High Resolution Biomedical Imaging Using Ultrasonic Metamaterials

Ultrasound biomedical imaging is used routinely for the diagnosis of many diseases. It has the ability show detailed structures of soft tissues, and has the advantage over other imaging methods, such as CT or MRI imaging, of being relatively cheap and portable. It is thus a good method for many diagnostic clinical settings. The current resolution of ultrasonic imaging in the body is determined by various factors, including transducer design, frequency of operation, and depth of penetration required. However, there is a fundamental limit to the imaging performance of such systems - namely the diffraction limit. This sets the minimum spot size that a focused beam can achieve by conventional means, even in a perfect propagation medium. The present proposal aims to improve this by the use of metamaterials, which will be incorporated within an ultrasonic transducer system. These exotic materials are, in fact, made up of a complicated geometry, where the internal structure contains many sub-wavelength features. These can act together to make the material behave in a way that is totally different from normal structures. The result is that they can, for example, have a negative refractive index, noting that for conventional materials the value is always positive. Thus, a plate with flat parallel sides can focus ultrasound, provided it is designed correctly.

The research will identify the best designs that can be used at biomedical ultrasound frequencies, which in the present case will be 1-5 MHz. To date, acoustic metamaterials have been designed typically for much lower frequency, and for use in air. In this project, novel new designs are proposed, which will first be modelled theoretically, and then constructed using high-resolution additive manufacturing (3D printing) techniques. Once built, the new structures will be tested with biomedical ultrasound transducers, and their performance in imaging systems determined. In this way, it is hoped to produce a new approach to diagnostic ultrasound, with resolution enhancement that could be useful for cardiovascular disease, prostate and skin cancer diagnosis.