Code Division Multiple Element Synthetic Aperture Transmission

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 lowers the frame rate and may result in motion artifacts. This paper, describes a method in which all transmitting centers can be excited at the same time and separated at the receiver to yield a full image in only one transmission. Hereby the benefits from traditional STA can be utilized and a high frame rate can be maintained, which thereby prevents motion artifacts. The different centers are excited using mutually orthogonal codes. The signal at the receiver is 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 finite impulse response channels and estimated using a maximum likelihood technique. The method was verified using simulations in 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 simulated and the axial resolution was 0.23 mm (-3 dB) and 0.31 mm (-6 dB). lateral resolution 0.53 mm (-3 dB) and 0.71 mm (-6 dB) and maximum lateral sidelobe level less than 44 dB. Conventional STA is given as a reference with the same aperture setup using 4 emissions excited with a single cycle sinusoid at 7 MHz. The axial resolution is here 0.23 mm (-3 dB) and 0.31 mm (-6 dB), lateral resolution 0.53 mm (-3 dB) and 0.71 mm (-6 dB) and maximum lateral sidelobe level less than 44 dB