RESOLUTION IN A B-MODE ULTRASONOGRAM

This study is a small part of a larger study which is evaluating the effects of B-mode processing on ultrasonograms of the abdomen. Since we examine innovative as well as conventional processing algorithms, we have recently studied the effects of lateral beam profile filters on ultrasonograms of objects which simulate the texture found in abdominal scans. Finite beam widths, associated with all conventional ultrasonic transducers, significantly degrade the lateral resolution obtained in a B-mode ultrasonogram. Software filters, constructed from transducer beam profiles, have been proposed as a means for improving lateral resolution. Unfortunately the filters designed in this manner perform poorly when applied to textured medical ultrasound images. In 1974 McSherry and Keller (1) briefly described a minimum mean-square error filter designed for application to bistable cardiac images. If this filter were applied to conventional gray scale ultrasonograms, the large side lobes characteristic of the filter would have produced ringing in the resulting image. Noise and win­ dowing problems in beam profile filter design were handled heuris­ tically in a recent paper by Hundt and Trantenberg (2). They col­ iected beam profile data from a B-mode display and then derived a frequency domain filter from the data. B-mode display data is characterized by poor spatial resolution and non-linear A to D gray level assignment. These were therefore incorporated in the beam profile measurements, and increased the noise content of the data. This clearly hampers successful filter design. In this study we wish to examine the effects of lateral ueam profile filters applied to A-line data from textured objects. This is accomplished by collecting r-f waveforms from test objects and then using this data to simulate on a computer the functions 167