A method to increase the image resolution and dynamic range is to use the acquired data from several emissions (lines) and to beamform the collected RF signals treating the focal point in transmit as a virtual source of a spherical wave. The transducer is swept mechanically over the region of interest to scan a full volume. The same beamformation procedure is applied both in the azimuth and the elevation planes. This paper presents a study of the influence of the position of the transmit focus on the image resolution, the signal-to-noise ratio and penetration depth. The investigation is based on simulations and measurements. The system used in this work is a research scanner developed at the department. The transducer is a 7.5 MHz linear array with a pitch of 208 µm and a fixed focus in the elevation direction at 25 mm. The field is simulated for points placed at every 5 mm between 10 and 150 mm depths. 100 different positions of the transmit focus are investigated. For every transmit focus the image is beamformed and evaluated. Finally the gain in signal-to-noise ratio and penetration depth are investigated experimentally for the setup, with which the best resolution is achieved. Simulations indicate that the size of the point spread function at a depth of 60 mm is decreased from 3 mm to 0.66 mm and from 4 mm to 2.5 mm in the azimuth and elevation planes, respectively. The gain in signal-to-noise ratio measured in a tissue mimicking phantom is 10 dB. The penetration depth increases from 70 to 100 mm. The method can be applied in applications, where the image quality is of prime importance, such as in the classification of atherosclerotic lesions in the carotid artery.
Ultrasonics, 2006, Vol 44
Virtual Sources; Post beamforming; Synthetic Aperture Ultrasound