Speckle reduction in ultrasonic imaging using two-dimensional phase-insensitive receiving arrays.

Speckle noise, which is inherent in all coherent energy forms, can limit the quality of ultrasonic images. This speckle can be reduced by adding images formed by transducers with different characteristics and the amount of this speckle reduction is dependent on the correlation between the transducers' responses. A function, called the generalized correlation function, was developed that found the correlation between arbitrary transducers as a function of their transducer response. Simulation studies were done with both spatial and frequency compounding and the function accurately predicted the correlation found in experiments. A pulse-echo imager, which utilizes two-dimensional phase-insensitive receiving arrays, was developed to reduce speckle in ultrasonic images. Three receiving arrays were designed, constructed and tested, each containing individually addressable elements. The elements were combined coherently to form subapertures with varying size, shape and overlap. Images were made of a point-like scatter, a uniformly scattering material, a tissue-mimicking phantom, with simulated cysts, and liver tissue sample with different scattering strengths. The resolution characteristics of the arrays were found including a new measure called the homogeneous resolution cell size that gives a measure of the effective point spread function of the transducer for imaging in essentially homogeneous tissues. In addition to this, the speckle signal-to-noise ratio was measured, as was the speckle spot size. These parameters were used to find the normalized contrast-to-speckle ratio (CSR/d), a measure of the ability of the system to detect low-contrast lesions. Incoherent summation of images from small groups of elements was found, in comparison to coherent summation, to decrease the speckle noise by a factor of up to four, while maintaining enough resolution to show almost a factor of two improvement in image quality as measured by CSR/d. Image quality was found to be affected by the size and shape of the subapertures used to form the image. It was found that there was significant correlation between non-overlapping elements used as receivers with a common transmitter. This was found experimentally as evidenced by the measure of 16.7 independent views when 64 nonoverlapping subapertures were used to form the image. The impulse response was computed for an annular segment, the element shape for one of the arrays, and used to predict the correlation of the array using the developed correlation function. The simulations confirmed the correlation between elements and yielded a value for effective number of views of 13.5. Overall, it was shown that two-dimensional receiving arrays can be used to reduce speckle and improve image quality, and that this behavior can be modeled accurately.Ph.D.Biomedical engineeringMedical imagingUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/162190/1/8920537.pd