Second harmonic imaging using synthetic aperture sequential beamforming

The paper investigates Second Harmonic Imaging (SHI) using Synthetic Aperture Sequential Beamforming (SASB). The investigation is made by an experimental Synthetic Aperture Real-time Ultrasound System (SARUS). A linear array transducer is used to scan 4 wires at the image depths of {22.5, 47.5, 72.5, 97.5} mm, respectively. Three different experiments are made using three different transmit foci at 10 mm, 25 mm and 50 mm. A 2-cycle sine wave with a center frequency of 5 MHz is used as the excitation. The SHI is achieved by using Pulse Inversion (PI) technique. The data received with and without PI from SARUS are beamformed using Dynamic Receive Focusing (DRF) and SASB by a Beamformation Toolbox. The Full Widths at Half Maximum (FWHM) in both the lateral and axial directions for these four wire targets using different imaging algorithms (DRF, DRF+SHI, SASB and SASB+SHI) are calculated and shown in the paper. The Full Width at One Tenth Maximum (FWOTM) is also investigated. By combining SASB and SHI, the lateral resolution is improved by 66%, 35% and 46% for FWHM, and 52%, 20% and 29% for FWOTM, compared to DRF, DRF+SHI and SASB, respectively. The axial resolution is improved 24% on average by SHI.