High Frequency Backscatter Measurements of Bovine Tissues with Unfocused and Focused Transducers

In order to improve resolution of ultrasonic imaging, high frequency scanners which are employed in the range 20 MHZ to 50 MHz are needed. As a result, it is critical to obtain data on ultrasonic scattering and attenuation in this frequency range. At these high frequencies, it is not feasible to make scattering measurements with unfocused transducers; therefore focused transducers are needed. Using the standard substitution method to calculate the backscatter coefficient, as is used with unfocused transducers, yields erroneous results for focused transducers. The assumption that the reflected echo from a perfect reflector in the far field can be calculated as though the transducer acted like a point source is not valid for focused transducers. Therefore, a method is presented for focused transducers where the flat reflector is substituted by a particulate reference medium whose backscatter coefficient is well known and documented. As a validation study, measurements are made with IO MHz focused and unfocused transducers. Preliminary backscatter results at IO MHz indicate a good agreement with previous work. For bovine myocardium, a backscatter coeffIcient of 6.35 x IOe f .02 mm-’ sr“ was obtained for unfocused transducer and 4. I x IO-” f I I mm-’ si’ for the focused transducer. These measurements will be extended up to 30 MHz using focused and unfocused transducers for bovine myocardium, kidney, and liver. Introduction The ultrasonic backscatter coefficient, as a function of frequency, is a useful parameter in describing the scattering efficiency of a material. In clinical ultrasound, the backscattered echoes in the frequency range l-20 MHz are used to analyze biological tissues. However, there is a need to explore ultrasound imaging at a higher frequency range in order to improve image resolution and quality. High frequency measurements of backscatter have been conducted on blood (2). For frequencies greater than 20 MHz, focused transducers are needed to ensure adequate signal-tonoise ratio since high frequency unfocused transducers have reduced SNR due to small aperture sizes to keep measurements fairly close to the transducer. This paper explores backscatter measurements from bovine tissues at 10 MHz using focused and unfocused transducers. The methods described in this paper will be used to extend the backscattering measurements up to 30 MHz and comparisons will be made between focused and unfocused transducers