Ultrasonic focus localization via nonlinear harmonic source detection

Accurate ultrasonic focus localization is paramount for the safety and efficacy of the focused ultrasound (FUS) applications. Nonlinear effects at the focus generate frequency-multiplied harmonics, establishing it as the dominant harmonic source. Consequently, detecting these nonlinear harmonics enables localization of the harmonic source, corresponding to the FUS focus. This study introduces and validates this focus localization mechanism experimentally using two complementary systems. For the sound field scanning system, the focus position determined by the peak amplitude of the acoustic field was compared with that derived from the time difference of arrival (TDOA) calculations of harmonic signals. For the phased-array ultrasound system, the geometric focus location derived from the B-mode image of the FUS transducer edge was compared with locations obtained by TDOA calculation and harmonic source reconstruction. Experimental results demonstrate that harmonic source localization accurately determines the FUS focus. TDOA calculations achieved superior axial accuracy compared to linear array-based harmonic reconstruction (0.182 mm vs 0.438 mm axial deviation). Results consistently show sub-wavelength-scale agreement between harmonic-based localization and reference standards. This work establishes nonlinear harmonics as reliable acoustic signatures for focus tracking, offering significant potential for therapeutic FUS monitoring applications.