Anomaly Detection and Artifact Recovery in PET Attenuation-Correction Images Using the Likelihood Function

In dual modality PET/CT, CT data are used to generate the attenuation correction applied in the reconstruction of the PET emission image. This requires converting the CT image into a 511-keV attenuation map. Algorithms for making this transformation require assumptions about the makeup of material within the patient. Anomalous material such as contrast agent administered to enhance the CT scan confounds conversion algorithms and has been observed to result in inaccuracies, i.e., inconsistencies with the true 511-keV attenuation present at the time of the PET emission scan. These attenuation artifacts carry through to the final attenuation-corrected PET emission image and can resemble diseased tissue. We propose an approach to correcting this problem that employs the attenuation information carried by the PET emission data. A likelihood-based algorithm for identifying and correcting of contrast is presented and tested. The algorithm exploits the fact that contrast artifacts manifest as too-high attenuation values in an otherwise high quality attenuation image. In a separate study, the performance of the loglikelihood as an objective-function component of a detection/correction algorithm, independent of any particular algorithm was mapped out for several imaging scenarios as a function of statistical noise. Both the full algorithm and the loglikelihood performed well in studies with simulated data. Additional studies including those with patient data are required to fully understand their capabilities.

Successful transplantation of human hepatic stem cells with restricted localization to liver using hyaluronan grafts

Cell therapies are potential alternatives to organ transplantation for liver failure or dysfunction but are compromised by inefficient engraftment, cell dispersal to ectopic sites, and emboli formation. Grafting strategies have been devised for transplantation of human hepatic stem cells (hHpSCs) embedded into a mix of soluble signals and extracellular matrix biomaterials (hyaluronans, type III collagen, laminin) found in stem cell niches. The hHpSCs maintain a stable stem cell phenotype under the graft conditions. The grafts were transplanted into the livers of immunocompromised murine hosts with and without carbon tetrachloride treatment to assess the effects of quiescent versus injured liver conditions. Grafted cells remained localized to the livers, resulting in a larger bolus of engrafted cells in the host livers under quiescent conditions and with potential for more rapid expansion under injured liver conditions. By contrast, transplantation by direct injection or via a vascular route resulted in inefficient engraftment and cell dispersal to ectopic sites. Transplantation by grafting is proposed as a preferred strategy for cell therapies for solid organs such as the liver.

Analysis of a novel offset cone-beam computed mammotomography system geometry for accomodating various breast sizes

We evaluate a newly developed dedicated cone-beam transmission computed mammotomography (CmT) system configuration using an optimized quasi-monochromatic cone beam technique for attenuation correction of SPECT in a planned dual-modality emission and transmission system for pendant, uncompressed breasts. In this study, we perform initial CmT acquisitions using various sized breast phantoms to evaluate an offset cone-beam geometry. This offset geometry provides conjugate projections through a full 360 degree gantry rotation, and thus yields a greatly increased effective field of view, allowing a much wider range of breast sizes to be imaged without truncation in reconstructed images. Using a tungsten X-ray tube and digital flat-panel X-ray detector in a compact geometry, we obtained initial CmT scans without shift and with the offset geometry, using geometrical frequency/resolution phantoms and two different sizes of breast phantoms. Acquired data were reconstructed using an ordered subsets transmission iterative algorithm. Projection images indicate that the larger, 20 cm wide, breast requires use of a half-cone-beam offset scan to eliminate truncation artifacts. Reconstructed image results illustrate elimination of truncation artifacts, and that the novel quasi-monochromatic beam yields reduced beam hardening. The offset geometry CmT system can indeed potentially be used for structural imaging and accurate attenuation correction for the functional dedicated breast SPECT system.

Target localization using scanner-acquired SPECT data

Target localization using single photon emission computed tomography (SPECT) and planar imaging is being investigated for guiding radiation therapy delivery. Previous studies on SPECT-based localization have used computer-simulated or hybrid images with simulated tumors embedded in disease-free patient images where the tumor position is known and localization can be calculated directly. In the current study, localization was studied using scanner-acquired images. Five fillable spheres were placed in a whole body phantom. Sphere-to-background 99mTc radioactivity was 6:1. Ten independent SPECT scans were acquired with a Trionix Triad scanner using three detector trajectories: left lateral 180°, 360°, and right lateral 180°. Scan time was equivalent to 4.5 min. Images were reconstructed with and without attenuation correction. True target locations were estimated from 12 hr SPECT and CT images. From the 12 hr SPECT scan, 45 sets of orthogonal planar images were used to assess target localization; total acquisition time per set was equivalent to 4.5min. A numerical observer localized the center of the targets in the 4.5 min SPECT and planar images. SPECT-based localization errors were compared for the different detector trajectories. Across the four peripheral spheres, and using optimal iteration numbers and postreconstruction smoothing, means and standard deviations in localization errors were 0.90 ± 0.25 mm for proximal 180° trajectories, 1.31 ± 0.51 mm for 360° orbits, and 3.93 ± 1.48 mm for distal 180° trajectories. This rank order in localization performance is predicted by target attenuation and distance from the target to the collimator. For the targets with mean localization errors < 2 mm, attenuation correction reduced localization errors by 0.15 mm on average. The improvement from attenuation correction was 1.0 mm on average for the more poorly localized targets. Attenuation correction typically reduced localization errors, but for well-localized targets, the detector trajectory generally had a larger effect. Localization performance was found to be robust to iteration number and smoothing. Localization was generally worse using planar images as compared with proximal 180° and 360° SPECT scans. Using a proximal detector trajectory and attenuation correction, localization errors were within 2 mm for the three superficial targets, thus supporting the current role in biopsy and surgery, and demonstrating the potential for SPECT imaging inside radiation therapy treatment rooms.

Combination of converging collimators for high-sensitivity brain SPECT

The objective of this study, which is related to human brain SPECT, was to increase the sensitivity of a triple-camera SPECT system and reduce statistical noise in reconstructed images using a combination of converging collimators. The reason for combining collimators is to ensure both high sensitivity and sufficient sampling without trading off spatial resolution.

METHODS

A high-sensitivity half-cone-beam (HCB) collimator, designed specifically for brain imaging, was combined with other collimators and compared with conventional parallel-beam and fanbeam circular orbit acquisitions. For comparison, previously studied HCB collimation with a circle-and-helix data acquisition trajectory was also included in this study. Simulations of the Hoffman 3-dimensional brain phantom were performed to calculate the efficiencies of collimators and their combinations and to quantitatively evaluate reconstruction bias, statistical noise, and signal-to-noise ratios in the reconstructed images. Experimental brain phantom data were also acquired and compared for different acquisition types. Finally, a patient brain scan was obtained with a combination of HCB and fanbeam collimators and compared with a triple-fanbeam circular orbit acquisition.

RESULTS

A combination of 2 HCB collimators and 1 fanbeam collimator, compared with a triple-fanbeam collimator, can increase the photon detection efficiency by 27% and by more than a factor of 2, compared with triple-parallel-hole collimation, with equal spatial resolution measured on the axis of rotation. Quantitative analysis of reconstruction bias and visual analysis of the images showed no signs of sampling artifacts. Reconstructed images in the simulations, experimental brain phantom, and patient brain scans showed improved quality with this collimator combination due to increased sensitivity and reduced noise. Lesion visibility was also improved, as confirmed by signal-to-noise ratios. Alternatively, triple-HCB circle-and-helix acquisition has also shown competitive results, with a slight disadvantage in axial sampling and implementation procedure.

CONCLUSION

Combined HCB and fanbeam collimation is a promising approach for high-sensitivity brain SPECT.

Characterizing the MTF in 3D for a Quantized SPECT Camera Having Arbitrary Trajectories

The emergence of application-specific 3D tomographic small animal and dedicated breast imaging systems has stimulated the development of simple methods to quantify the spatial resolution or Modulation Transfer Function (MTF) of the system in three dimensions. Locally determined MTFs, obtained from line source measurements at specific locations, can characterize spatial variations in the system resolution and can help correct for such variations. In this study, a method is described to measure the MTF in 3D for a compact SPECT system that uses a 16 × 20 cm(2) CZT-based compact gamma camera and 3D positioning gantry capable of moving in different trajectories. Image data are acquired for a novel phantom consisting of three radioactivity-filled capillary tubes, positioned nearly orthogonally to each other. These images provide simultaneous measurements of the local MTF along three dimensions of the reconstructed imaged volume. The usefulness of this approach is shown by characterizing the MTF at different locations in the reconstructed imaged 3D volume using various (1) energy windows; (2) iterative reconstruction parameters including number of iterations, voxel size, and number of projection views; (3) simple and complex 3D orbital trajectories including simple vertical axis of rotation, simple tilt, complex circle-plus-arc, and complex sinusoids projected onto a hemisphere; and (4) object shapes in the camera's field of view. Results indicate that the method using the novel phantom can provide information on spatial resolution effects caused by system design, sampling, energy windows, reconstruction parameters, novel 3D orbital trajectories, and object shapes. Based on these measurements that are useful for dedicated tomographic breast imaging, it was shown that there were small variations in the MTF in 3D for various energy windows and reconstruction parameters. However, complex trajectories that uniformly sample the breast volume of interest were quantitatively shown to have slightly better spatial resolution performance than more simple orbits.

Quantitative Evaluation of Half-Cone-Beam Scan Paths in Triple-Camera Brain SPECT

In this study related to human brain SPECT imaging, simulation of half-cone-beam (HCB) collimation with different scan paths is performed and compared with simulated fan-beam and parallel-hole circular orbit acquisitions of disk-phantom projection data. Acquisition types are quantitatively evaluated based on the photon detection efficiency, the root-mean-squared error, contrast and signal-to-noise ratio measurements of the reconstructed images. We demonstrate that a triple-camera SPECT system with half-cone-beam collimators and circle-and-helix scan paths can offer up to a 26% efficiency increase over fan-beam, and up to a 128% increase over parallel-hole collimators for equal spatial resolutions, and display no visible axial sampling artifacts in reconstructed disk-phantom images. In addition, we perform qualitative experimental evaluation of triple-HCB circle-and-helix acquisition using a Hoffman 3D brain phantom. Reconstructed brain phantom images show improved quality due to reduced noise and no apparent sampling artifacts. Triple-HCB circle-and-helix SPECT has a potential for improved brain imaging, producing higher image quality with a smaller reconstruction error and better lesion detectability due to increased efficiency for equal spatial resolution compared to conventional fan-beam and parallel-hole SPECT.

Brain SPECT Simulation Using Half-Cone-Beam Collimation and Single-Revolution Helical-Path Acquisition

In this study related to human brain SPECT imaging, simulation of half-cone-beam collimation and helical-path data acquisition is performed. We discuss problems related to circular-orbit acquisition using cone-beam collimation, such as shoulder interference resulting in object truncation, and insufficient sampling of the object resulting in axial distortions in the reconstructed images. We demonstrate that a triple-camera SPECT system with half-cone-beam collimation and single-revolution helical-path acquisition eliminates both issues and offers substantially improved sampling and almost artifact-free reconstruction of the object.

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.

Intertumoral differences in hypoxia selectivity of the PET imaging agent 64Cu(II)-diacetyl-bis(N4-methylthiosemicarbazone)

Cu-Diacetyl-bis(N(4)-methylthiosemicarbazone) (Cu-ATSM) is a recently developed PET imaging agent for tumor hypoxia. However, its accuracy and reliability for measuring hypoxia have not been fully characterized in vivo. The aim of this study was to evaluate (64)Cu-ATSM as a hypoxia PET marker by comparing autoradiographic distributions of (64)Cu-ATSM with a well-established hypoxia marker drug, EF5.

METHODS

R3230 mammary adenocarcinomas (R3230Ac), fibrosarcomas (FSA), and 9L gliomas (9L) were used in the study. EF5 and Hoechst 33342, a vascular perfusion marker, were administered to the animal for immunohistochemical analysis. (64)Cu-ATSM microPET and autoradiography were performed on the same animal. The tumor-to-muscle ratio (T/M ratio) and standardized uptake values (SUVs) were characterized for these 3 different types of tumors. Five types of images-microPET, autoradiography, EF5 immunostaining, Hoechst fluorescence vascular imaging, and hematoxylin-and-eosin histology-were superimposed, evaluated, and compared.

RESULTS

A significantly higher T/M ratio and SUV were seen for FSA compared with R3230Ac and 9L. Spatial correlation analysis between (64)Cu-ATSM autoradiography and EF5 immunostained images varied between the 3 tumor types. There was close correlation of (64)Cu-ATSM uptake and hypoxia in R3230Ac and 9L tumors but not in FSA tumors. Interestingly, elevated (64)Cu-ATSM uptake was observed in well-perfused areas in FSA, indicating a correlation between (64)Cu-ATSM uptake and vascular perfusion as opposed to hypoxia. The same relationship was observed with 2 other hypoxia markers, pimonidazole and carbonic anhydrase IX, in FSA tumors. Breathing carbogen gas significantly decreased the hypoxia level measured by EF5 staining in FSA-bearing rats but not the uptake of (64)Cu-ATSM. These results indicate that some other (64)Cu-ATSM retention mechanisms, as opposed to hypoxia, are involved in this type of tumor.

CONCLUSION

To our knowledge, this study is the first comparison between (64)Cu-ATSM uptake and immunohistochemistry in these 3 tumors. Although we have shown that (64)Cu-ATSM is a valid PET hypoxia marker in some tumor types, but not for all, this tumor type-dependent hypoxia selectivity of (64)Cu-ATSM challenges the use of (64)Cu-ATSM as a universal PET hypoxia marker. Further studies are needed to define retention mechanisms for this PET marker.

Evaluation of fully 3-D emission mammotomography with a compact cadmium zinc telluride detector

A compact, dedicated cadmium zinc telluride (CZT) gamma camera coupled with a fully three-dimensional (3-D) acquisition system may serve as a secondary diagnostic tool for volumetric molecular imaging of breast cancers, particularly in cases when mammographic findings are inconclusive. The developed emission mammotomography system comprises a medium field-of-view, quantized CZT detector and 3-D positioning gantry. The intrinsic energy resolution, sensitivity and spatial resolution of the detector are evaluated with Tc-99m (140 keV) filled flood sources, capillary line sources, and a 3-D frequency-resolution phantom. To mimic realistic human pendant, uncompressed breast imaging, two different phantom shapes of an average sized breast, and three different lesion diameters are imaged to evaluate the system for 3-D mammotomography. Acquisition orbits not possible with conventional emission, or transmission, systems are designed to optimize the viewable breast volume while improving sampling of the breast and anterior chest wall. Complications in camera positioning about the patient necessitate a compromise in these two orbit design criteria. Image quality is evaluated with signal-to-noise ratios and contrasts of the lesions, both with and without additional torso phantom background. Reconstructed results indicate that 3-D mammotomography, incorporating a compact CZT detector, is a promising, dedicated breast imaging technique for visualization of tumors < 1 cm in diameter. Additionally, there are no outstanding trajectories that consistently yield optimized quantitative lesion imaging parameters. Qualitatively, imaging breasts with realistic torso backgrounds (out-of-field activity) substantially alters image characteristics and breast morphology unless orbits which improve sampling are utilized. In practice, the sampling requirement may be less strict than initially anticipated.

Dosimetry and radiographic analysis of 131I-labeled anti-tenascin 81C6 murine monoclonal antibody in newly diagnosed patients with malignant gliomas: a phase II study

The objective was to perform dosimetry and evaluate dose-response relationships in newly diagnosed patients with malignant brain tumors treated with direct injections of (131)I-labeled anti-tenascin murine 81C6 monoclonal antibody (mAb) into surgically created resection cavities (SCRCs) followed by conventional external-beam radiotherapy and chemotherapy.

METHODS

Absorbed doses to the 2-cm-thick shell, measured from the margins of the resection cavity interface, were estimated for 33 patients with primary brain tumors. MRI/SPECT registrations were used to assess the distribution of the radiolabeled mAb in brain parenchyma. Results from biopsies obtained from 15 patients were classified as tumor, radionecrosis, or tumor and radionecrosis, and these were correlated with absorbed dose and dose rate. Also, MRI/PET registrations were used to assess radiographic progression among patients.

RESULTS

This therapeutic strategy yielded a median survival of 86 and 79 wk for all patients and glioblastoma multiforme (GBM) patients, respectively. The average SCRC residence time of (131)I-mu81C6 mAb was 76 h (range, 34-169 h). The average absorbed dose to the 2-cm cavity margins was 48 Gy (range, 25-116 Gy) for all patients and 51 Gy (range, 27-116 Gy) for GBM patients. In MRI/SPECT registrations, we observed a preferential distribution of (131)I-mu81C6 mAb through regions of vasogenic edema. An analysis of the relationship between the absorbed dose and dose rate and the first biopsy results yielded a most favorable absorbed dose of 44 Gy. A correlation between decreased survival and irreversible neurotoxicity was noted. A comparative analysis, in terms of median survival, was performed with previous brachytherapy clinical studies, which showed a proportional relationship between the average boost absorbed dose and the median survival.

CONCLUSION

This study shows that (131)I-mu81C6 mAb increases the median survival of GBM patients. An optimal absorbed dose of 44 Gy to the 2-cm cavity margins is suggested to reduce the incidence of neurologic toxicity. Further clinical studies are warranted to determine the effectiveness of (131)I-mu81C6 mAb based on a target dose of 44 Gy rather than a fixed administered activity.

Phase 1 trial study of 131I-labeled chimeric 81C6 monoclonal antibody for the treatment of patients with non-Hodgkin lymphoma

We report a phase 1 study of pharmacokinetics, dosimetry, toxicity, and response of (131)I anti-tenascin chimeric 81C6 for the treatment of lymphoma. Nine patients received a dosimetric dose of 370 MBq (10 mCi). Three patients received an administered activity of 1480 MBq (40 mCi), and 2 developed hematologic toxicity that required stem cell infusion. Six patients received an administered activity of 1110 MBq (30 mCi), and 2 developed toxicity that required stem cell infusion. The clearance of whole-body activity was monoexponential with a mean effective half-life of 110 hours (range, 90-136 hours) and a mean effective whole-body residence time of 159 hours (range, 130-196 hours). There was rapid uptake within the viscera; however, tumor uptake was slower. Activity in normal viscera decreased proportional to the whole body; however, tumor sites presented a slow clearance (T(1/2), 86-191 hours). The mean absorbed dose to whole-body was 67 cGy (range, 51-89 hours), whereas the dose to tumor sites was 963 cGy (range, 363-1517 cGy). Despite lack of a "blocking" antibody, 1 of 9 patients attained a complete remission and 1 a partial remission. These data demonstrate this radiopharmaceutical to be an encouraging agent for the treatment of lymphoma particularly if methods to protect the normal viscera are developed.

Mammotomography with pinhole incomplete circular orbit SPECT

Dedicated mammotomography with pinhole incomplete circular orbit (PICO) SPECT imaging of an uncompressed pendant breast was evaluated with small, very-high-stopping-power pinhole apertures. Comparisons were made with planar pinhole scintimammography. Enhanced 3-dimensional imaging performance with very-high-stopping-power apertures is thought to ultimately yield improved sensitivities for lesion detection and identification in breast disease.

METHODS

Pinhole collimators made of high-density and high atomic number (184)W or depleted (238)U, with aperture diameters from 1 to 4 mm, were used to image 0.6- and 1.0-cm-diameter spherical lesions in a pendulous, uncompressed breast phantom in planar and PICO-SPECT modes. The breast was centered on the horizontal axis of rotation of an incomplete circular orbit. Lesion, breast and body, and myocardial activities (L:B:M) were included in the phantoms to simulate clinical imaging conditions with (99m)Tc (140 keV). Lesion contrasts and signal-to-noise ratios (SNRs) for all apertures were determined for near clinical acquisition times for L:B:M ratios of 12:1:20 and 7:1:25. A set of minidisks inserted in the breast phantom was scanned to determine sampling limitations at depth from the nipple. In an initial study, a patient with biopsy-confirmed breast carcinoma was injected with 960 MBq (99m)Tc-tetrofosmin and scanned 2 h later with planar pinhole and PICO-SPECT techniques.

RESULTS

Overall, for PICO-SPECT imaging there were small differences in measured counting rate sensitivity (4.9%) and lesion contrast (8.8%) with larger SNR differences (20.8%) between tungsten and depleted uranium pinhole materials at this energy and these lesion sizes. Backgrounds from simulated myocardial uptake had minor contributions in all reconstructed image volumes because of the rapid sensitivity fall-off for pinhole apertures. An optimal aperture diameter between 2 and 3 mm was determined from peak SNR, indicating that these aperture sizes may have the best performance for lesions as small as 0.6 cm in diameter with activity concentration ratios of (99m)Tc similar to those currently seen in patients. Both lesions were visualized with PICO-SPECT better than with planar pinhole imaging, with respective contrast improvements >20 times the values obtained from planar imaging for the same pinholes. In the patient study, higher contrast (>6) visualization of the active tumor periphery was obtained with PICO-SPECT than with planar imaging.

CONCLUSION

These results indicate that the enhanced spatial resolution of smaller apertures outweighs the loss in sensitivity in small lesion identification with PICO-SPECT. Although the imaging differences between investigated aperture types are small and some limitations to this imaging approach exist, dedicated PICO-SPECT of the breast appears to be an improved technique compared with conventional planar pinhole scintimammography. This technique provides enhanced contrast and SNR for imaging small lesions with the high-resolution pinhole apertures along with 3-dimensional localization of the lesions.

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.

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.

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.

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.

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.

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.

Bayesian reconstruction and use of anatomical a priori information for emission tomography

A Bayesian method is presented for simultaneously segmenting and reconstructing emission computed tomography (ECT) images and for incorporating high-resolution, anatomical information into those reconstructions. The anatomical information is often available from other imaging modalities such as computed tomography (CT) or magnetic resonance imaging (MRI). The Bayesian procedure models the ECT radiopharmaceutical distribution as consisting of regions, such that radiopharmaceutical activity is similar throughout each region. It estimates the number of regions, the mean activity of each region, and the region classification and mean activity of each voxel. Anatomical information is incorporated by assigning higher prior probabilities to ECT segmentations in which each ECT region stays within a single anatomical region. This approach is effective because anatomical tissue type often strongly influences radiopharmaceutical uptake. The Bayesian procedure is evaluated using physically acquired single-photon emission computed tomography (SPECT) projection data and MRI for the three-dimensional (3-D) Hoffman brain phantom. A clinically realistic count level is used. A cold lesion within the brain phantom is created during the SPECT scan but not during the MRI to demonstrate that the estimation procedure can detect ECT structure that is not present anatomically.

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.

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.

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.

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.

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.

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.

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.