QUANTITATIVE RESULTS IN ULTRASONIC TOMOGRAPHY OF LARGE OBJECTS USING LINE SOURCES AND CURVED DETECTOR ARRAYS

A tomographic reconstruction technique valid for line sources, curved detector arrays, and large objects is presented. For acquisitions involving a curved detector array inverse diffraction is first used to propagate the field back to a straight line and then the standard filtered backpropagation (FBP) algorithm is employed to reconstruct the image. Using inverse diffraction the measured field can be accurately propagated all the way back to the reconstruction area. Thus an essential improvement is obtained compared to using the approximate backpropagation of Rytov data contained in the FBP algorithm, which becomes inaccurate when the distance from the measurement surface to the reconstruction area is large. We apply this technique to measured data and show that it gives reconstructions of high quality, both with respect to geometry and velocity. We also show that when the illuminating wave is cylindrical rather than plane, segmentation of the image can be used in combination with inverse diffraction and FBP reconstruction to obtain high-quality images of large objects. The size of each image segment must be such that the cylindrical wave locally behaves like a plane wave across it.