Cardiac In-vivo Measurements Using Synthetic Transmit Aperture Ultrasound

This paper investigates the feasibility of acquiring cardiac images using synthetic transmit aperture (STA) ultrasound. Focusing in STA is done by beamforming all points in the image for every emission, creating a low-resolution image. The low-resolution images for each emission are summed, effectively achieving dynamic transmit and receive focusing. The purpose of this paper is to acquire in-vivo cardiac images using STA to investigate image quality and the effect of tissue motion. For the in-vivo experiments, a 3 MHz and a 3.5 MHz transducers were used with 64 and 128 elements, respectively, together with the RASMUS experimental ultrasound scanner. Both transducers have a pitch of half a wavelength. To ensure an adequate signal-to-noise ratio, a 20 mus non-linear frequency modulated chirp and a 7-element de-focused virtual source were used for transmission. The number of virtual sources used in each scan sequence is equal to the number of transducer elements. A pulse repetition frequency of 4500 Hz was used, allowing a frame rate of 78 and 39 frames/s for the 64 and 128 element transducer. As the heart walls move with a speed of up to 80 mm/s, a movement of the tissue of several wavelengths is possible during an STA scan sequence using 64 emissions. To investigate the possibility of reducing the susceptibility to tissue motion, a sparse scan sequence using only 9 virtual source emissions is used to reduce the required scan-time. A shorter scan sequence will reduce the tissue motion between the first and last emission, and allows a frame rate of up to 555 frames/s. The sparse sequence is interleaved with the full sequence to allow a better comparison between the two techniques. A measurement of a point spread phantom shows a FWHM for the full scan sequences of 1.29 mm and 0.66 mm for the 64 and 128 element transducers, and an average side-lobe level of -47.25 dB and -58.42 dB respectively. The sparse scan sequences have a FWHM of 1.24 mm and 1.10 mm for the 64 and - 128 element transducers, and an average side-lobe level of -39.48 dB and -42.13 dB respectively. The four image sequences presented in this paper show a cross-section of the ventricle and atrium, where the walls of the ventricles and the atrioventricular valves are visible. The dynamic of the beating heart is visible during the recorded sequences. Distortions are seen in the images created by the full scan sequence, which are attributed to be caused by tissue motion. The images created by the sparse sequence show a reduced contrast, but also a reduction of the distortion caused by tissue motion.