Synthetic aperture imaging using a semi-analytic model for the transmit beams

Many modern high-end scanners use some form for coherent synthesis of image lines by combining beams acquired with different transmissions such as retrospective dynamic transmit focusing (Acuson / Siemens), nSIGHT (Philips), and Zone imaging (Zonare). To synthesize the final image, beams acquired by different transmissions are coherently summed. The inter-beam delays and interpolation function vary across implementations. There are two major strategies described in literature to calculate the inter-beam delays and to uniformly focus both transmit and receive beams throughout the field of view - using virtual sources, and by applying spatial matched filtration. The virtual source model is precise, when the transmit is either strongly focused (f-number 1, 2) or images are formed using circular or spherical waves. The model becomes inadequate for higher f-numbers. The spatial matched filtration can be used both for strongly and weakly focused transmissions, but requires the measurement and storage of the response of point targets within the limits of the transmit beam. This paper presents a semi-analytic model for the transmitted field, which can be applied to synthetic transmit imaging. The model is more precise than the virtual source concept, does not require the measurement of the transmit field as matched filtration methods do, and can be applied to both strongly and weakly focused transmissions. Furthermore, the model is applicable to tissue harmonic and contrast enhanced ultrasound imaging. The paper presents the development of the model using the principles of diffraction, and its validation using computer simulations and measurements on a phantom.