Multiple transmit focusing using modified orthogonal Golay codes for small scale systems

Recently, coded excitation with orthogonal Golay codes is used to increase the frame rate and the SNR of ultrasound imaging according to R.Y. Chiao and L.J. Thomas (2000), M. O'Donnell (1992) and Y.M. Yoo et al. (2001). An application of orthogonal Golay codes may be to use them for simultaneous transmit multi-zone (STMZ) focusing, in which orthogonal Golay codes can be simultaneously transmitted along the same scan line, each focused at different depths. This method achieves multiple transmit focusing with no accompanying decrease in the imaging frame rate. However, coded excitation system using Golay codes is in trouble to use for handheld, portable ultrasound systems due to the large number of correlators since it should have correlators at all active channels to compress the received signals without any degradation. In this paper, we present STMZ focusing method using modified orthogonal Golay codes for small scale systems, in which 2(N-1) (N is the number of active channels) correlators can be saved by performing compression after beamforming. By inverting the precedence of compression and beamforming when using coded excitation and dynamic receive focusing, the error is introduced as stated in R.T. Bjerngaard and J.A. Jensen (2002). However, since the error decreases with depth and the coded excitation is usually used to provide far field of depth, the presented method may be the possibility of reducing complexity of the receiver structure without introducing errors. To verify the presented method, we performed experiments with a commercial ultrasound scanner and acquired A-scan and B-scan images of a tissue-mimicking phantom.