Attenuation Estimation and Temperature Imaging Using Backscatter for Extracorporeal HIFU Treatment Planning

For HIFU to be widely applicable in the clinic, problems relating to treatment planning, delivery and monitoring need to be resolved. The characterisation of the acoustic and thermal properties of specific tissues is an important pre-requisite to determining the optimal exposure parameters for individual treatments. We describe a preliminary evaluation of two methods that may be of use in deriving such planning information prior to HIFU. Both methods have been implemented on a diagnostic ultrasound scanner. One is backscatter attenuation estimation (BAE), which uses pulse-echo data and an axial beam translation substitution method to estimate the average attenuation coefficient of tissue overlying the region to be treated. The second method is backscatter temperature imaging (BTI) applied to a non-lesioning test exposure, which is normally used to determine the focal position but here the observed peak temperature rise is employed to provide an estimate of all case-specific losses involved in delivering a dose of thermal energy. HIFU lesioning experiments were performed in ex vivo bovine liver tissue, and used to test the ability of BAE and BTI to provide accurate information for adjusting the HIFU power so as to compensate for varying ultrasonic attenuation by overlying tissues (mimicked by gels with different attenuation properties). HIFU-induced lesions were made according to two types of exposure regime, (a) without regard for the differing attenuation coefficients of gels placed between the HIFU transducer and the tissue to be treated, and (b) after adjusting the HIFU drive power according to corrections derived from BAE and BTI measurements. Both correction methods were found to improve lesion size reproducibility, as measured from cut lesioned tissue specimens, although BTI performed better than BAE. The paper discusses likely reasons for this, as well as problems to be overcome if either method is to be clinically useful.