Adaptive and nonlinear ultrasound imaging

Two major topics in clinical ultrasound imaging are addressed in this thesis. The first is phase aberration correction. Phase aberrations arise due to sound velocity inhomogeneities within the body. These inhomogeneities distort propagating wavefronts, reducing contrast in the image and degrading image quality. In the limit that the effects of the aberration can be modeled as a phase screen, correlation-based processing can be used to measure and remove the effects of the aberration. In many cases, however, the aberration cannot be fully modeled as a phase screen. Then, a general aberration correction system capable of removing image artifacts left after correlation-based processing must be developed. Here, a two-step aberration correction procedure is developed. First, correlation based processing is used to correct for phase errors. An improved method is developed. Its performance is demonstrated on phantom images degraded by aberrations. In the second step, an algorithm called PARCA (Parallel Adaptive Receive Compensation Algorithm) is developed to remove residual image artifacts after phase correction. The efficacy of PARCA to improve images of both phantoms as well as clinical images is shown. Also, a discussion of two-dimensional arrays for aberration correction is presented.; The second topic is contrast enhancement in nonlinear imaging. Contrast agents, such as bubbles, are used in ultrasound to enhance backscatter from blood. To increase contrast between these agents and tissue, nonlinear methods such as harmonic imaging can be used. Contrast is limited, however, by tissue second harmonic signals. Here, we show that a major source of this signal is nonlinear propagation through tissue. In addition, we present methods to suppress this second harmonic generation. One simple approach is to decrease the f/number of the imaging system. Simulations show that doubling the size of the array while keeping total power output constant, decreases propagating second harmonic generation. A second approach uses active noise cancellation to suppress second harmonics. A specific method, the Harmonic Cancellation System (HCS), is developed and presented as an example. In simulations and experiments, HCS decreased second harmonic generation by over 30 dB.