Correction of Phase Aberrations in Medical Ultrasound Images Using Signal Redundancy

Abstract

The pulse-echo technique has been widely used in medical ultrasound imaging. This technique uses an array of transducer elements to transmit a focused beam into the body, and each element then becomes a receiver to collect the echoes. The received echoes from each element are dynamically focused to form an image. Focusing on transmission and reception is performed assuming that the wave propagation speed inside the body is the same everywhere. Unfortunately, the speed inside the body is not constant; it varies from 1470 m/s to greater than 1600 m/s. This speed variation will result in increased side lobes and degraded lateral resolution. Aberration phenomena and their extent in tissue have been evaluated in many works (O'Donnell & Flax 1988; Zhu & Steinberg, 1992; Shmulewitz et al., 1993; Robinson et al., 1994; Hinkelman et al., 1998). The degradation might be tolerable if the frequency is not very high and the aperture size of the array is not very large. However, higher frequencies and larger apertures have been used to improve lateral resolution of ultrasound images. But the resolution improvement cannot be achieved beyond a certain limit, because both larger aperture and higher frequency make the system more sensitive to propagation velocity variations in the body. For example, the four transverse abdominal scan images shown in the first row of Fig.1 were formed with a 64-element linear array using four different aperture sizes (9 mm, 12 mm, 18 mm, and 27 mm) to form each single beam in the image. The array pitch was 1.0 mm and the pulse had a 3.5 MHz centre frequency and 2 MHz bandwidth. The Superior-Mesenteric-Artery (SMA) and the Aorta (A) are the main objects in these images. Because of the shape of the rectus muscles (speed ~ 1580 m/s) and the fat layers (speed ~ 1450 m/s) at this position, the distortions caused by phase aberration in these images can be easily seen. In the 9-mm aperture image, the superior-mesenteric-artery is almost doubled but the artery wall can still be recognized. When the aperture size becomes larger, the distortion becomes worse. Phase aberration is one of the most important factors that limit improvement to lateral resolution of ultrasound imaging systems. Successful correction of phase aberrations will make it possible to improve the lateral resolution of images. Phase aberration corrected images using the near-field-signal-redundancy algorithm (Li, 1997) are shown in the second row of Fig.1 and the lateral resolution is improved when the aperture size becomes larger. In this chapter, the near-field-signal-redundancy algorithm is described in details. But first, a review of some related methods developed for phase-aberration correction is given. The