Properties and performance modelling of finite buffer M/G/1/K networks

Finite buffer, single-server queueing systems and networks are difficult to analyze since the length of time a customer spends in the system does not follow the Markovian property. A two-moment approximation schema is developed for the probability distribution of M/G/1/K systems and extended to the analysis of M/G/1/K   queueing networks. The general purpose of this paper is to develop a flexible and practical transform-free approach for computing the probability distribution and performance measures of the system as well as identify the underlying properties of these systems. It is shown that for most performance measures, a sigmoid or S-shaped curve with an inflection point at ρ=1$\rho =1$ appears as K→∞$K\to \infty$. This has direct implications for the analysis and optimization of such systems. The performance modelling of the M/G/1/K queueing networks of general topologies along with extensive numerical results accompany the paper along with the linear concave performance measures for these systems.     

Pitfalls in quantitative contrast echocardiography: the steps to quantitation of perfusion

Current methods used clinically to assess myocardial perfusion are invasive and expensive. As the technology of ultrasound imaging improves, CE may provide a relatively inexpensive, noninvasive means of quantitating myocardial perfusion. Issues regarding stability of microbubble contrast agents must be studied more closely under physiologic conditions. As such, encapsulated microbubbles may provide more stability under physiologic pressures than free gas microbubbles. Introducing high concentrations of contrast, either by hyperconcentrating the contrast agent or by increasing the injection rate, may provide greater stability under physiologic conditions. Further, before quantitative statement of tissue perfusion can be made, the relationship between tracer concentration and system response must be established. Further, a "linear" postprocessing ultrasound setting does not eliminate this requirement as data must still undergo nonlinear transformation during log compression and time-gain compensation. Additionally, issues regarding "electronic thresholding" must be explored more extensively in vivo. Commercial ultrasound scanners, in their present form, may not offer adequate sensitivity for absolute quantitative studies. Further development of modified ultrasound systems may provide sufficient sensitivity for quantitative perfusion imaging. CE offers a potentially powerful tool in the clinical management of patients with ischemic heart disease. Conventional coronary angiography provides information on the size of a lesion, but accompanying tissue perfusion distal to the lesion cannot be determined. Doppler ultrasonography determines velocity of blood flow in large vessels but does not offer the potential to quantitate tissue perfusion. Clearly, CE has a place in the future of diagnostic imaging. The recent work of Ito et al. demonstrated the qualitative potential of CE in the identification of "areas at risk" in patients who had undergone thrombolysis or percutaneous transluminal coronary angioplasty after an acute myocardial infarction. With further improvement in the ultrasound imaging techniques and microbubble stability, CE may offer an inexpensive, noninvasive means of assessing myocardial perfusion.     

The clinical assessment of cancer tests: a statistical approach to the prediction of "false positive" rates from controls

As the sensitivity of the new in vitro cancer detection techniques increases, a larger number of "false positive" results can be expected from control populations apparently free from malignant disease at the time of the test. An attempt is made to predict the number of unexplained positives, together with the expected age and sex distribution, using published cancer registration statistics. The predicted numbers are compared with the observed numbers of unexplained positives in two clinical trials of the MEM test.     

A microcomputer system for processing natural languages

This correspondence describes a microcomputer system, called ?BE (for microprocessor-based English), for processing natural languages. Its techniques and facilities, however, should be extendible to other languages. By using the microprocessor as special purpose hardware for several functions of a natural language processor (in particular, hashing and parsing), the system aids computational linguists by increasing the computational power available for natural language investigations. An overview of ?BE is given followed by a more detailed discussion of the hasher and parser. The paper concludes with specific applications of the microprocessor system to computational linguistics. ?BE is not based on a particular theory of language so that the system's user may define his own theory and investigate its consequences. ?BE contains facilities and capabilities to assist such investigations.     

A national survey of the weight loss in pig carcasses between slaughter and 24 hours post mortem

A national survey was carried out in Great Britain in 1978/79 to determine the weight loss occurring in pig carcasses between slaughter and 24 hours post mortem. A total of 20,600 carcasses were measured over a 12-month period, the sample covering forty-eight slaughtering plants and the main market weights of pig. At 24 hours post mortem, the overall mean weight loss was estimated to be 2·27% of the hot weight. The figure differed little between market weights or between times of the year, but there were important differences between slaughtering plants.     

A dual method for maximum entropy restoration

A simple iterative dual algorithm for maximum entropy image restoration is presented. The dual algorithm involves fewer parameters than conventional minimization in the image space. Mini-computer test results for Fourier synthesis with inadequate phantom data are given.     

Portable Device for Detection of Petit Mal Epilepsy

We have developed and tested a portable device that analyzes the electroencephalogram (EEG) to determine if petit mal epilepsy waveforms are present. Clinicians should rimd it useful in diagnosing seizure activity of their patients. The micropower, battery-operated, portable device indicates a seizure has occurred if three criteria are satisfied: 1) frequencies of 2.5-7 Hz, 2) large amplitude waves, and 3) minimum number of waves per second. Levels and counts are adjustable, thus insuring high reliability against noise artifacts and permitting each subject to be individually fitted. The device has shown promise in giving the patient a possible mechanism of seizure control or suppression.