Impact of element pitch on synthetic aperture ultrasound imaging.

Abstract

Synthetic aperture imaging was introduced in medical ultrasound to obtain high-quality images. In synthetic aperture ultrasound imaging, spherical transmit waves illuminate a target region from different positions, resulting in low-resolution images for each transmission. By coherent compounding of the resulting low-resolution images, a high-resolution image is obtained. Multiple steered receiving beams need to be created to obtain each low-resolution image and, thus, grating lobes should influence the image quality. In the present study, an array ultrasonic probe with a small element pitch was introduced to reduce the influences of grating lobes, and the effect of element pitch on image quality was examined in detail. A linear array ultrasonic probe at a nominal center frequency of 7.5MHz with an element pitch of 0.1mm has been introduced. This probe does not produce grating lobes within the imaging region in theory because the element pitch of this probe is half of the ultrasonic wavelength. The contrast of an ultrasonic image was evaluated using a cyst phantom. The contrasts obtained by synthetic aperture imaging with element pitches of 0.1 and 0.2mm were 4.88 and 4.69dB, respectively, which were similar to the 4.67dB obtained by conventional beamforming with focused transmit beams, when the number of transmissions was 121. The contrast obtained with an element pitch of 0.1mm was similar (4.34dB) even when the number of transmissions was decreased to 61. However, the contrast obtained with an element pitch of 0.2mm showed a larger degradation (3.77dB) at 31 transmissions. Even with larger element pitches, good image contrast could be obtained when the number of transmissions was large. This is because echoes from grating lobes are incoherent among transmissions, and they are suppressed by compounding low-resolution images obtained by individual transmissions. On the other hand, an array probe with smaller element pitches achieves good image contrast even with a smaller number of transmissions and, thus, it would be preferable to realize a higher frame rate.