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At least 847 records · Page 47Linked to original sources
PubMed2005

Three-dimensional assessment of MR imaging-guided percutaneous cryotherapy using multi-performer repeated segmentations: the value of supervised learning.

RATIONALE AND OBJECTIVES: Accurate and reproducible segmentations of two-dimensional images are an important prerequisite for assessing tumor ablations three dimensionally (3D). We evaluated whether supervised learning methods would improve multiperformer repeated segmentations of magnetic resonance images (MRI) obtained before and after MRI-guided cryotherapy of renal cell carcinoma. MATERIALS AND METHODS: Three medical students independently performed five manual segmentations of a biopsy-proven renal cell carcinoma that was treated with percutaneous MRI-guided cryotherapy. Using pretreatment (T2-weighted fast recovery fast spin echo [FRFSE]) and posttreatment (T1-weighted, fat-suppressed, dynamically enhanced) MRIs, regions of tumor cryonecrosis were segmented. The same tasks were repeated after an experienced abdominal radiologist provided supervised learning. Segmentation sensitivity was compared with an estimated 3D-ground truth via voxel counts for regions of tumor, both before and after treatment, and for the regions of cryonecrosis. The sensitivity of each repeated segmentation was compared against the estimated ground truth using sensitivity, overlap index, and volume (mL). RESULTS: Supervised learning significantly improved posttreatment segmentation sensitivity (P = .03). With supervised learning, the ranges of the performance metrics over the segmentation performers were: pretreated tumor, sensitivity 0.902-0.999, overlap index 0.935-0.961, and volume 19.15-23.71 mL; posttreated tumor, sensitivity 0.923-0.991, overlap index 0.952-0.981, and volume 20.67-22.70 mL; in the ablation zone, sensitivity 0.938-0.969, overlap index 0.940-0.962, and volume 31.79-32.36 mL. CONCLUSIONS: Supervised learning improved multiperformer repeated segmentations of MRIs obtained before and after MRI-guided percutaneous cryotherapy of renal cell carcinoma. These methods may prove useful in aiding the 3D assessment of percutaneous tumor ablations.

Aged↗
PubMed2006

A fuzzy c-means (FCM)-based approach for computerized segmentation of breast lesions in dynamic contrast-enhanced MR images.

RATIONALE AND OBJECTIVES: Accurate quantification of the shape and extent of breast tumors has a vital role in nearly all applications of breast magnetic resonance (MR) imaging (MRI). Specifically, tumor segmentation is a key component in the computerized assessment of likelihood of malignancy. However, manual delineation of lesions in four-dimensional MR images is labor intensive and subject to interobserver and intraobserver variations. We developed a computerized lesion segmentation method that has the advantage of being automatic, efficient, and objective. MATERIALS AND METHODS: We present a fuzzy c-means (FCM) clustering-based method for the segmentation of breast lesions in three dimensions from contrast-enhanced MR images. The proposed lesion segmentation algorithm consists of six consecutive stages: region of interest (ROI) selection by a human operator, lesion enhancement within the selected ROI, application of FCM on the enhanced ROI, binarization of the lesion membership map, connected-component labeling and object selection, and hole-filling on the selected object. We applied the algorithm to a clinical MR database consisting of 121 primary mass lesions. Manual segmentation of the lesions by an expert MR radiologist served as a reference in the evaluation of the computerized segmentation method. We also compared the proposed algorithm with a previously developed volume-growing (VG) method. RESULTS: For the 121 mass lesions in our database, 97% of lesions were segmented correctly by means of the proposed FCM-based method at an overlap threshold of 0.4, whereas 84% of lesions were correctly segmented by means of the VG method. CONCLUSION: Our proposed algorithm for breast-lesion segmentation in dynamic contrast-enhanced MRI was shown to be effective and efficient.

Algorithms↗
PubMed2004

Gender differences in the value of ST-segment depression during adenosine stress testing.

Previous studies have suggested that ST-segment depression with adenosine myocardial perfusion imaging (MPI) may be a marker of significant coronary artery disease (CAD). It is unclear if the significance of ST depression differs between men and women. We investigated the diagnostic accuracy of ST-segment depression with adenosine radionuclide MPI as a marker of significant CAD in men and women. Consecutive patients who had angina or suspected CAD and underwent an adenosine stress test and subsequent angiography were retrospectively analyzed. The inclusion criteria were met by 959 patients. Mean age was 64 +/- 11 years, and 43% were women. ST depression occurred in 7.6% of the cohort and more often in women (64% women vs 36% men, p <0.001). Among men and women, patients with ST-segment depression had a significantly higher peak rate-pressure product, more chest pain, and a higher ejection fraction in response to the adenosine infusion compared with those without ST-segment depression. ST-segment depression occurred more often in the presence of stenotic lesions (>/=50% and >/=70%), and left main or 3-vessel disease, regardless of gender. Transient ischemic dilation occurred more often in men with ST-segment depression. The logistic regression analysis demonstrated that the only significant predictors of left main or 3-vessel CAD were gender, an abnormal result on MPI, transient ischemic dilation, and ST-segment depression. In conclusion, ST-segment depression during adenosine MPI is an important marker of angiographically significant CAD in men and women. The presence of ST-segment depression is associated with left main disease and 3-vessel CAD.

Adenosine↗
PubMed2004

Software for interactive segmentation of the carotid artery from 3D black blood magnetic resonance images.

A semiautomatic algorithm for segmenting organ surfaces from 3D medical images is presented in this work. The algorithm is based on a deformable model, and allows the user to initialize the model by combining and molding primitive shapes such as cylinders and spheres to form an initial approximate model of the organ surface. The initial model is automatically deformed to better fit organ boundaries. The algorithm was applied to segment the carotid bifurcation from 3D black blood magnetic resonance (MR) images of 5 subjects. The algorithm-segmented surfaces were compared to surfaces segmented manually by an experienced user. On average, approximately 3 min were required to segment an image using the algorithm, whereas 1h was required for manual segmentation. The average distance between corresponding points on the manually and algorithm-segmented surfaces was 0.37 mm, whereas the average maximum distance was 2.03 mm. Moreover, algorithm-segmented surfaces exhibited less intra-operator variability than those segmented manually.

Algorithms↗
PubMed2003

Arthropod-like expression patterns of engrailed and wingless in the annelid Platynereis dumerilii suggest a role in segment formation.

The origin of animal segmentation, the periodic repetition of anatomical structures along the anteroposterior axis, is a long-standing issue that has been recently revived by comparative developmental genetics. In particular, a similar extensive morphological segmentation (or metamerism) is commonly recognized in annelids and arthropods. Mostly based on this supposedly homologous segmentation, these phyla have been united for a long time into the clade Articulata. However, recent phylogenetic analysis dismissed the Articulata and thus challenged the segmentation homology hypothesis. Here, we report the expression patterns of genes orthologous to the arthropod segmentation genes engrailed and wingless in the annelid Platynereis dumerilii. In Platynereis, engrailed and wingless are expressed in continuous ectodermal stripes on either side of the segmental boundary before, during, and after its formation; this expression pattern suggests that these genes are involved in segment formation. The striking similarities of engrailed and wingless expressions in Platynereis and arthropods may be due to evolutionary convergence or common heritage. In agreement with similarities in segment ontogeny and morphological organization in arthropods and annelids, we interpret our results as molecular evidence of a segmented ancestor of protostomes.

Animals↗
PubMed2005

Is the outcomes of early ST-segment resolution after thrombolytic therapy in acute myocardial infarction always favorable?

AIMS: To determine whether the magnitude of ST-segment resolution after thrombolytic therapy (TT) predicts short- and long-term outcomes in an unselected population of patients with an acute myocardial infarction (AMI). BACKGROUND: Recent studies suggest that resolution of ST-segment elevation (STE) on the 2-hour post-TT electrocardiogram (ECG) is a useful predictor of prognosis. However, these studies were restricted to clinical trials where only 15% to 20% of the patients receiving TT were often enrolled. METHODS: The present study evaluated an unselected consecutive group of patients who received TT. All clinical, investigational, and follow-up data had been collected in a prospective manner. The analysis of ECGs was done retrospectively with the reader blinded to the clinical course. STE at 80 milliseconds after the J point was measured on the baseline and 90-minute ECG using a hand-held caliper. The resolution of STE was categorized as complete (>or=70%), partial (30% to <70%), and none (<30%) as has been done in previous studies. RESULTS: Three hundred fifty-two patients (250 men, 102 women; age, mean+/-SEM, 61.8+/-1.0 years; peak creatine kinase, 1938+/-185 micromol/L; door to needle time, 50.0+/-6 minutes, <30 minutes, 50%; <45 minutes, 70%) with AMIs who received TT were included in the study. Inhospital deaths and recurrent AMI/postinfarct angina revealed no significant association with increasing ST-segment resolution (P>.05). A 70% or higher ST-segment resolution was associated with a significantly lower incidence of inhospital congestive heart failure (CHF) and CHF/death (P<.05). Similarly, with a 70% or higher ST-segment resolution, there was a lower incidence in the 1-year outcomes of CHF and death/CHF. However the 1-year occurrences of unstable angina or recurrent AMIs taken singly did not bear a correlation to increasing magnitudes of ST-segment resolution (P>.05). Although as a composite measure, there was an increasing trend with ST-segment resolution. CONCLUSIONS: Magnitude of ST-segment resolution after TT appears to demonstrate a dichotomous relationship to measured outcomes. Although there is a lower incidence of death/CHF with increasing ST-segment resolution, there appears to be a higher likelihood for recurrent AMI/unstable angina.

Canada↗
PubMed2006

Segmentation of anatomical structures in chest radiographs using supervised methods: a comparative study on a public database.

The task of segmenting the lung fields, the heart, and the clavicles in standard posterior-anterior chest radiographs is considered. Three supervised segmentation methods are compared: active shape models, active appearance models and a multi-resolution pixel classification method that employs a multi-scale filter bank of Gaussian derivatives and a k-nearest-neighbors classifier. The methods have been tested on a publicly available database of 247 chest radiographs, in which all objects have been manually segmented by two human observers. A parameter optimization for active shape models is presented, and it is shown that this optimization improves performance significantly. It is demonstrated that the standard active appearance model scheme performs poorly, but large improvements can be obtained by including areas outside the objects into the model. For lung field segmentation, all methods perform well, with pixel classification giving the best results: a paired t-test showed no significant performance difference between pixel classification and an independent human observer. For heart segmentation, all methods perform comparably, but significantly worse than a human observer. Clavicle segmentation is a hard problem for all methods; best results are obtained with active shape models, but human performance is substantially better. In addition, several hybrid systems are investigated. For heart segmentation, where the separate systems perform comparably, significantly better performance can be obtained by combining the results with majority voting. As an application, the cardio-thoracic ratio is computed automatically from the segmentation results. Bland and Altman plots indicate that all methods perform well when compared to the gold standard, with confidence intervals from pixel classification and active appearance modeling very close to those of a human observer. All results, including the manual segmentations, have been made publicly available to facilitate future comparative studies.

Algorithms↗
PubMed2006

A hierarchical 3D segmentation method and the definition of vertebral body coordinate systems for QCT of the lumbar spine.

We have developed a new hierarchical 3D technique to segment the vertebral bodies in order to measure bone mineral density (BMD) with high trueness and precision in volumetric CT datasets. The hierarchical approach starts with a coarse separation of the individual vertebrae, applies a variety of techniques to segment the vertebral bodies with increasing detail and ends with the definition of an anatomic coordinate system for each vertebral body, relative to which up to 41 trabecular and cortical volumes of interest are positioned. In a pre-segmentation step constraints consisting of Boolean combinations of simple geometric shapes are determined that enclose each individual vertebral body. Bound by these constraints viscous deformable models are used to segment the main shape of the vertebral bodies. Volume growing and morphological operations then capture the fine details of the bone-soft tissue interface. In the volumes of interest bone mineral density and content are determined. In addition, in the segmented vertebral bodies geometric parameters such as volume or the length of the main axes of inertia can be measured. Intra- and inter-operator precision errors of the segmentation procedure were analyzed using existing clinical patient datasets. Results for segmented volume, BMD, and coordinate system position were below 2.0%, 0.6%, and 0.7%, respectively. Trueness was analyzed using phantom scans. The bias of the segmented volume was below 4%; for BMD it was below 1.5%. The long-term goal of this work is improved fracture prediction and patient monitoring in the field of osteoporosis. A true 3D segmentation also enables an accurate measurement of geometrical parameters that may augment the clinical value of a pure BMD analysis.

Absorptiometry, Photon↗
PubMed2004

Evaluation of atlas selection strategies for atlas-based image segmentation with application to confocal microscopy images of bee brains.

This paper evaluates strategies for atlas selection in atlas-based segmentation of three-dimensional biomedical images. Segmentation by intensity-based nonrigid registration to atlas images is applied to confocal microscopy images acquired from the brains of 20 bees. This paper evaluates and compares four different approaches for atlas image selection: registration to an individual atlas image (IND), registration to an average-shape atlas image (AVG), registration to the most similar image from a database of individual atlas images (SIM), and registration to all images from a database of individual atlas images with subsequent multi-classifier decision fusion (MUL). The MUL strategy is a novel application of multi-classifier techniques, which are common in pattern recognition, to atlas-based segmentation. For each atlas selection strategy, the segmentation performance of the algorithm was quantified by the similarity index (SI) between the automatic segmentation result and a manually generated gold standard. The best segmentation accuracy was achieved using the MUL paradigm, which resulted in a mean similarity index value between manual and automatic segmentation of 0.86 (AVG, 0.84; SIM, 0.82; IND, 0.81). The superiority of the MUL strategy over the other three methods is statistically significant (two-sided paired t test, P < 0.001). Both the MUL and AVG strategies performed better than the best possible SIM and IND strategies with optimal a posteriori atlas selection (mean similarity index for optimal SIM, 0.83; for optimal IND, 0.81). Our findings show that atlas selection is an important issue in atlas-based segmentation and that, in particular, multi-classifier techniques can substantially increase the segmentation accuracy.

Algorithms↗
PubMed2005

Atlas-based hippocampus segmentation in Alzheimer's disease and mild cognitive impairment.

This study assesses the performance of public-domain automated methodologies for MRI-based segmentation of the hippocampus in elderly subjects with Alzheimer's disease (AD) and mild cognitive impairment (MCI). Structural MR images of 54 age- and gender-matched healthy elderly individuals, subjects with probable AD, and subjects with MCI were collected at the University of Pittsburgh Alzheimer's Disease Research Center. Hippocampi in subject images were automatically segmented by using AIR, SPM, FLIRT, and the fully deformable method of Chen to align the images to the Harvard atlas, MNI atlas, and randomly selected, manually labeled subject images ("cohort atlases"). Mixed-effects statistical models analyzed the effects of side of the brain, disease state, registration method, choice of atlas, and manual tracing protocol on the spatial overlap between automated segmentations and expert manual segmentations. Registration methods that produced higher degrees of geometric deformation produced automated segmentations with higher agreement with manual segmentations. Side of the brain, presence of AD, choice of reference image, and manual tracing protocol were also significant factors contributing to automated segmentation performance. Fully automated techniques can be competitive with human raters on this difficult segmentation task, but a rigorous statistical analysis shows that a variety of methodological factors must be carefully considered to insure that automated methods perform well in practice. The use of fully deformable registration methods, cohort atlases, and user-defined manual tracings are recommended for highest performance in fully automated hippocampus segmentation.

Aged↗
PubMed2006

Automatic anatomical brain MRI segmentation combining label propagation and decision fusion.

Regions in three-dimensional magnetic resonance (MR) brain images can be classified using protocols for manually segmenting and labeling structures. For large cohorts, time and expertise requirements make this approach impractical. To achieve automation, an individual segmentation can be propagated to another individual using an anatomical correspondence estimate relating the atlas image to the target image. The accuracy of the resulting target labeling has been limited but can potentially be improved by combining multiple segmentations using decision fusion. We studied segmentation propagation and decision fusion on 30 normal brain MR images, which had been manually segmented into 67 structures. Correspondence estimates were established by nonrigid registration using free-form deformations. Both direct label propagation and an indirect approach were tested. Individual propagations showed an average similarity index (SI) of 0.754+/-0.016 against manual segmentations. Decision fusion using 29 input segmentations increased SI to 0.836+/-0.009. For indirect propagation of a single source via 27 intermediate images, SI was 0.779+/-0.013. We also studied the effect of the decision fusion procedure using a numerical simulation with synthetic input data. The results helped to formulate a model that predicts the quality improvement of fused brain segmentations based on the number of individual propagated segmentations combined. We demonstrate a practicable procedure that exceeds the accuracy of previous automatic methods and can compete with manual delineations.

Adult↗
PubMed2005

Expression of 'segmentation' genes during larval and juvenile development in the polychaetes Capitella sp. I and H. elegans.

Polychaete annelids and arthropods are both segmented protostome invertebrates. To investigate whether the segmented body plan of these two phyla share a common molecular ground pattern, we report the developmental expression of orthologues of the arthropod segment polarity genes engrailed (en), hedgehog (hh), and wingless (wg/Wnt1) in larval and juvenile stages of the polychaete annelid Capitella sp. I and en in a second polychaete, Hydroides elegans. Temporally, neither Wnt1 nor hh are detected in the segmented region of the larval body until after morphological segmentation is apparent. Expression of CapI-Wnt1 is limited to a ring of ectoderm marking the future anus during larval segmentation. CapI-hh is expressed in a ring of the hindgut internal to that of CapI-Wnt1, as well as in a subset of ventral nerve cord neurons, anterior gut tissue, and mesoderm. In both H. elegans and Capitella sp. I, en is expressed in a spatially and temporally dynamic manner in segmentally iterated structures as well as a population of cells that migrate internally from ectoderm to mesoderm, possibly representing a population of ecto-mesodermal precursors. Significantly, the expression patterns we report for wg, en, and hh orthologues in Capitella sp. I and for en in larval development of H. elegans are not comparable to the highly conserved ectodermal segment polarity pattern observed in arthropods at any life history stage, consistent with distinct origins of segmentation between annelids and arthropods.

Amino Acid Sequence↗
PubMed1998

Correlation between exercise-induced ischemic ST-segment depression and myocardial blood flow quantified by positron emission tomography.

BACKGROUND: Ischemic ST-segment depression (horizontal or downsloping) is the most common manifestation of exercise-induced myocardial ischemia. The mechanisms responsible for these types of ST-segment depression are largely unknown. We investigated the relation of these 2 types of exercise-induced ST-segment depression to changes in regional myocardial blood flow (RMBF) by using exercise positron emission tomography (PET). METHODS AND RESULTS: The RMBF was measured with nitrogen-13 ammonia PET both at rest and during low-level supine bicycle exercise in 27 patients with angiographically proven coronary artery disease and in 6 healthy volunteers. ST-segment depression was measured from the isoelectric PR segment 80 ms after the J point. Exercise-induced horizontal ST-segment depression > or =0.1 mV was observed in 9 patients and downsloping depression > or =0.1 mV was observed in 18 patients. Multivessel disease was more frequent and areas of exercise-induced ischemia were larger in patients with downsloping depression than in patients with horizontal depression (P < .02, P < .05). In patients with horizontal ST-segment depression, RMBF in ischemic areas and in surrounding areas increased by a similar amount (31%+/-29% and 32%+/-16%) with exercise. In patients with downsloping ST-segment depression, RMBF was unchanged or decreased in ischemic areas (10%+/-24%) but increased in surrounding areas (46%+/-27%) with exercise. In healthy volunteers, RMBF increased in all areas (56%+/-30%) with exercise. CONCLUSIONS: Compared with horizontal changes in ST-segment morphology, downsloping changes may better indicate severe ischemia and greater differences in the increase of blood flow with exercise in the ischemic myocardium and in the surrounding areas.

Adult↗
PubMed1996

Transient autonomic dysfunction precedes ST-segment depression in patients with syndrome X.

Increased sympathetic drive has been suggested to play a role in the pathogenesis of syndrome X (angina pectoris, positive exercise testing, and angiographically normal coronary arteries). Heart rate variability (HRV) studies have shown that patients with syndrome X have an imbalance in autonomic nervous system activity (sympathetic predominance). However, it is not known if transient ST-segment depression which occurs in syndrome X during daily activities is related to this autonomic nervous system dysfunction. This study investigates the relation between the response of the autonomic nervous system, as assessed by HRV analysis, and the occurrence of transient ST-segment depression during 24-hour ambulatory electrocardiographic monitoring in 23 patients (4 men and 19 women, mean age 55 +/- 6 years) with syndrome X. The frequency-domain variables of HRV low-frequency (0.04 to 0.15 Hz) and high-frequency (0.15 to 0.40 Hz) power were measured at 6-minute intervals during the 30 minutes preceding the onset of transient ST-segment depression. Fourteen patients (61%) had > or = 1 episode of ST-segment depression in the 24 hours, whereas the remaining 9 patients (39%) had no significant ST-segment change. HRV measures differed according to whether or not ST-segment depression was associated with increased heart rate. Episodes of ST-segment depression associated with increased heart rate were preceded by a reduction of high-frequency power and an increase in the low-frequency--high-frequency ratio, whereas episodes of ST-segment depression not associated with increased heart rate showed no significant HRV changes. Low-frequency power remained unchanged irrespective of heart rate. Thus, in patients with syndrome X, a sympathovagal imbalance (sympathetic predominance due to vagal tone withdrawal) precedes episodes of ST-segment depression that are associated with an increased heart rate.

Autonomic Nervous System↗
PubMed1999

Magnesium secretion by the distal segment of the Malpighian tubules of the black field cricket Teleogryllus oceanicus.

The short distal segment of unstimulated Teleogryllus Malpighian tubules secreted hyperosmotic fluid containing primarily Mg (125mmoll(-1)), Cl (242mmoll(-1)) and Na (43mmoll(-1)). Remarkably, the volume secreted by the distal segment in unit time was independent of segment length, i.e. the volume was constant regardless of the length of the segment. Magnesium was secreted at a rate of 75.5pmolmin(-1)mm(-1); the highest rate recorded for any epithelium. Low concentrations of K (20mmoll(-1)) were present but almost no P or S. Ca (2.5mmoll(-1)) concentration was higher than in the main segment. The short distal segment secreted 100% of the Mg, 54% of the Cl and 23% of the Na secreted by the whole tubule. The main segment secreted fluid containing primarily K (199mmoll(-1)), Cl (149mmoll(-1)), Na (104mmoll(-1)) and P (48mmoll(-1)) with very low concentrations of Ca (1mmoll(-1)) and S. The main segment appeared to reabsorb a small fraction of the Mg secreted by the distal segment. The fluid secreted by the whole tubule was isosmotic and alkaline, approximately pH8.

Journal Article↗
PubMed1993

Morphologic and functional alterations of intestinal segments following urinary diversion.

The current study was undertaken to determine the effect of chronic urinary diversion on both the morphology and absorptive function of intestine. Adult female Wistar rats (N = 8) underwent urinary diversion by interposing a 10-12 cm. segment of distal ileum between the bladder and sigmoid colon following urethral ligation. Control animals (N = 8) underwent sham laparotomies. Three months later, the diverted segment and a 10-12 cm. segment of distal ileum in control animals were studied. The intestinal segments were studied morphologically by both light microscopy and transmission electron microscopy. By using a previously reported in vivo intestinal perfusion model, the transport properties of the intestinal segments were examined. There were prominent and consistent morphological alterations in the diverted segments when compared with normal ileum. These changes consisted of marked mucosal atrophy evidenced by loss of microvilli with decreased villi-to-crypt ratio in diverted segments. Diverted ileum secreted more sodium and absorbed less chloride than normal ileum though this difference was not statistically significant. Water flux into the intestinal lumen was higher in diverted bowel (p = .06). The absorption of ammonium, potassium, and urea, as well as bicarbonate secretion, was the same in the diverted ileum as in normal ileum. There was also no significant difference in pH change or osmolar flux between the two groups. In conclusion, the results of this study indicate that despite morphological changes as a result of chronic exposure of urine, intestinal segments continue to transport urinary solutes similar to normal nondiverted segments. This represents the first convincing evidence that the reason for the low incidence of metabolic alterations following urinary diversion is not due to decreased absorptive capacity of the intestinal mucosa.

Animals↗
PubMed2001

Spatial scale of motion segmentation from speed cues.

For the accurate perception of multiple, potentially overlapping, surfaces or objects, the visual system must distinguish different local motion vectors and selectively integrate similar motion vectors over space to segment the retinal image properly. We recently showed that large differences in speed are required to yield a percept of motion transparency. In the present study, to investigate the spatial scale of motion segmentation from speed cues alone, we measured the speed-segmentation threshold (the minimum speed difference required for 75% performance accuracy) for 'corrugated' random-dot patterns, i.e. patterns in which dots with two different speeds were alternately placed in adjacent bars of variable width. In a first experiment, we found that, at large bar widths, a smaller speed difference was required to segment and perceive the corrugated pattern of moving dots, while at small bar-widths, a larger speed difference was required to segment the two speeds and perceive two transparent surfaces of moving dots. Both the perceptual and segmentation performance transitions occurred at a bar width of around 0.4 degrees. In a second experiment, speed-segmentation thresholds were found to increase sharply when dots with different speeds were paired within a local pooling area. The critical pairing distance was about 0.2 degrees in the fovea and increased linearly with stimulus eccentricity. However, across the range of eccentricities tested (up to 15 degrees ), the critical pairing distance did not change much and remained close to the receptive field size of neurons within the primate primary visual cortex. In a third experiment, increasing dot density changed the relationship between speed-segmentation thresholds and bar width. Thresholds decreased for large bar widths, but increased for small bar widths. All of these results are well fit by a simple stochastic model, which estimates the probabilities of having identical or different motion vectors within a local pooling area whose size is the same as that of primate V1 neurons. Altogether, these results demonstrate that speed-based segmentation can function well, even at small spatial scales (i.e. high-spatial frequencies of spatial corrugation) and thereby emphasizes the critical role of a local pooling process early in the cortical motion-processing pathway.

Cues↗
PubMed2000

The effect of resting ST segment depression on the diagnostic characteristics of the exercise treadmill test.

OBJECTIVES: The aim of this study is to demonstrate the effect of resting ST segment depression on the diagnostic characteristics of the exercise treadmill test. BACKGROUND: Previous studies evaluating the effect of resting ST segment depression on the diagnostic characteristics of exercise treadmill test have been conducted on relatively small patient groups and based only on visual electrocardiogram (ECG) analysis. METHODS: A retrospective analysis of data collected prospectively was performed on consecutive patients referred for evaluation of chest pain. One thousand two hundred eighty-two patients without a prior myocardial infarction underwent standard exercise treadmill tests followed by coronary angiography, with coronary artery disease defined as a 50% narrowing in at least one major epicardial coronary artery. Sensitivity, specificity, predictive accuracy and area under the curve of the receiver operating characteristic (ROC) plots were calculated for patients with and without resting ST segment depression as determined by visual or computerized analysis of the baseline ECG. RESULTS: Sensitivity of the exercise treadmill test increased in 206 patients with resting ST segment depression determined by visual ECG analysis compared with patients without resting ST segment depression (77 +/- 7% vs. 45 +/- 4%) and specificity decreased (48 +/- 12% vs. 84 +/- 3%). With computerized analysis, sensitivity of the treadmill test increased in 349 patients with resting ST segment depression compared with patients without resting ST segment depression (71 +/- 6% vs. 42 +/- 4%) and specificity decreased (52 +/- 9% vs. 87 +/- 3%) (p < 0.0001 for all comparisons). There was no significant difference in the area under the curve of the ROC plots (0.66-0.69) or the predictive accuracy (62-68%) between the four subgroups. CONCLUSIONS: The diagnostic accuracy and high sensitivity of the exercise treadmill test in a large cohort of patients with resting ST segment depression and no prior myocardial infarction support the initial use of the test for diagnosis of coronary artery disease. The classification of resting ST segment depression by method of analysis (visual vs. computerized) did not affect the results.

Chest Pain↗