Hair-sized lens helps look in blood vessels

A tiny measurement system that incorporates a lens as thick as two human hairs has been developed by researchers to investigate the force exerted on the wall of an artery as blood whooshes past. In a research paper published today in the Institute of Physics publication Journal of Micromechanics and Microengineering, Dr Rob Keynton and colleagues at the University of Louisville, Kentucky and Michigan Technological University, USA describe how they have designed and made an integrated miniature acoustical lens-transducer system that will focus ultrasound (high-frequency sound) very precisely, enabling medical researchers to look at very fine detail.

Very accurate imaging systems that use high-frequency ultrasound generated by a transducer are widely used in medical diagnostics and a variety of non-destructive testing applications (such as examining how a crack is spreading inside a material). Focusing the ultrasound beam means that a smaller region is examined by more intense sound waves, so that much more detailed information – and a sharper image – can be obtained.

A number of research groups have developed acoustical lenses to focus sound waves; the significant advance made by Dr Keynton and his colleagues is that they have managed to create extremely small lenses attached directly to the transducer generating the sound. An individual transducer crystal (made from PZT, lead-zirconate-titanate) with an attached wire was placed in a holding well – in effect, a recessed hole – and a liquid plastic (epoxy or Plexiglass) poured into the well. Sophisticated and highly accurate micromachining techniques were then used to produce the concave structure needed to focus the ultrasound. The combined lens and transducer device is just 260 micrometres thick, with the lens itself being 160 micrometres thick with a diameter of 930 micrometres. For comparison, a human hair is around 100 micrometres wide.

The device is already being used in medical research by Dr Keynton´s group to look at the shear stress (dragging force) of blood on the artery walls and to see how this is connected to the development of cardiovascular diseases or conditions such as intimal hyperplasia – when grafts block up again after a vascular bypass procedure. Another likely medical application is in dermatology, where high-frequency ultrasound could be used to investigate skin cancers, telling the surgeon how deep a tumour is and where its edges are before surgery takes place.

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