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R S Cobbold

Publications and source records attributed to R S Cobbold.

At least 55 records · Page 3Linked to original sources
PubMed1989

Role of models in understanding and interpreting clinical Doppler ultrasound.

Mathematical and physical models are essential tools in both fundamental and clinically applied Doppler ultrasound research. In this paper we illustrate a variety of models and show how they can be used to understand and interpret clinical Doppler ultrasound signals, particularly from stenosed arteries. The physical models discussed include both steady and pulsatile flow systems, and also a flow visualization technique that can be used to interpret the Doppler signals at a fundamental hemodynamic level. The mathematical models deal with three different aspects of the Doppler signal: models that describe the mechanism of ultrasound scattering by blood, a model to stimulate the returned Doppler signal and a model that may be used to aid in the analysis of clinical recordings. Each of these models provides a more complete understanding of blood flow through normal and stenosed vessels and contributes to the interpretation of clinical Doppler signals.

Computer Simulation↗
PubMed1988

A comparative study and assessment of Doppler ultrasound spectral estimation techniques. Part I: Estimation methods.

When compared to the classical Discrete Fourier Transform (DFT) or Fast Fourier Transform (FFT) approach, modern estimation methods offer the potential for achieving significant improvements in estimating the power density spectrum of Doppler ultrasound signals. Such improvements, for example, might enable minor flow disturbances to be detected, thereby improving the sensitivity in arterial disease assessment. Specifically, reduction in the variance and bias can be achieved, and this may enable disturbed flow to be detected in a more sensitive manner. The approach taken here, is to consider spectral estimation methods as a problem of fitting an assumed model to the Doppler signal. The models described assume that the signal is stationary. Since the Doppler signal is generally nonstationary, it is assumed that a short enough time window interval can be chosen over which the signal can be considered stationary. We shall review the various methods and when appropriate, relate them to the nature of the Doppler signal.

Arterial Occlusive Diseases↗
PubMed1988

A comparative study and assessment of Doppler ultrasound spectral estimation techniques. Part II: Methods and results.

Various alternative spectral estimation methods are examined and compared in order to assess their possible application for real-time analysis of Doppler ultrasound arterial signals. Specifically, five general frequency domain models are examined, including the periodogram, the general autoregressive moving average (ARMA) model which has the autoregressive (AR) and moving average (MA) models as special cases, and Capon's maximum likelihood spectral model. A stimulated stationary Doppler signal with a known theoretical spectrum was used as the reference test sequence, and white noise was added to enable various signal/noise conditions to be created. The performance of each method representative of each spectral model was assessed using both qualitative and quantitative schemes that convey information related to the bias and variance of the spectral estimates. Three integrated performance indices were implemented for quantitative analysis. The relative computational complexity for each algorithm was also investigated. Our results indicate that both the AR(Yule-Walker) and ARMA(singular value decomposition) models of orders (8) and (4,4), respectively, show good agreement with the theoretical spectrum, and yield estimates with variances considerably less than the Fast Fourier Transform (FFT). Preliminary results obtained with these methods using a clinical, non-stationary Doppler signal supports these observations.

Algorithms↗
PubMed1988

Comparison of four digital maximum frequency estimators for Doppler ultrasound.

The performance of four methods for digitally estimating the maximum frequency waveform from the Doppler ultrasound spectrum, are described. The methods investigated are: a percentile method, D'Alessio's threshold crossing method [D'Alessio T. (1985) "Objective" algorithm for maximum frequency estimation in Doppler spectral analysers. Med. Biol. Engng and Comput. 23, 63-68.], a modified threshold crossing method, and a new hybrid algorithm. Evaluations of the variance and bias were performed using stationary simulated continuous wave (CW) Doppler signals of different bandwidths and signal/noise ratios (SNR) of 9 and 17 dB. Furthermore, a simulated nonstationary Doppler signal, similar to that from a normal internal carotid artery, was also used to compare the various methods. Overall, it was found that the modified threshold method and the new hybrid method have the best performance over a wide range of signal and noise hybrid method have the best performance over a wide range of signal and noise conditions; however, D'Alessio's method also performs well for low SNR's.

Computer Simulation↗
PubMed1986

A new pulsatile flow visualization method using a photochromic dye with application to Doppler ultrasound.

A nonintrusive method for the visualization of pulsatile flow velocity profiles is described. The method is based on the use of a photochromic dye that is added to the fluid being studied and a nitrogen laser which excites the dye producing a marker "line" whose movement can be photographed. A microcomputer is used as a system controller, to coordinate the system timing and to manage the data transfer. The method used for analysis of the photographs to determine the velocity profiles is described. Examples are presented of instantaneous velocity profiles obtained from velocity waveforms that are similar to those of the femoral artery. In addition, application of the system for studying the relationship between Doppler ultrasound spectral recordings and flow velocity profiles is discussed.

Biomedical Engineering↗
PubMed1986

Comparison of CW Doppler ultrasound spectra with the spectra derived from a flow visualization model.

The methods and results of a study to determine the accuracy of continuous wave (CW) Doppler spectral recordings by comparison to the spectra derived from the flow profiles photographed simultaneously in a pulsatile flow visualization model are reported in this paper. A pulsatile pump produced a flow velocity waveform, similar to that seen in the human femoral artery, in a quartz glass tube. The velocity profiles, which were made visible by using a photochromic dye/laser technique, were photographed, and at the same time the instantaneous Doppler spectra were recorded. A comparison of the Doppler data and the photographed profiles gave the following results. The Doppler spectrograms and those reconstructed from the flow visualization data were quite similar. Excellent agreement was observed between the instantaneous maximum and mean Doppler waveforms. Individual spectra showed some differences and these differences were quantified by the novel application of certain statistical shape descriptor coefficients that are based on the estimation of the higher order moments of the spectra. The Doppler spectra are generally more skewed towards higher frequencies, narrower, and more peaked than the flow visualization spectra. Analysis of the assumptions and various sources of error lead to the conclusion that the differences were probably caused by ultrasound beam nonuniformity and the effects of refraction, causing a reduction of the beam field response at the tube edges. It is concluded that provided certain precautions are taken in the measurement technique, the CW Doppler ultrasound spectra fairly accurately represent the true velocity profile.

Blood Flow Velocity↗
PubMed1986

Influence of spectral broadening on continuous wave Doppler ultrasound spectra: a geometric approach.

A model is presented that enables the detailed effects of spectral broadening to be calculated for a continuous wave (CW) Doppler system by using geometric boundary arguments. The model assumes a uniform distribution of isotropic scatterers and treats the transmitter and receiver crystals as incremental sources and receivers. Detailed results for rectangular and circular geometries are presented in order to provide a physical understanding of the manner in which spectral broadening arises. Results are given for the circular geometry, to illustrate the manner in which the received spectrum is affected by the transducer size and distance from the vessel.

Blood Flow Velocity↗
PubMed1985

DIAG: a computer-assisted dermatologic diagnostic system--clinical experience and insight.

Over the past few decades, the computer has been utilized in various ways to assist physicians in formulating their differential diagnoses, usually on an experimental basis. Following a review of the three major technics employed in computer-assisted diagnosis, statistical pattern classification, production rules, and cognitive models, a new system (DIAG) is described. The sole function of DIAG is to assist in the formulation of the differential diagnosis of skin diseases. At the beginning of the fourth year of development, the accuracy of the program in reaching diagnoses has been shown, on many occasions, to be comparable to its expert counterparts. However, it has been demonstrated that the system is not utilized within the dermatology clinic routinely. In response to this, aspects of DIAG have been modified in order to improve its clinical acceptability. Until this acceptance is achieved, these tools will not fulfill their potential to affect patient care.

Diagnosis, Computer-Assisted↗
PubMed1985

Accuracy of carotid Doppler peak frequency analysis: results determined by receiver operating characteristic curves and likelihood ratios.

To determine the accuracy of peak Doppler frequency measurements in the diagnosis of extracranial carotid arterial disease and to apply the results to the assessment of the individual patient, 397 studies of continuous-wave Doppler frequency analysis were compared with the results of arteriography. The diagnostic accuracy of the measurement of peak frequency was determined in two ways. First, receiver operator characteristic curves were calculated for different threshold levels of peak frequency. The results showed that the measurement of the peak frequency can be related in only an approximate way to the severity of the arteriographic stenosis because of the uncertainty of the probe-to-vessel angle. Nevertheless, the overall maximum diagnostic accuracy was approximately 91% (Kappa = 0.82 +/- 0.05). Second, likelihood ratios were calculated to take into account the degree of abnormality of the peak frequency measurements. The probability of carotid disease increased as the peak frequency increased, giving likelihood ratio ranges from 0.03 to 25.71. To be able to apply the peak frequency measurements to the assessment of the individual patient in the vascular laboratory, our results were used to construct tables that show the posttest probability of carotid disease based on the clinically estimated pretest probability of disease and the measured peak frequency. It is concluded that the measurement of peak Doppler frequency is an adjunct in the diagnosis of carotid artery disease.

Aged↗
PubMed1984

Limitations in the accuracy of peak frequency measurements in the diagnosis of carotid disease.

Peak Doppler frequency is an index of the severity of carotid stenosis. Variability in this measurement is examined through in vitro and clinical studies. In vitro studies, using a carotid flow model, show that observers locate a stenosis and interpret the peak frequency differently, and each observer uses a different probe-vessel angle. Clinical studies support these findings. Comparison of 304 carotid Doppler studies with arteriograms demonstrates 90% overall clinical accuracy. Each observer has a consistent range of peak frequency measurements, yet the description of a discrete percent stenosis is limited by observer variability.

Blood Flow Velocity↗
PubMed1984

Spectral analysis of Doppler flow velocity signals: assessment of objectives, methods, and interpretation.

The objective of this paper is to review the theoretical basis and clinical application of electrical impedance plethysmography in the noninvasive evaluation of peripheral arterial and venous disease. Theoretical, experimental and clinical studies have not demonstrated a direct relationship between electrical impedance changes and limb volume changes. Potential sources of error have also been identified. This has led to the development of clinical tests based on impedance plethysmography for the detection of peripheral arterial disease, venous insufficiency and venous outflow obstruction. Impedance plethysmography, using the method of venous occlusion, is presently the most commonly employed noninvasive method for the detection of deep venous thrombosis.

Carotid Artery Diseases↗
PubMed1982

A critical study of ultrasound Doppler spectral analysis for detecting carotid disease.

The results of in vitro and in vivo studies to determine the application and limitations of frequency analysis for CW Doppler ultrasound assessment of carotid stenoses are reported. In the in vitro study, the peak Doppler frequency and a new spectral broadening index were determined proximal to, at, and distal to axisymmetric and asymmetrical model stenoses. Good correlations with percent area stenoses were found. In the clinical study, 162 cases were examined using a 4 MHz Doppler system and by standard four-vessel arteriography. Peak frequencies of greater than 3.8 KHz were diagnostic of internal carotid stenoses of 3.2 mm minimum lumen diameter or less, with a sensitivity of 92% and a specificity of 94%. Spectral broadening, evaluated by subjective grading, yielded similar results.

Carotid Artery Diseases↗