Spatial redefinition of ultrasound pressure fields using polycarbonate lenses: Model and experimental validation

Exposure of cells and tissues to an ultrasound field is well known to elicit bioeffects when the pressures employed exceed some threshold value. Previous studies have shown that biological outcomes such as apoptosis, necrosis, as well as molecular uptake processes, can all be induced using particular pressure regimes. Moreover, the ready facility to focus ultrasound extracorporeally using curved tranducers has led to the realization that clinically relevant targets such as tumors: even those at deep seated anatomical locations; can be effectively treated via a non-invasive ultrasonic protocol. One can imagine that targets [notionally small tumours] could easily vary in size, and therefore that a single transducer would not necessarily be able to produce a focal region that covers the full target extent without resorting to some more elaborate protocol. One suggestion might be to move the transducer controllably in order to shift the focus in 3D and thus cover a larger area of the target with an appropriate exposure. Alternatively, different transducers could be employed for different target sizes, or one could choose the very flexible but expensive option of phased arrays, which allows automatic and programmable 3D spatio-temporal control of the pressure field. A further [relatively inexpensive] method is to make a single, focused transducer more versatile by incorporating ultrasonic lenses to modify pressure field distribution. For the purposes of the present work, we have investigated this latter route with a view to exploiting it as an alternative to high intensity focused ultrasound (HIFU) for treatment, and with a main aim of reducing collateral damage arising from iatrogenic heating.