The effects of aberration, time-shift compensation, and spatial compounding on the discrimination of positive-contrast lesions in ultrasound b-scan images are investigated using a two-dimensional (2-D) array system and tissue-mimicking phantoms. Images were acquired within an 8:8 12-mm 2 field of view centered on one of four statistically similar 4-mm diameter spherical lesions. Each lesion was imaged in four planes offset by successive 45 rotations about the central scan line. Images of the lesions were acquired using conventional geometric focusing through a water path, geometric focusing through a 35-mm thick distributed aberration phantom, and time-shift compensated transmit and receive focusing through the aberration phantom. The views of each lesion were averaged to form sets of water path, aberrated, and time-shift compensated 4:1 compound images and 16:1 compound images. The contrast ratio and detectability index of each image were computed to assess lesion differentiation. In the presence of aberration representative of breast or abdominal wall tissue, time-shift compensation provided statistically significant improvements of contrast ratio but did not consistently affect the detectability index, and spatial compounding significantly increased the detectability index but did not alter the contrast ratio. Time-shift compensation and spatial compounding thus provide complementary benefits to lesion detection.
The effectiveness of adaptive beam formation using aberration correction has been demonstrated in ultrasonic b-scans of liver-mimicking scattering phantoms imaged with and without an intervening aberrator that produced wavefront distortion comparable to that of abdominal wall. Images of 4 mm diam spherical features (either positive or negative contrast lesions or scatterer-free cysts) in the uniform scattering background of the phantoms were produced at 3.0 MHz with a two-dimensional (80×80-element) array transducer system. Time-shift aberration was estimated from the scattering data and used to compensate both transmit and receive waveforms. Image improvements were assessed by comparison of feature contrast with and without aberration correction in individual images and by comparison of intensities in averages of independent, statistically identical images. Feature contrasts and borders were visibly and measurably improved, sometimes to near the water path results, using aberration correction, particularly when both transmit and receive corrections were applied. An efficient implementation of aberration correction was achieved by correction of multiple image scan lines with a single aberration estimate. Aberration correction using estimates from one-seventh the number of scan lines in 8 mm wide images produced improvements comparable to those achieved by individually estimating and correcting aberration in every scan line.
Parameters of ultrasonic aberration can be obtained from power spectra of scattering when individual scattering measurements from which the spectra are estimated have a common aberration and the same nominal geometry. However, the scattering volumes are then confined to a small spatial region and use of finitely many overlapping volumes that result in a nonzero variance is necessary for the measurements. Assuming the scattering is from a spatially uncorrelated medium, the variance of the spectral estimates is expressed as the product of the variance for a single measurement and a reduction factor that depends on the amount of overlap between each volume pair. This factor describes the rate of convergence and the accuracy of the estimates as a function of the number and the overlap of the scattering volumes. Assuming further that the individual volumes are localized by a Gaussian window and that the centers of the volumes are located on orbits of an icosahedral rotation group, the variance is minimized by adjusting the weight and radius of each orbit. Numerical evaluations using orbits formed by icosahedral vertices, face centers, and edge midpoints show that a significant reduction of variance can be achieved from volumes in a confined region.
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