line source and a fan-beam collimator was used for simultaneous transmission and emission data acquisition. All three detectors were equipped with fan-beam collimators. Thallium-201 myocardial scintigraphy was performed on phantom study and 19 patients. Transmission images, uncorrected and corrected emission images were iteratively reconstructed with a EM-ML algorithm. Attenuation map computed from the transmission data was utilized for the attenuation correction. For the phantom study, circumferential profile analysis was applied to both datasets of horizontal long-axis slices through the center of the phantom. The maximum profile value in the circumferential profile set to 100% in the normalized uncorrected and corrected profiles. The uncorrected circumferential profiles from cardiac insert model 7070 and RH-2 cardiophantom showed decrease in activity in basal regions which appeared improvement in the attenuation corrected profiles. In clinical study, the inferior-to-anterior activity ratio, changed from 0.78 +/- 0.10 to 0.97 +/- 0.11 on stress images in patients with inferior ischemia and from 0.96 +/- 0.12 to 1.15 +/- 0.13 on 4 hour delayed or rest images in normal cases. The anteroapical wall of the attenuation corrected images, however, showed a decrease in activity relative to the inferior wall in normal cases. The increase in activity in inferior wall on attenuation corrected images was observed frequently in clinical study but not in phantom study. A presence of scatter from the liver or bowels may cause the increase in activity in the inferior wall in clinical study. In conclusion, transmission scan is one of the useful methods for the attenuation correction. Scatter correction, however, is also necessary to make an accurate attenuation corrected images.

Heart↗

PubMed1986

Nonparametric methods for survival/sacrifice experiments.

In many carcinogenicity studies, the time to disease occurrence is not clinically observable; a survival/sacrifice experiment is considered for nonparametric inference about the rate of disease occurrence. A multistate model for disease development and death is considered and an algorithm of the EM type for maximum likelihood estimation is obtained. Questions of identifiability and estimability are addressed. Under the model, interval hazards for disease occurrence are identifiable for intervals defined by the sacrifice times. A score test is developed appropriate for the comparison of two groups with respect to disease development without need of any assumption concerning lethality of the disease concerned.

Animals↗

PubMed1996

Pattern-mixture models for multivariate incomplete data with covariates.

Pattern-mixture models stratify incomplete data by the pattern of missing values and formulate distinct models within each stratum. Pattern-mixture models are developed for analyzing a random sample on continuous variables y(1), y(2) when values of y(2) are nonrandomly missing. Methods for scalar y(1) and y(2) are here generalized to vector y(1) and y(2) with additional fixed covariates x. Parameters in these models are identified by alternative assumptions about the missing-data mechanism. Models may be underidentified (in which case additional assumptions are needed), just-identified, or overidentified. Maximum likelihood and Bayesian methods are developed for the latter two situations, using the EM and SEM algorithms, direct and interactive simulation methods. The methods are illustrated on a data set involving alternative dosage regimens for the treatment of schizophrenia using haloperidol and on a regression example. Sensitivity to alternative assumptions about the missing-data mechanism is assessed, and the new methods are compared with complete-case analysis and maximum likelihood for a probit selection model.

Algorithms↗

PubMed1986

The modelling of biological systems in three dimensions using the time domain finite-difference method: I. The implementation of the model.

A computer method has been developed which uses the time domain finite-difference (TDFD) algorithm to calculate the deposition of the electromagnetic (EM) field in three-dimensional biological models. This, the first of two papers, describes the algorithm and the computer programs developed. The method is demonstrated by calculating the penetration of the EM field from a rectangular waveguide radiating into a homogeneous model, the calculation being carried out in two dimensions for simplicity in this paper.

Computer Simulation↗

PubMed1996

A diagnostic algorithm for distinguishing the eosinophilia-myalgia syndrome from fibromyalgia and chronic myofascial pain.

OBJECTIVE: To develop a diagnostic algorithm for the eosinophilia-myalgia syndrome (EMS) that complements the existing case definition. METHODS: We conducted a retrospective study using data on 59 clinical and laboratory variables from a consecutive referral cohort of 91 patients with EMS meeting the Centers for Disease Control and Prevention case definition. Age and sex matched controls included 93 patients with fibromyalgia and 99 patients with chronic myofascial pain. The study period was March 1989 to April 1992. Recursive partitioning was used to create a diagnostic algorithm. RESULTS: In the 283 case patients and controls with disabling myalgias, 4 differentiating variables identified patients with EMS: extremity edema, leukocyte count > 12.5 x 10(9)/l, dyspnea, and absence of arthralgias. These 4 variables form a diagnostic algorithm that has a sensitivity of 95.6%, a specificity of 96.9%, and positive and negative predictive values of 93.5 and 97.9%, respectively. CONCLUSION: This algorithm is practical and can be easily applied in any medical setting. It also readily distinguishes EMS from other common myalgia syndromes.

Adult↗

PubMed1993

Emergency medical services priority dispatch.

STUDY OBJECTIVE: To test the ability of a locally designed priority dispatch system to safely exclude the need for advanced life support (ALS). DESIGN: Retrospective review of emergency medical services (EMS) incident records to determine how often the lone dispatch of basic life support (BLS) units, staffed with basic emergency medical technicians, subsequently required or involved ALS care. SETTING: A large centralized municipal EMS system with a tiered ALS/BLS ambulance response. All BLS units carry automated defibrillators. MEASUREMENTS: Consecutive EMS records (35,075) were reviewed by computerized search for ALS procedures. Records indicating ALS procedures were tabulated and then manually reviewed for the nature of and probable indication for the ALS intervention. INTERVENTION: Brief sequences of computer-stored questions that help dispatchers identify (or exclude) signs and symptoms indicating the need for ALS. RESULTS: The dispatch triage system spared ALS units from initial dispatch in 14,100 of the EMS incidents (40.2%), increasing their availability and use for more serious calls. Among these 14,100 cases, only 41 patients (0.3%) later received drugs such as nitroglycerin and naloxone; another 27 patients (0.2%) received resuscitative interventions such as epinephrine or defibrillation. Furthermore, on closer analysis, the immediate presence of a paramedic might have provided a true potential for advantage in outcome for only five or six patients (less than 0.04 of the 14,100 BLS dispatches). Meanwhile, many important operational, fiscal, and cost-effective patient care benefits were realized with this system. CONCLUSION: A computer-aided dispatch triage algorithm can facilitate improvements in both EMS system operations and prehospital patient care by safely and reliably identifying EMS incidents requiring only BLS.

Algorithms↗