1Institute of Cancer Research, UK
There is increasing interest in the use of ultrasound for its therapeutic potential, whether for increasing drug activation or uptake, or for creating volumes of thermally ablated tissue (High intensity Focused Ultrasound HIFU). These applications entail the use of higher powers and longer pulse lengths than are usual for ultrasound imaging, and thus carry with them different challenges for measurement of the acoustic fields. Therapeutic benefit is obtained through the thermal and mechanical effects induced in tissue. What is important here is to determine the pressure or intensity in the region being targeted, and thus to differentiate between “exposure” and “dose”.
Therapeutic ultrasound fields are usually mapped using piezo-electric or fibre optic hydrophone probes. The pressure distribution can also be visualised using Schlieren or holographic techniques. Intensity distributions can be inferred by imaging the heating of an acoustic absorber using an infra-red camera. Acoustic power is measured using radiation force of calorimetric techniques. Once the field has been characterised under free field conditions in water, it is necessary to “derate” it to estimate the level at the target site. Here, in situ intensity or pressure is usually used, but for HIFU ablation, thermal dose is the parameter most commonly quoted.
Representative field distributions and acoustic power measurements will be discussed, as will the validity of the thermal dose concept at the high temperatures used for thermal ablation.
Existing field characterisation methods are appropriate for many “low power” therapeutic ultrasound applications. However, the small focal regions, on-linear propagation and cavitation that are characyeristic of HIFU fields require new techniques to be developed. No fully satisfactory dosimetric parameter has yet been identified for therapeutic ultrasound applications.