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PubMed1989

Nonparametric comparison of entire ROC curves for computerized ECG left ventricular hypertrophy algorithms using data from the Framingham Heart Study.

A computer program may be capable of several different statements for left ventricular hypertrophy (eg, possible LVH, probable LVH, consistent with LVH), but such statements resulting from discretized levels of sensitivity/specificity would represent only isolated points on a receiver-operating characteristic (ROC) curve, which is a plot of all levels of sensitivity versus specificity. Even if two algorithms use the same discrete scales, their performances may not readily be compared. The authors present a comparison methodology for ROC curves using ROC area as a nonparametric measure of the ability of the algorithm to separate the two populations; the ROC area ranges from 0.5 (no ability) to 1.0 (perfect separation) and is unbiased if the normal versus abnormal populations have no common values for the measurement. The methodology compares the performance of ECG algorithms on the same population of cases by testing for significant differences of ROC areas and incorporating correlation of the algorithms in a nonparametric way. To illustrate this methodology, they use ECG and echocardiographic data from the Framingham Heart Study.

Algorithms↗
PubMed1992

Maximal dynamic range electrotactile stimulation waveforms.

A new method to measure the dynamic range of electrotactile (electrocutaneous) stimulation uses both steepest ascent (gradient) and one-variable-at-a-time methods to determine the waveform variables that maximize the subjective magnitude (intensity) of the electrotactile percept at the maximal current without discomfort for balanced-biphasic pulse bursts presented at a 15-Hz rate. The magnitude at the maximal current without discomfort is maximized by the following waveform (range tested in parentheses): number of pulses/burst = 6 (1-20), pulse repetition rate within a burst = 350 Hz (200-1500), and phase width = 150 microseconds (40-350). The interphase interval (separation between positive and negative phases in a biphasic pulse) does not affect dynamic range from 0-500 microseconds. The number of pulses/burst has a large effect on the perceived dynamic range when this is measured using a subjective-magnitude-based algorithm, whereas it has little effect on the traditional dynamic range measure, i.e., (maximal current without discomfort)/(sensation threshold current). The perceived stimulus magnitude at the maximal current without discomfort is approximately twice as strong with 6 pulses/burst as it is with 1 pulse/burst (a frequently-used waveform).

Bias↗
PubMed1992

Electrical impedance tomography. Determination of the boundary of an object inserted into a water-filled cylinder.

In order to circumvent the electrode position determination problem in static electrical impedance tomography, it is possible to insert the object to be imaged into a water-filled cylinder on which the electrodes are at fixed and known positions. It has previously been shown that if the boundary of the internally placed object and the conductivity of the salty water in the cylinder are known, then a significant improvement in the conductivity image of the object is obtained. An algorithm for finding the boundary of an internally placed object is developed based on the finite element method (FEM). The boundary is assumed to obey a parametric model and the parameters are estimated by inverting a matrix representing the sensitivity of the boundary voltage measurements to parameter variations. The algorithm assumes that the object's internal conductivity is uniform and known. Simulation studies show that if the internal conductivity is not uniform to the extent found in the arm cross-sections, up to 9% error in the boundary, as measured from a centrally placed reference point, may result. It is also shown that if previous knowledge about the boundary shape is used to model the boundary with fewer numbers of parameters, then the boundary may be found with less error.

Algorithms↗
PubMed1991

Geometric quantitative coronary arteriography. A comparison of unsubtracted and dual energy-subtracted images.

The application of dual energy (DE) subtraction techniques to quantitative coronary arteriography (QCA) has the advantage of removing the tissue signal surrounding the vessel profile. We have compared the performance of two geometric QCA algorithms on DE-subtracted and -unsubtracted images to determine, for each, if DE subtraction is advantageous. The two algorithms under study were an edge detection algorithm and a Fourier analysis-based algorithm. For each algorithm, linear regression analysis was performed of measured cross-sectional area (CSA) versus actual CSA of coronary vessel phantoms. The edge detection algorithm was found to have improved precision (P less than .05) when applied to the DE-subtracted images. The Fourier analysis algorithm, however, was not effected by the DE subtraction. Among the unsubtracted image results, the Fourier measurements were more accurate (P less than .05) than the edge detection measurements. We conclude that the benefits to edge detection QCA of DE tissue subtraction outweigh the disadvantages of increased image noise and possible misregistration artifacts. However, the Fourier algorithm is relatively insensitive to tissue signal variations.

Algorithms↗
PubMed1991

Use of immunoglobulin heavy-chain and light-chain measurements in a multicenter trial to investigate monoclonal components: II. Classification by use of computer-based algorithms.

We describe a computer algorithm for classifying serum monoclonal proteins (MC) based on serum protein electrophoresis (SPE) and the automated measurement of kappa and lambda light chains and IgG, IgA, and IgM. We developed the algorithm by using a large database of unselected samples containing MC collected in a multicenter study. The performance of the algorithm was optimized by using iterative computational procedures and was tested on both the development database and on an independent set of MC-containing samples. With the development database, the algorithm correctly classified 50% and misassigned 2.5% of the MC. Where the MC were present in concentrations greater than 10 g/L, the rate of successful classification increased to 72% with 3% misclassification. When the algorithm was tested on a group of 101 MC-containing samples from an independent source, 67% were correctly classified and 8% misclassified, half of the latter being unusual IgD myelomas. We discuss the scope for the application of the algorithm in routine laboratory practice involving personal computer software.

Algorithms↗
PubMed1988

Improved algorithms for automatic bone histomorphometry on a numerized image analysis system.

Quantitative histological methods have proved to be the most effective methods in bone disease research. Faster and more accurate techniques are currently needed. We have developed a simple digitized image analysis system which allows accurate measurements of trabecular bone mass. The algorithm is based on the 'four-connected sets' mathematical theory. Given a numerized image displayed by a CCD camera, the algorithm recognizes all possible four-connected sets and provides area measurements. The first procedure automatically eliminates small, irrelevant profiles (wrinkles, cell nuclei, etc.) while larger profiles are erased interactively. The second procedure similarly erases the artefactual defects within the trabeculae (artefactual cracks or empty osteocytic lacunae). The method was shown to be very accurate and time-saving.

Algorithms↗
PubMed1984

A comparison of the artificial pancreas (glucose controlled insulin infusion system) and a manual technique for assessing insulin sensitivity during euglycaemic clamping.

Two main methods are available for assessing insulin sensitivity with the hyperinsulinaemic euglycaemic clamp technique: one employs a glucose-controlled insulin infusion system (the Biostator) with automatic feedback control; the second depends on frequent glucose measurement and the use of an algorithm and a pocket calculator ('manual') to determine the glucose infusion rate. The amount of glucose infused is a measure of insulin sensitivity. The efficiency of the two methods was compared in nine normal subjects (seven lean, two obese). After an overnight fast subjects were infused with insulin at 50 mU X kg-1 X h-1 for 2 h; this rate was doubled during the first 10 min for the manual technique. Blood glucose averaged 4.7 +/- 0.1 and 4.8 +/- 0.1 mmol/l from 0 to 120 min for Biostator and manual techniques and did not deviate significantly from the desired level. Variability of the clamp was also similar over the same period (coefficient of variation 5.1 +/- 0.6% and 6.4 +/- 0.7%, Biostator and manual). Glucose infused to maintain steady state from 60 to 120 min was higher, however, with the manual than the Biostator method (5.7 +/- 0.6 versus 4.4 +/- 0.6 mg X kg-1 X min-1, p less than 0.01) even when the loading dose was omitted, although the two methods correlated closely (p less than 0.05). Glucose infusion rate varied more from minute to minute with the Biostator (coefficient of variation 28.8 +/- 3% versus 12.2 +/- 2.1%). Steady-state serum insulin levels (30-120 min) were the same during both methods.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1991

Classification of postoperative cardiac patients: comparative evaluation of four algorithms.

Four classification algorithms based on Bayes' rule for minimum error are compared by evaluating their ability to recognize high- and normal-risk cardio-surgical patients. These algorithms differ in the modelling of the probability density function (pdf) for each class and include: (a) two parametric algorithms based on the assumption of normal pdf; (b) two non-parametric algorithms using Parzen multidimensional approximation of pdf with normal kernels. In each case, classes with both equal and different covariance matrices were considered. A set of 200 patients in the 6 h immediately following cardiac surgery has been used to test the performance of the algorithms. For each patient the three measured variables most effective in representing the difference between the two classes were considered. We found that the two algorithms which explicitly incorporate the information on the different sample covariance between the physiological variables existing in the two classes generally provide better recognition of high- and normal-risk patients. Of these two algorithms the parametric one appears extremely attractive for practical applications, since it exhibits slightly better performance in spite of its great simplicity.

Algorithms↗
PubMed1990

How to study the kinetic depth effect experimentally.

Sperling, Landy, Dosher, and Perkins (1989) proposed an objective 3D shape identification task with 2D artifactual cues removed and with full feedback (FB) to the subjects to measure KDE and to circumvent algorithmically equivalent KDE-alternative computations and artifactual non-KDE processing. (1) The 2D velocity flow-field was necessary and sufficient for true KDE. (2) Only the first-order (Fourier-based) perceptual motion system could solve our task because the second-order (rectifying) system could not simultaneously process more than two locations. (3) To ensure first-order motion processing, KDE tasks must require simultaneous processing at more than two locations. (4) Practice with FB is essential to measure ultimate capacity (aptitude) and, thereby, to enable comparisons with ideal observers. Experiments without FB measure ecological achievement--the ability of subjects to extrapolate their past experience to the current stimuli.

Algorithms↗
PubMed1992

Prenatal screening for Down's syndrome with use of maternal serum markers.

BACKGROUND: Approximately 35 percent of all cases of Down's syndrome in fetuses can be detected by measuring maternal serum alpha-fetoprotein during the second trimester in the general population of pregnant women. Recent case-control studies indicate that this detection rate could be approximately doubled by measuring serum levels of unconjugated estriol and chorionic gonadotropin, which are abnormally low and abnormally high, respectively, in women carrying fetuses affected by Down's syndrome. METHODS: We prospectively screened 25,207 women and adolescents in the second trimester of pregnancy and assigned each a risk of fetal Down's syndrome with an algorithm that took into account measurements of all three serum markers in combination with maternal age. On this basis, 1661 subjects (6.6 percent) were initially assigned a second-trimester risk of fetal Down's syndrome of at least 1 in 190, and 962 (3.8 percent) were offered amniocentesis for chromosomal analysis after verification of gestational age. Gestational age was determined on the basis of the first day of the last menstrual period or, when available, by ultrasonography. RESULTS: Among the 760 women and adolescents who chose amniocentesis, 20 cases of fetal Down's syndrome were detected, along with 7 other chromosomal disorders. There was 1 additional case of fetal Down's syndrome among the 202 women who chose not to have amniocentesis. The rate of detection of Down's syndrome was thus 58 percent (21 of 36 expected cases), and the frequency of identifying a fetus with Down's syndrome in women undergoing amniocentesis was 1 per 38 amniocenteses (95 percent confidence interval, 1 in 25 to 1 in 62). CONCLUSIONS: Measuring serum alpha-fetoprotein, chorionic gonadotropin, and estriol is more effective in screening for fetal Down's syndrome than measuring maternal serum alpha-fetoprotein alone. Such an expanded protocol can readily be incorporated into existing prenatal screening programs.

Adolescent↗
PubMed1986

Two-view motion analysis: a unified algorithm.

We present a linear algorithm for determining the three-dimensional rotation and translation of a rigid object from two time-sequential perspective views using point correspondences. The algorithm is different from existing ones in two respects. First, various measures for combating noise are incorporated. Second, the algorithm is unified in the sense that, assuming that the surface assumption holds, it can handle both the case of nonzero translation and the case of zero translation.

Algorithms↗
PubMed1990

Simultaneous measurements of blood pH, pCO2, pO2 and concentrations of hemoglobin and its derivates--a multicenter study.

During the last few years a need for simultaneous measurements of pH, pCO2, pO2, total hemoglobin concentration, oxygen saturation, carboxyhemoglobin, and methemoglobin has been emphasized. Besides the direct use of such measured quantities a series of algorithms has been developed, especially to describe series of oxygen parameters of the blood. A multicenter study involving 20 hospital centers in Denmark and Sweden was conducted. ABL pH/blood gas analyzers and OSM3 HEMOXIMETERs from Radiometer A/S, Denmark were used. Each center was requested to perform daily quality control procedures using the QUALICHECK quality control system from RADIOMETER. Blood samples were analyzed simultaneously on both types of analyzers using local procedures. During the study 11,700 blood data sets were collected. The quality control procedures showed that the analyzers performed according to specifications with a few exceptions during the study. Measured values of hematocrit were obtained for some samples allowing for calculation of a regression equation for hematocrit and total hemoglobin. Average values for total hemoglobin from arterial and venous samples showed 7.2 mmol/L which is well below the typical normal value of 9.3 mmol/L. This was reflected in the calculated value for total oxygen (oxygen content). The measured quantities made calculation of 2,3-diphosphoglycerate, cDPG, possible. Based on the DPG results, an internal quality procedure has been suggested. Oxygen saturation was calculated based on actual values of pH and pO2 and the standard oxyhemoglobin dissociation curve, and then compared to the measured oxygen saturation. The discrepancy between the calculated and measured values for the different types of blood samples showed that calculation of oxygen saturation should be used with great care for pO2 values below 10 kPa and especially for capillary and umbilical blood samples.

2,3-Diphosphoglycerate↗
PubMed1992

Image comparison techniques for use with megavoltage imaging systems.

In this paper we describe software facilities for enabling patient positioning studies using the megavoltage imaging system developed at the Royal Marsden Hospital and Institute of Cancer Research. The study focuses on the use of the system for three purposes: patient position verification (by comparing images taken at treatment simulation with megavoltage images taken at treatment time); reproducibility studies (by analysing a set of megavoltage images); and set-up correction (by adjusting the set-up until the megavoltage image obtained at treatment registers with the simulation image). The need is discussed for suitably presented simulator images, a method of determining field boundaries and the possibility of delineating soft-tissue interfaces. Several algorithms of different types, developed specifically for the purpose of intercomparison of planar projection images, are presented. The techniques employed and their usefulness, in both the qualitative and the quantitative sense, are discussed. The results are presented of a phantom and clinical study, to evaluate the rigour and reproducibility of the algorithms. These results indicate that measurements can be made to an accuracy of about 1-2 mm, with a similar value for interobserver reproducibility for the best image comparison techniques available.

Algorithms↗
PubMed1989

Correcting for the thermodynamic characteristics of a body plethysmograph.

A constant volume body plethysmograph is used to measure changes in total body volume by relating pressure changes measured inside a closed chamber to changes in its volume. The relationship between pressure and volume, however, is complicated by the fact that rapid changes in pressure are accompanied by changes in the temperature of the gas in the plethysmograph. The rate at which the temperature of the gas subsequently approaches that of the environment affects the frequency response of the plethysmograph as a volume measuring instrument. In this paper it is shown that the transient response in plethysmograph pressure to a step change in volume must be a multi-component function, due to the gas near the walls of the plethysmograph approaching thermal equilibrium with the environment at a faster rate than gas near the center. The step response of a 200 L plethysmograph was determined experimentally and found to be well described empirically by a sum of two decaying exponential functions of time. The fitted exponentials were used to construct an algorithm for digitally correcting a measured pressure signal for the effects of the plethysmograph response. When applied to a hypothetical volume waveform over a single breath, an almost perfect correction was obtained.

Animals↗
PubMed1989

Using composite health status measures to assess the nation's health.

Research in progress at the National Center for Health Statistics for evaluating the usefulness of composite measures of health status for assessing the nation's health is described. Three measures suitable for use in the general population, the Health Insurance Experiment-Functional Limitations (HIE-FL), the Health Utility Index (HUI), and the Quality of Well-being (QWB) scale, have been mapped to data collected in the 1980 National Health Interview Survey (NHIS). Analysis using current algorithms for making composite function status measures according to the QWB methods suggests that traditional single indicators of health tend to overestimate the level of health by about 10%. When symptoms and problems are added to the composite function score, the overestimate as measured by the single indicator is at least 50%. The authors are continuing to validate these algorithms, to develop similar ones for the HIE-FL and HUI, and to extend the analysis to data collected in 1977, 1979, and 1984. Current results indicate that to realize fully the benefits of composite measures, well-established, valid, and reliable measures of health-related quality of life should be included as part of the regular NHIS data collection procedures.

Activities of Daily Living↗
PubMed1987

Skeletal measurements using a flying spot digital imaging device.

A flying spot digital imaging unit was devised to measure skeletal length and angles. This device uses a spinning chopper wheel and fixed slit collimator in front of a conventional X-ray tube to produce a scanning pencil beam that passes through the patient and onto an electronic detector. The beam scans the patient transversely and the device moves longitudinally, creating a digital image with a skin dose of less than 2 mrad (0.02 mGy). Patients can be imaged both when upright and when recumbent. A large field of view makes it possible for the entire length of the spine or lower extremities to be included in a single image. Built-in computer algorithms are used to make measurements of skeletal lengths and angles. The accuracy of the digital device was tested by making measurements of known angles and lengths using protractors and rulers. Skeletal measurements were then made on a number of patients. Interobserver error was tested using both patient and nonpatient images. The following patient measurements were made: Cobb angle in 166 patients with scoliosis; knee joint angles in 120 patients undergoing knee surgery; lower extremity lengths in 78 patients with length inequalities; bony fragment angulation and displacement in 30 patients with healing fractures. These studies have shown the device to be accurate with low interobserver error, while delivering much lower patient dose than more conventional methods. The ability to manipulate window levels and widths enables visualization of thick and thin body parts on the same image. Our experience both in the laboratory and with patients has been encouraging, and the method appears to have several advantages over more conventional techniques.

Bone and Bones↗
PubMed1992

Hemodynamic and oxygen transport patterns for outcome prediction, therapeutic goals, and clinical algorithms to improve outcome. Feasibility of artificial intelligence to customize algorithms.

A generalized decision tree or clinical algorithm for treatment of high-risk elective surgical patients was developed from a physiologic model based on empirical data. First, a large data bank was used to do the following: (1) describe temporal hemodynamic and oxygen transport patterns that interrelate cardiac, pulmonary, and tissue perfusion functions in survivors and nonsurvivors; (2) define optimal therapeutic goals based on the supranormal oxygen transport values of high-risk postoperative survivors; (3) compare the relative effectiveness of alternative therapies in a wide variety of clinical and physiologic conditions; and (4) to develop criteria for titration of therapy to the endpoints of the supranormal optimal goals using cardiac index (CI), oxygen delivery (DO2), and oxygen consumption (VO2) as proxy outcome measures. Second, a general purpose algorithm was generated from these data and tested in preoperatively randomized clinical trials of high-risk surgical patients. Improved outcome was demonstrated with this generalized algorithm. The concept that the supranormal values represent compensations that have survival value has been corroborated by several other groups. We now propose a unique approach to refine the generalized algorithm to develop customized algorithms and individualized decision analysis for each patient's unique problems. The present article describes a preliminary evaluation of the feasibility of artificial intelligence techniques to accomplish individualized algorithms that may further improve patient care and outcome.

Algorithms↗
PubMed1986

Estimation of drug binding parameters.

Many methods have been suggested and tested to estimate the association constants and binding capabilities of ligand-macromolecule interactions from experimental data. This problem is a subset of the general problem of parameter estimation for nonlinear algebraic models where both the independent and dependent variables are subject to measurement error. It is often difficult to anticipate the effect on the parameter estimates that is caused by error in the primary measurements. In this work, a computer algorithm is described which finds the maximum likelihood estimate for the true values of the parameters and also estimates for the values of the measurements. It is applied to experimental binding data in two examples for fitting the association constants and binding capacities.

Dicumarol↗