In conventional synthetic transmit aperture imaging (STA) the image is built up from a number of low resolution images each originating from consecutive single element firings to yield a high resolution image. This may result in motion artifacts making flow imaging problematic. This paper describes a method in which all transmitting centers can be excited at the same time and separated at the receiver. Hereby the benefits from traditional STA can be utilized and a high fframe rate can be maintained and the images are not influenced by motion artifacts. The different centers are excited using mutually orthogonal codes. The total signal at the receiver is then a linear combination of the transmitted signals convolved with the corresponding pulse-echo impulse response. The pulse-echo impulse responses for the different elements are modeled as FIR channels and estimated using a maximum likelihood technique. The method was verified using Field II. A 7 MHz transducer was simulated with 128 receiving elements and 64 transmitting elements divided into subapertures so that 4 virtual transmission centers were formed. The point spread function was measured and the axial resolution was 0.2312 mm (-3dB) and 0.3083 mm (-6dB), lateral resolution 0.5301 mm (-3dB) and 0.7068 mm (-6dB) and maximum lateral sidelobe level less than 44 dB. Conventional STA is given as a reference with the same setup excited with a single cycle sinusoid at 7 MHz with axial resolution 0.2312 mm (-3dB) and 0.3083 mm (-6dB), lateral resolution 0.5301 mm (-3dB) and 0.7068 mm (-6dB) and maximum lateral sidelobe level less than 44 dB.
Proceedings of Spie Medical Imaging Meeting, Ultrasonic Imaging and Signal Processing, 2004, p. 300-306