Ultrasonic propagation in mammalian cell suspensions based on a shell model.

The interaction of ultrasonic waves with individual cells has been modelled on the basis that the cells can be represented by viscous liquid spheres surrounded by a viscoelastic shell (the membrane) immersed in a viscous fluid. The computational model includes thermal waves and requires 22 input parameters. Many of the parameters are not available in the literature and a detailed discussion is given on the procedures by which the values used in the model calculations were chosen. In spite of the arbitrariness of the choice of many of the parameter values, the computations show surprisingly good agreement with experimental measurements of ultrasonic attenuation in animal cell suspensions. The model has been used here to investigate different aspects of the interaction of ultrasound with the cells. It is found that the membrane is important only between 0.5 and 30 MHz and contributes less than 15% to the attenuation. Absorption is shown to be an important feature to include, while the scattering contribution to the attenuation is less than 1% at 3 MHz. The thermal effects are important at frequencies below 1 MHz and contribute some 65% to the attenuation at 100 MHz.

A spectral approach to ultrasonic scattering from human tissue: methods, objectives and backscattering measurements.

The dearth of information on the physical processes involved in the propagation of ultrasound in tissue and the growing need for such information are discussed. The physical nature of the ultrasonic diagnostic process in considered in terms of a wave phenomenon and the limitations and advantages of frequency spectral analysis as a means of obtaining information are briefly analysed. A description is given of an experimental measuring system using a time-gate to select echoes scattered from a particular volume at a depth in soft tissues. The influence of attenuation by overlying tissue and the choice of the duration of the acceptance gate on the frequency spectra obtained are considered. The paper reports some backscattering measurements on formalin-fixed samples of human fat, liver and spleen in the frequency range 0-5-5-0 MHz. The results suggest that the approach may have diagnostic value in a clinical situation, in the characterization of structure in specific volumes of soft human tissue.

Evolution equations and ultrasonic wave propagation in biological tissues.

Fundamental research in ultrasonic diagnostics is often limited by the relatively simple models of propagation invoked in what is largely an empirical science. Careful discussion of the problem reveals the need for more sophisticated propagation theories as the basis of measurement techniques and for the interpretation of experimental results. It is thus important for dialogue between theoreticians and experimentalists in the field. The complexity of ultrasonic wave propagation in tissue arises from a combination of factors. First, there is the biochemical sophistication of the media concerned. Second, there is the variety of physical phenomena involved: the diffractive nature of the ultrasonic field, the presence of absorption, the presence of large scale inhomogeneities and small scale scatterers, and the possibility of finite amplitude propagation effects. It has tended to be the custom to deal with each of these problems on an individual basis with a second feature being introduced as a perturbation of the results obtained for the first. The present authors have been concerned to find a unified approach which will permit each of the effects to be taken into account in relation to the others. This approach is based on the application of two-dimensional evolution equations modelling ultrasonic propagation in non-cavitating soft tissues. The model incorporates all the propagation phenomena known from experimental studies, indicating a need for knowledge of nine material parameters for a complete description. It thus provides a basis for numerical investigation of the relative significance of the parameters under different conditions. This will permit identification of those that should be known experimentally with high precision and those that have a minor role in the propagation phenomena.

Numerical considerations for interface reflections in medical ultrasound.

The original concept of the echo mechanism in diagnostic medical ultrasound--of step impedance discontinuities--is investigated in terms of the numerical implications of more refined models. The effect of a discontinuity in the attenuation coefficients is found to have as much significance as the impedance discontinuity at low reflection interfaces. An approximate analytical model is developed for spatially varying changes in the impedance. Numerical calculations are presented for reflections from delta-function and Gaussian-envelope RF pulses for two different models: an impedance gradient, and a connective tissue layer. The models used are well documented in acoustics textbooks, but the numerical results for typical tissue parameters show that a wide variety of interface structures may give rise to reflected amplitudes in the same range as that determined by the step impedance model. It is suggested that experimental investigation of interface structures may increase our understanding of the tissue: ultrasound interaction in diagnostic processes.

Perceptual studies on ultrasonic B-scan textures.

A pilot study of the perceptual characteristics of ultrasonic textured images is described. Scans of four models performed on four real-time machines optimised for display of a normal liver were used. A trial with 22 observers indicated that the model that gave images closest to the liver image varied between machines. A second, paired similarity test with five observers using all the model images was performed, with a cluster analysis of a multidimensional scaling procedure. This suggested that the prominent features of the textural images are often more closely related to the machines than to the models. Considerable further work is needed to confirm these pilot results and to identify the visual cues that are most significant in textured images.

Factors affecting the choice of preamplification for ultrasonic hydrophone probes.

This paper gives a systematic analysis of the effects of including an integrated (built-in) preamplifier into the ultrasonic piezoelectric probes (hydrophones) that are finding increasing use in biomedical applications. The design parameters considered include the end-of-cable sensitivity, gain, dynamic range, power supply requirements, construction intricacy, and cost. The rationale behind the inclusion of a preamplifier is given, and it is shown that the additional complexity introduced with the preamplifier into the measurement chain may not be warranted in all applications. Both the drawbacks and advantages of hydrophone preamplification are demonstrated, especially for the case of high pressure amplitude ultrasonic field measurements. Guidelines are developed for the potential user to identify the need for preamplification and the factors that influence the selection of the appropriate circuitry.

Investigation of ultrasound axially traversing the human eye.

A ray tracing model for ultrasonic propagation through the human eye, including the lens, has been developed on the assumptions of lossless media and non-reflecting interfaces. Measurement of the distribution of an ultrasonic beam before and after traversing specimens of human eyes in vitro, and of the velocity of ultrasound in the various dissected media, has permitted some comparison of the predictions of the model with experiment. The agreement is good although there are significant limitations involved and these are discussed. For imaging systems the effect of the eye arises largely from the lens which acts as a defocussing lens of focal length approx. 13.5 cm. Although the experiments were performed at approx. 4 MHz, the validity of the ray tracing model is largely frequency independent and will be appropriate at the higher frequencies commonly used in ophthalmology.

Ultrasonic wave fluctuations through tissue: an experimental pilot study.

The fundamental role of the assumption of plane wave ultrasonic propagation in attenuation and scattering measurements on soft tissues is discussed. The evidence for and against the validity of the assumption is shown to be inconclusive. An experimental procedure for direct assessment, by measurement of the ultrasonic amplitude and phase fluctuations occurring after an interrogating plane wave has passed through a tissue specimen, is described. The phase measurements impose significant demands on the mechanical precision, temperature stability and electronic timing capabilities of the measurement system. Measurements are reported on two specimens of fresh beef liver. One exhibited significant fluctuations of amplitude and phase which were reduced by 50% when the effects of thickness variations in the specimen were removed. The other specimen exhibited relatively small fluctuations. The feasibility of making such measurements has been demonstrated and the detailed features of the experimental procedure-which requires considerable care-have been outlined. Although more measurements are needed for definitive conclusions, these will require the development of a reliable technique for determining the presence of gaseous microbubbles in tissue in vitro. The preliminary conclusion that some samples of liver may exhibit low levels of wavefront disruption is consistent with recent evidence in the literature.

Measurement of ultrasonic exposure with radiation force and thermal methods.

It is the purpose of this paper to discuss briefly the factors that influence the interaction of the ultrasonic field and medium being irradiated, and to collect together the various techniques that have been suggested for measuring energetic parameters of the ultrasonic field. The calorimetric and radiation force methods are then discussed in more detail. The former is discussed as a technique whose capabilities have not been fully exploited while the latter demands attention both because of its simplicity and its controversial theoretical basis. Areas in which further work is needed are indicated in the summary and conclusion. It is hoped that the paper will be of practical use to workers in the field.

Some scientific and technical aspects of medical ultrasonics.

The development of diagnostic applications of ultrasound has tended to follow the evolution of electric technology, and while this development as resulted in benefit to the patient, the relatively unsophisticated scientific ideas have been adequate. The present paper reviews briefly some elements of the development of diagnostic imaging relation to their scientific bases. The discussion is particularly concerned with the popular grey-scale pulse-echo systems and the philosophy behind future developments that may lead to successful pursuance of tissue differentiation, and the optimisation of instrumentation. Of necessity, the discussion touches the more fundamental areas of measurement of the ultrasonic parameters of tissues, the development of tissue models, the measurement of transducer field characteristics, the studies in visual perception; and is briefly linked to biological effects and therapy.