A speckle target adaptive imaging technique in the presence of distributed aberrations.

Acoustic velocity inhomogeneities in tissue result in aberration of ultrasound images. These aberrations can be modeled as a near field thin phase screen or as a distributed aberrator. The effect of a near field thin phase screen is to time shift the received echo at each element, while distributed aberrators result in both pulse distortions and time shifts from element to element. Most current techniques for the correction of distributed aberrators are limited to application on point targets. A new technique is proposed which uses multiple transmits from spatially shifted transmit apertures (the translating transmit aperture algorithm), in conjunction with phase conjugate filters, to correct for distributed aberrations in the presence of speckle targets. The performance of the translating transmit aperture algorithm in improving the correlation between signals received by elements of different spatial separations is measured, and factors affecting the performance of this technique are investigated in simulation and experiment.

Parallel processing techniques for the speckle brightness phase aberration correction algorithm.

The speckle brightness adaptive algorithm has previously been implemented in approximately real-time on low frequency, one-dimensional arrays. To increase the speed of this technique, a temporally parallel algorithm and a spatially parallel algorithm are described. Theoretical analyses, simulation results and experimental measurements are presented for these algorithms. Theoretical predictions indicate that these techniques increase the correction speed, but some decrease in the accuracy of the compensating phase estimate occurs. Simulation results indicate that these parallel algorithms perform well at removing the effects of phase aberration. Preliminary experimental measurements demonstrate the correction speed improvements achievable with these algorithms.

Two dimensional ultrasonic beam distortion in the breast: in vivo measurements and effects.

Two dimensional arrival time data was obtained for the propagation of ultrasound across the breasts of 7 female volunteers. These profiles were extracted through the use of cross-correlation measurements and a simulated annealing process that maintained phase closure while aligning the data. The phase aberration measured in two dimensions had a larger magnitude than previously reported phase aberration measured in one dimension in the breast. A point spread function generation computer program was used to demonstrate the system response degrading effects of the measured phase aberration and the usefulness of current one dimensional phase aberration correction techniques. The results indicate that two dimensional correction algorithms are necessary to restore the system performance losses due to phase aberration.

The impact of acoustic velocity variations on target detectability in ultrasonic images of the breast.

Experimentally obtained ultrasonic phase aberration profiles in the breast were used to investigate the impact of acoustic velocity variations on images of simulated breast lesions. The imaging properties of several phased-array transducers with varying frequencies and geometries were studied as phase error profiles of increasing magnitude were introduced. The targets studied were anechoic lesions of various sizes. The results indicate that phase aberrations significantly degrade the contrast of ultrasonic breast images, especially for high-resolution scanning systems.

In vivo measurements of ultrasonic beam distortion in the breast.

The phase aberrations encountered by ultrasonic pulses propagating through breast tissue in twenty-two female volunteers were measured. The experiments were designed to assess the impact of these aberrations on clinical ultrasonic image quality for a variety of transducer and imaging geometries. The phase aberration profiles of a given patient were correlated with the amount of parenchymal tissue determined from that patient's mammogram. These data are useful in assessing the image quality achievable with conventional ultrasonic imaging systems, and the potential application of adaptive ultrasonic imaging systems. The results indicate that phase aberrations significantly degrade breast image quality for typical transducer frequencies and sizes.

A real-time adaptive ultrasonic imaging system.

The performance of an experimental, adaptive, phased array imaging system in improving the quality of abdominal organ images in healthy volunteers was assessed. Trials were conducted under usual clinical imaging conditions and when phase aberrators of known shape and magnitude were introduced into the imaging system. The system, which uses local target brightness as a quality factor, was able to improve clinical image quality when aberrators introduced externally were present and had a negligible effect on image quality when they were not. On several of the volunteers, the phase aberrations were measured directly across the abdomen. The implications of these results for the future clinical application of adaptive imaging systems and the future system modifications are reported.