<FONT face=Verdana>X waves may be used in ultrasound imaging to obtain very high frame rate. But the spatial resolution of the image with a linear imaging mode decreases as compared to conventional focused imaging. The nonlinear fields of limited-diffraction X waves are calculated and studied by solving the KZK equation using a time-domain numerical method. The result indicates that the nonlinear fields of broadband and band-limited X waves are both limited-diffraction beams. Moreover

the second harmonic component has narrower mainlobe and lower sidelobes than the fundamental field. Utilizing the second harmonic of X waves in ultrasound imaging

higher spatial resolution would be obtained throughout the entire imaging range. A theoretical analysis verifies the numerical calculation result.