High frequency nonlinear imaging

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Nonlinear imaging has already shown improved image resolution compared to fundamental imaging at lower frequencies (2–4 MHz). The required steps to obtain nonlinear images at high frequencies are presented here. The transmit frequency was from 20 to 60 MHz and on receive, pulse-inversion, the first, second, and the third harmonic were used for imaging. Experiments were conducted using a wire phantom and in vitro from a human femoral artery. PVDF transducer (6 mm diameter, 10 mm focal length, central frequency 42 MHz, −6 dB bandwidth 38 MHz) was used with a filtered amplifier (46.0 and 50.6 dBm output). The pulse-inversion experiment at 40 and 60 MHz had lateral resolution (−6 dB contours) 64 and 52 μm and axial 43 and 31 μm. The fundamental experiments at 20, 30, 40, and 60 MHz had lateral resolutions 104, 89, 74, 57 μm and axial 72, 86, 46, 35 μm. The second harmonic at 40 and 60 MHz had lateral resolution 62 and 47 μm and axial 52 and 38 μm. The third harmonic at 60 MHz had lateral resolution 48 μm and axial 42 μm. Nonlinear tissue images had improved resolution compared to fundamental imaging, though the improvement was small for the second harmonic experiment.