Minimally invasive and non-invasive therapeutic ultrasound treatments can be used to ablate, necrotize, and/or otherwise damage tissue. High-intensity focused ultrasound (HIFU), for example, is used to thermally or mechanically damage tissue. HIFU thermal treatments increase the temperature of tissue at a focal region such that the tissue quickly forms a thermally coagulated treatment volume. HIFU treatments can also cause mechanical disruption of tissue with well-demarcated regions of mechanically emulsified treatment volumes that have little remaining cellular integrity.
A current trend in HIFU medical technologies is to use two-dimensional multielement phased arrays with the elements distributed over a segment of a spherical surface. Each element of such an array is controlled independently, which makes it possible to electronically steer the focus in space, to create a complex field configuration in the form of several foci, and to minimize the heating of acoustic obstacles (for instance, ribs) while maintaining high intensities at the focus. The arrays can also be utilized to improve the quality of focusing in inhomogeneous tissue using time reversal methods, as well as to trace the region of treatment, which shifts due to respiration. In many HIFU applications, the acoustic intensity in-situ can reach several tens of thousands of watts per square centimeter (W/cm2), causing nonlinear propagation effects. Nonlinear effects can result in formation of weak shocks in the ultrasound waveform, which fundamentally change the efficiency of ultrasound thermal action on tissue, and can lead to new biological effects of a non-thermal nature.
The present technology is directed to methods for characterizing nonlinear ultrasound fields and associated systems and devices. In several embodiments, for example, a method of calculating output of a HIFU device comprises treating a target site with a multi-element HIFU array. In some embodiments, the array comprises a generally spherical segment. The method can further include simulating a field of the array by setting a boundary condition for the array. Setting a boundary condition can include simplifying at least one geometrical aspect of the generally spherical segment (e.g., modeling a multi-element spherical array as a single-element flat transducer). By modeling the nonlinear effects using the simplified boundary condition, effects of the HIFU treatment parameters can be more readily discerned.
Mathematical models other than the Westervelt or Khokhlov-Zabolotskaya-Kuznetsov (KZK) equations can be used to simplify nonlinear HIFU effects. Also, a lookup table can be associated with results from the Westervelt or other equation in addition to, or in place of, results from the KZK equation.