A focused limited-diffraction beam for high-resolution imaging

Unfocused limited-diffraction beams such as Bessel beams and X waves have been studied extensively to obtain a large depth of field for medical imaging. In this study, a focused zeroth-order Bessel beam j₀(αr), where α = 1202.45 m⁻¹ and r is the radius, produced with a 50 mm diameter and 2.5-MHz transducer and with an acoustic lens of 50-mm focal length (f-number = 1) was used to obtain a C-mode image of an object in water (0.6mm wavelength). The results of the study showed that at the focal distance, the lateral resolution of the image obtained with an unapodized transducer in both transmission and reception was lower than that obtained with an unapodized beam in transmit but the j₀ Bessel beam was used in receive. Beam plots of the unapodized beam and the j₀ Bessel beam at the focal distance showed that the lateral beamwidths were about 0.78 and 0.59 mm, respectively. The 0.59-mm beamwidth of the focused Bessel beam is better than that of the diffraction limit (1.22 × wavelength × f-number = 0.732 mm). The decreased beamwidth of the j₀ Bessel beam results in a higher lateral image resolution while the sidelobes of the beam were suppressed by the focused unapodized transmit beam.