Interaction of ultrasound with soft tissue and biological fluids from the statistical physics perspective Seminar

In this work I have tried to bring together essential information about available experimental and theoretical results, complementing these with a practical approach to modelling. Conscious of increasing specialisation in medical ultrasound (both for diagnostic and treatment purposes) and recent vast advances in instrumentation techniques I thought it appropriate to restrict the scope of the talk to the general aspects of the interaction of ultrasound with tissue and concentrate on the means for theoretical understanding of the involved physics.

The most important aspect of this is the information that should be originally contained in the equations of motion. These must incorporate essential properties of the media to include comprehensive description of practical applications, and also follow from the basic physical principles. The scientific basis of such a propagation medium as soft tissue is largely incomplete. A proper mathematical description of ultrasound in tissue depends on the knowledge of the functional roles of bio-molecules, their spatial structure, types of chemical bonds between the atoms, interaction of protein structures of different molecules, etc. Unfortunately, advances in understanding of the detailed events underlying these very complex processes are still limited. Despite the awareness that the empirically observed features of tissue result from changes in its molecular structure under the action of the imposed ultrasound field, there is currently no satisfactory approach to analysing these changes and to constructing the quantitative theory.

There is a strong demand for a statistical mechanics approach unifying mechanics and thermodynamics of soft tissue, and for the theoretical calculation of specific heats, absorption and such transport coefficients as shear viscosity and heat conductivity of tissue. However, even the qualitative understanding of the nature of the accompanying irreversible thermodynamic transformations is fragmentary. As a result, the available theoretical tools are largely restricted to various phenomenological theories of irreversible thermodynamics and analogies with continuous mechanics. These thermodynamic theories offer different definitions of irreversible entropy depending on the properties of the media that need to be modeled. With the mechanisms controlling the change of the state of the molecules unknown, it can be very difficult to sort out which irreversible model is appropriate and even to understand the relationship between these models.

As of now, physics of soft tissue appears a poorly ordered mixture of theoretical and experimental observations. This work attempts to provide a more systematic approach to some of the mentioned problems of soft tissue from the standpoint of statistical mechanics and irreversible thermodynamics.

I am exceptionally grateful to to my co-workers from UCL, ICR and Oxford for updating me on the literature and helpful discussions and am pleased to take the opportunity to acknowledge my debt to them.