Focused beams for high-resolution imaging and other applications

Analytical solutions were obtained for limited diffraction beams such as Bessel beams and X waves, as well as for conventional plane wave, with or without focus in both rectangular and polar coordinates. Using a focused plane wave in transmit and a focused Bessel beam in receive, a high-resolution C-mode image that breaks the diffraction limit of the conventional focused plane wave was obtained using a wave source of 50-mm diameter, 2.5-MHz frequency, 50-mm focal length, and 0.6-mm wavelength for an object at the focus. The two-way (pulse-echo) full-width-at-half-maximum (FWHM) lateral resolution of the image obtained was 0.56 mm. As comparison, the FWHM lateral resolution of the image obtained with the focused plane wave in both transmit and receive was 0.66 mm. The diffraction limit calculated with the FWHM of the square (two-way) of the Jinc function was 0.62 mm. Besides high image resolution, the focused Bessel beam also has a larger depth-of-field and its higher sidelobes were suppressed by the focused plane wave in transmit. In addition, using the analytical solutions developed, an ultrasound image (beam pattern) was produced using a commercial Verasonics 32x32 Matrix Array for visual (retina) stimulation of the brain to help blind people to see objects.