Clinical use and evaluation of coded excitation in B-mode images

Use of long encoded waveforms can be advantageous in ultrasound imaging, as long as the pulse compression mechanism ensures low range sidelobes and preserves both axial resolution and contrast. A coded excitation/compression scheme was previously presented by our group, which is based on a predistorted FM excitation and a mismatched compression filter designed for medical ultrasonic applications. The attenuation effect, analyzed in this paper using the ambiguity function and simulations, dictated the choice of the coded waveform. In this study clinical images, images of wire phantoms, and digital video demonstrate the applicability, clinical relevance, and improvement attained with the proposed scheme. A commercial scanner (B-K Medical 3535) was modified and interfaced to a software configurable transmitter board and to a sampling system with a 2 GB memory storage. The experimental system was programmed to allow alternating excitation on every second frame. That offers the possibility of direct comparison of the same set of image pairs; one with pulsed and one with encoded excitation. Abdominal clinical images from healthy volunteers were acquired and statistically analyzed by means of the auto-covariance matrix of the image data. The resolution laterally is retained, axially is improved, while there is a clear increase in penetration. Phantom images using the proposed FM-based scheme as well as complementary Golay codes were also acquired, in order to quantitatively evaluate the characteristics of the compressed output and its stability to attenuation. Images of a wire phantom in water show that the range sidelobes resulting from pulse compression are below the acoustic noise, which was at 50 dB for the pulsed images. For images acquired from an attenuation phantom, the proposed compression scheme was robust to frequency shifts resulting from attenuation. The range resolution is improved 12% by the coded scheme compared to a 2-cycle pulse excitation. For the maximum acquisition depth of 15 cm, where the coded excitations are primarily intended, the improvement in SNR was more than 10 dB, while the resolution was retained.