Biomedical subjects

J E Bowsher

Publications and source records attributed to J E Bowsher.

16 recordsLinked to original sources

PubMed2006

Performance of dedicated emission mammotomography for various breast shapes and sizes.

We evaluate the effect of breast shape and size and lesion location on a dedicated emission mammotomography system developed in our lab. The hemispherical positioning gantry allows ample flexibility in sampling a pendant, uncompressed breast. Realistic anthropomorphic torso (which includes the upper portion of the arm) and breast phantoms draw attention to the necessity of using unique camera trajectories (orbits) rather than simple circular camera trajectories. We have implemented several novel three-dimensional (3D) orbits with fully contoured radius-of-rotation capability for compensating for the positioning demands that emerge from different breast shapes and sizes. While a general orbit design may remain the same between two different breasts, the absolute polar tilt range and radius-of-rotation range may vary. We have demonstrated that using 3D orbits with increased polar camera tilt, lesions near the chest wall can be visualized for both large and small sized breasts (325 ml to 1,060 ml), for a range of intrinsic contrasts (three to ten times higher activity concentration in the lesion than breast background). Overall, nearly complete 3D acquisition schemes yield image data with relatively high lesion SNRs and contrasts and with minimal distortion of the uncompressed breast shape.

Breast↗

PubMed2002

Modeling the axial extension of a transmission line source within iterative reconstruction via multiple transmission sources.

Reconstruction algorithms for transmission tomography have generally assumed that the photons reaching a particular detector bin at a particular angle originate from a single point source. In this paper, we highlight several cases of extended transmission sources, in which it may be useful to approach the estimation of attenuation coefficients as a problem involving multiple transmission point sources. Examined in detail is the case of a fixed transmission line source with a fan-beam collimator. This geometry can result in attenuation images that have significant axial blur. Herein it is also shown, empirically, that extended transmission sources can result in biased estimates of the average attenuation, and an explanation is proposed. The finite axial resolution of the transmission line source configuration is modeled within iterative reconstruction using an expectation-maximization algorithm that was previously derived for estimating attenuation coefficients from single photon emission computed tomography (SPECT) emission data. The same algorithm is applicable to both problems because both can be thought of as involving multiple transmission sources. It is shown that modeling axial blur within reconstruction removes the bias in the average estimated attenuation and substantially improves the axial resolution of attenuation images.

Algorithms↗

PubMed2002

Analytic determination of the pinhole collimator's point-spread function and RMS resolution with penetration.

Pinhole collimators are widely used to image small organs and animals. The pinhole response function (PRF) of knife-edge pinhole collimators has been estimated previously using geometric constructions without considering penetration and using "roll-off" models that employ an exponential model for the flux. An analytic expression for the PRF on the imaging plane that includes the effect of aperture penetration is derived in this paper by calculating the flux for photons passing through the aperture and those passing through the attenuating material. The PRF is then used to approximate the angular-dependent root-mean-square resolution in the directions parallel and perpendicular to the tilt of the point source. The corresponding aspect ratio is then obtained. The formulas are then compared with experimental data.

Computer Simulation↗

PubMed2001

An EM algorithm for estimating SPECT emission and transmission parameters from emissions data only.

A maximum-likelihood (ML) expectation-maximization (EM) algorithm (called EM-IntraSPECT) is presented for simultaneously estimating single photon emission computed tomography (SPECT) emission and attenuation parameters from emission data alone. The algorithm uses the activity within the patient as transmission tomography sources, with which attenuation coefficients can be estimated. For this initial study, EM-IntraSPECT was tested on computer-simulated attenuation and emission maps representing a simplified human thorax as well as on SPECT data obtained from a physical phantom. Two evaluations were performed. First, to corroborate the idea of reconstructing attenuation parameters from emission data, attenuation parameters (mu) were estimated with the emission intensities (lambda) fixed at their true values. Accurate reconstructions of attenuation parameters were obtained. Second, emission parameters lambda and attenuation parameters mu were simultaneously estimated from the emission data alone. In this case there was crosstalk between estimates of lambda and mu and final estimates of lambda and mu depended on initial values. Estimates degraded significantly as the support extended out farther from the body, and an explanation for this is proposed. In the EM-IntraSPECT reconstructed attenuation images, the lungs, spine, and soft tissue were readily distinguished and had approximately correct shapes and sizes. As compared with standard EM reconstruction assuming a fix uniform attenuation map, EM-IntraSPECT provided more uniform estimates of cardiac activity in the physical phantom study and in the simulation study with tight support, but less uniform estimates with a broad support. The new EM algorithm derived here has additional applications, including reconstructing emission and transmission projection data under a unified statistical model.

Algorithms↗

PubMed2001

Analytic determination of pinhole collimator sensitivity with penetration.

Pinhole collimators are widely used to image small organs and animals. The sensitivity of knife-edge pinhole collimators has been previously estimated using an "effective diameter" formulation and experimentally described using a sin(x) theta fit, where theta is the angle between the line segment from the center of the aperture to the photon source and its projection onto the plane of the aperture. An analytic form of the sensitivity of the pinhole collimator is derived in this paper. A numerical formula for predicting the sin(x) theta form of the sensitivity is calculated from the analytic form. Experimental data are compared with the theoretical estimate and the sin(x) theta prediction. The agreement is excellent.

Tomography, Emission-Computed, Single-Photon↗

PubMed1997

Fully Bayesian estimation of Gibbs hyperparameters for emission computed tomography data.

In recent years, many investigators have proposed Gibbs prior models to regularize images reconstructed from emission computed tomography data. Unfortunately, hyperparameters used to specify Gibbs priors can greatly influence the degree of regularity imposed by such priors and, as a result, numerous procedures have been proposed to estimate hyperparameter values from observed image data. Many of these procedures attempt to maximize the joint posterior distribution on the image scene. To implement these methods, approximations to the joint posterior densities are required, because the dependence of the Gibbs partition function on the hyperparameter values is unknown. In this paper, we use recent results in Markov chain Monte Carlo (MCMC) sampling to estimate the relative values of Gibbs partition functions and using these values, sample from joint posterior distributions on image scenes. This allows for a fully Bayesian procedure which does not fix the hyperparameters at some estimated or specified value, but enables uncertainty about these values to be propagated through to the estimated intensities. We utilize realizations from the posterior distribution for determining credible regions for the intensity of the emission source. We consider two different Markov random field (MRF) models-the power model and a line-site model. As applications we estimate the posterior distribution of source intensities from computer simulated data as well as data collected from a physical single photon emission computed tomography (SPECT) phantom.

Algorithms↗

PubMed1997

Improved Bayesian image estimation for digital chest radiography.

PURPOSE: Previously, we have shown that Spatially Varying Bayesian Image Estimation (SVBIE) can be used to reduce scatter and improve contrast-to-noise ratios (CNR) in digital chest radiographs with no degradation of image resolution. This previous algorithm used a model for scatter compensation that was derived for emission tomography. Here, we develop and evaluate a new iterative SVBIE technique that incorporates a scatter model derived for projection radiography. MATERIALS AND METHODS: Portable digital radiographs of an anthropomorphic chest phantom were obtained along with quantitative scatter measurements using a calibrated photostimulable phosphor system. The new iterative SVBIE technique was applied to the phantom image to reduce scatter. Scatter fraction reduction, CNR improvement, and resolution degradation were evaluated. RESULTS: Residual scatter fractions were reduced to less than 2% in the lungs and 30% in the mediastinum at 14 iterations. CNR was improved by approximately 50% in the lung region and 187% in the mediastinum. Resolution was not degraded. CONCLUSIONS: The new SVBIE technique can reduce scatter to levels far below those provided by an antiscatter grid and can increase CNR without loss of resolution. The new technique outperforms the previous Bayesian techniques.

Bayes Theorem↗

PubMed1994

Bayesian restoration of chest radiographs. Scatter compensation with improved signal-to-noise ratio.

OBJECTIVES: The authors introduce a Bayesian algorithm for digital chest radiography that increases the signal-to-noise ratio, and thus detectability, for low-contrast objects. METHOD: The improved images are formed as a maximum a posteriori probability estimation of a scatter-reduced (contrast-enhanced) image with decreased noise. Noise is constrained by including prior knowledge of image smoothness. Variations between neighboring pixels are penalized for small variations (to suppress Poisson noise), but not for larger variations (to avoid affecting anatomical structure). The technique was optimized to reduce residual scatter in digital radiographs of an anatomical chest phantom. RESULTS: The contrast in the lung was improved by a factor of two, whereas signal-to-noise ratio was improved by a factor of 1.8. Image resolution was unaffected for objects with a contrast greater than 2%. CONCLUSION: This statistical estimation technique shows promise for improving object detectability in radiographs by simultaneously increasing contrast, while constraining noise.

Algorithms↗

PubMed1994

An artificial neural network approach to quantitative single photon emission computed tomographic reconstruction with collimator, attenuation, and scatter compensation.

A spatially variant technique for quantitative single photon emission computed tomographic (SPECT) image reconstruction using an artificial neural network (ANN) is presented. This network was developed to simultaneously compensate for collimator, attenuation, and scatter effects during the reconstruction process. The network was trained using a supervised scheme which implemented the generalized delta rule. Training ended once the mean-squared error (MSE) between the ideal and reconstructed images converged to a minimum. After training, the ANN weights were held constant and could be used to reconstruct source distributions other than those used while training. In the absence of noise when only collimator effects were present, reconstruction of a Hoffman brain phantom had a 89% reduction in MSE compared to standard filtered backprojection. When collimator-and-attenuation and collimator-attenuation-and-scatter trials were tested against filtered backprojection with Chang attenuation compensation, the corresponding ANN reconstructions demonstrated 85% and 86% decreases in MSE, respectively. With noise present, and with standard noise reduction filters implemented prior to reconstruction, the ANN reconstructions displayed up to a 50% decrease in MSE compared to filtered backprojection reconstructions for 200,000 count data. These results demonstrate that an ANN can be used to reconstruct SPECT images with improved quantitative accuracy.

Biophysical Phenomena↗

PubMed1994

A theoretical model of independence for nursing home elders.

Dependence-oriented care is very much a part of nursing home care in the United States. Yet independence is a salient part of American culture and is desired by nursing home elders. Such a dichotomy sets the stage for a loss of competence, a loss of control, and a subsequent loss of well-being in nursing home elders. Potential reasons for dependence-oriented care are identified in this article, and a theoretical model of independence for frail elders is presented as a guide for professional caregivers. Findings from previous research studies related to the model suggest perceived situational expectancy of control of valued reinforcements as a mediator of the relationship between behavioral competence and psychological well-being. Findings from the literature propose the subjective environment as a mediator of the relationship between behavioral competence and perceived situational expectancy of control of valued reinforcements. Using the model, there is the potential to understand more fully the process of independence in nursing home elders and to guide professional caregivers through this process as care is delivered.

Aged↗

PubMed1994

Preventing diabetic foot ulcers.

Approximately 6% of the U.S. diabetic population (14 million) experiences lower extremity amputation due to diabetic foot ulcers. Investigators have demonstrated that the amputation rate can be reduced through specific preventive measures. However, these findings are not consistent across studies. Integration of research findings into the model of care for the chronically ill suggests that the inconsistency may be due to the lack of inclusion of key individual and environmental variables. In this critical review of the literature, a model of care for the chronically ill is provided to direct prevention, diagnosis, and treatment of diabetic foot ulcers.

Diabetic Foot↗

PubMed1993

Scatter compensation for digital chest radiography using maximum likelihood expectation maximization.

RATIONALE AND OBJECTIVES: An iterative maximum likelihood expectation maximization algorithm (MLEM) has been developed for scatter compensation in chest radiography. METHODS: The MLEM technique produces a scatter-reduced image which maximizes the probability of observing the measured image. We examined the scatter content and the low-contrast signal-to-noise ratio (SNR) in digital radiographs of anatomical phantoms before and after compensation. RESULTS: MLEM converged to an accurate (6.4% RMS residual scatter error) estimate within 12 iterations. Both contrast and noise were increased in the processed images as iteration progressed. In the lung, contrast was increased 108% and SNR was improved by 10%. In the retrocardiac region, contrast was increased 180% while SNR decreased by 6%. CONCLUSIONS: This is the first report of a post-acquisition scatter compensation technique which can increase SNR. These results suggest that statistical estimation techniques can enhance image quality and quantitative accuracy for digital chest radiography.

Algorithms↗

PubMed1991

Treatment of Compton scattering in maximum-likelihood, expectation-maximization reconstructions of SPECT images.

This paper studies the extent to which lesion contrasts and signal-to-noise ratios in maximum-likelihood, expectation-maximization estimates of SPECT images can be improved by considering Compton scattering when calculating the photon detection probability matrix. Matrices are generated using a Monte Carlo code that realistically models a SPECT imaging system. For cold lesions having true contrasts of one, thirty-six combinations of three lesion sizes, three lesion locations, and four projection-data count levels are considered. Both hot and cold, 2-cm-radius lesions having various values of true contrast are studied at a few count levels and lesion locations. With scatter in the matrix, scatter effects are completely removed from the central regions of lesions whose radius is greater than the full width at half maximum of the spatial resolution. Resolution recovery is initially slower but eventually superior. Percent improvements in contrast are greater for lower-contrast cold lesions and higher-contrast hot lesions. In most cases, signal-to-noise ratios are comparable or better. Correct absolute activity levels are obtained.

Humans↗

PubMed1990

Behavioral competence as a predictor of perceived control in nursing home elders.

The purpose of this study was to describe the relationship between behavioral competence and perceived control in nursing home elders. The literature strongly supports perceived control as a primary determinant of psychological well-being in nursing home residents. Identifying predictors of this salient variable will enhance the development of interventions to increase and maintain control in nursing home elders. A nonrandom sample of 134 cognitively intact nursing home elders with poor to excellent self-rated physical health was selected from 13 nursing homes in four counties of a southeastern state. Instruments used included the Desired Control Measure and the Philadelphia Geriatric Center Multilevel Assessment Instrument. Regression analysis demonstrated that components of behavioral competence, time use and self-rated health, explained 34% of the variance of perceived control. Additional research questions were identified.

Activities of Daily Living↗

PubMed1990

Personal control and other determinants of psychological well-being in nursing home elders.

The purpose of this correlational study was to identify predictors of psychological well-being in nursing home elders. Predictors considered were two dimensions of personal control, physical health, functional health, socioeconomic status, length of stay in the nursing home, and interaction variables. The nonrandom sample consisted of 302 cognitively intact nursing home elders with poor to excellent self-rated physical health. Instruments used included the Life Satisfaction Index A, Affect Balance Scale, Desired Control Measure, The Index of Activities of Daily Living, and The Two Factor Index of Social Position. Regression analysis demonstrated that dimensions of personal control and self-rated physical health explained 33% (p less than .001) of the variance of psychological well-being. Findings suggested predictor variables to guide the promotion and maintenance of psychological well-being in the subjects of this sample.

Aged↗

PubMedDate not supplied

Toward an understanding of three control constructs: personal control, self-efficacy, and hardiness.

The purpose of this paper is to further the understanding of three control models by describing their origins, definitions, generality, process, antecedents, and consequences. Each is applied to a clinical situation within a case study of a 98-year-old Caucasian woman living in a nursing facility. Similarities and differences among the models are reviewed. Implications for clinical practice and research are presented.

Adaptation, Psychological↗