Acoustic field of strongly focused HIFU transducer operating at 20 MHz - comparison of public domain numerical simulators with experimental observations

Recently, a new modality for dermatological intervention involving therapeutic ultrasound with integrated optical imaging has been demonstrated for treatments of actinic keratosis, basal cell carcinoma, vascular malformations etc. Such modality requires very confined focal zones which are typically generated by strongly focused transducers operating at relatively high frequency (e.g., 20 MHz). Several public domain solvers exist, but they are however typically used and validated for simulation of HIFU devices for treatment of deep pathologies, therefore operating with larger f-numbers and lower frequencies. The objective of this work is to evaluate selected solvers against experimental data, when used for a strongly focused transducer (f-
$\mathbf{number}\boldsymbol{=0.75)}$
operating at 20 MHz. Three simulators are evaluated: HIFUbeam-KZK, HIFUbeam-WAPE and HITU-Simulator. The experimental data consist of peak pressure measurements around the focal zone in axial and radial direction using a Fiber Optic Hydrophone as well as schlieren imaging. Both modules of the HIFUbeam package seem to underestimate value of the peak pressure around the focal zone, and simultaneously overestimating the size of the focal zone, especially along the axial direction. HITU-Simulator, on the other hand, shows good agreement with experimental data when it comes to both absolute pressure values as well as location of pre- and side-lobes. It is therefore a preferable simulator when it comes to simulation of acoustic fields from strongly focused high-frequency transducers.