Multiple-frequency phased array pattern synthesis for HIFU surgery

We present a simulation/experimental study to evaluate and optimize the focusing capabilities of a phased array prototype when excited by multiple-frequency components. A multiple-focus multiple-frequency pattern synthesis algorithm for phased arrays has been developed and tested using linear simulations in Matlab. The algorithm maintains the precise phase relationship between the frequency components at each focal spot to achieve a desirable therapeutic outcome. Preliminary simulations indicate that the focal region can be shaped based on the alignment and phase of multiple-frequency components. The pattern synthesis algorithm is experimentally validated with a 3.5 MHz, 64-element prototype designed for small-animal and superficial therapeutic HIFU applications (Imasonic, Inc) which has a 52% fractional bandwidth, allowing for therapeutic output in the frequency range of 2.7-4.6 MHz. Validation with hydrophone measurements at the focal locations showed that there is in increase in harmonic generation at the focal point with the frequency mixed patterns when compared to a conventional single frequency excitation pattern. This increased non-linearity, will allow for increased thermal absorption at the focal point, thus allowing for larger treatment volumes with the same total power or reduced treatment time per shot when compared to the single frequency case. Ex vivo experiments with fresh porcine liver were conducted to study the effect of multiple-frequency patterns when compared with conventional single frequency patterns during lesion formation. The lesion size was increased for the multiple-frequency patterns when compared the single frequency pattern at normalized power with respect to each other. In conclusion, wideband piezocomposite array transducers, together with multi-channel arbitrary waveform generators are enabling technologies which allow for complex, multiple-focus, multiple-frequency HIFU patterns. These patterns can enhanced the focal gain with proper phase alignment. Furthermore, multiple frequency patterns have been shown to be able to increase the harmonic generation at the focal spot, thus improving local absorption. Our early results with ex vivo porcine liver indicate that multiple frequency excitation can enhance the therapeutic gain at the focal points.