ULTRASOUND SPECTROSCOPY

Abstract

An ultrasound spectrometer was designed, constructed and used to measure the frequency dependence of forward-scattered ultrasound from biological specimens. A piezoelectric transducer was continuously tuned through the frequency range of 150 to 400 MHz, producing ultrasound of the same frequency. Pulse modulation of the input signal permitted a frequency resolution of 2 MHz. The received pulse was detected at various temporal positions of its amplitude, thereby allowing measurement of the interference of the scattered and unscattered ultrasound radiation. Because of system nonlinearities all received signals were calibrated with respect to the attenuation of ultrasound in water over the system frequency range. The attenuation of water over the frequency range of 150 to 400 MHz was consequently measured and the values agreed very well with figures given in the literature. Forward-scattering experiments were performed with both physical objects and biological specimens. Sapphire spheres and plastic cylinders exhibited the expected Mie scattering resonant structure. Planar glass plates showed the commonly observed Fabry-Perot resonant structure. Measurement of the resonant frequencies agreed well with theoretically-predicted values. The biological specimens consisted of various cell suspensions of densities on the order of 100 million cells per milliliter. Because of the high cell densities necessary for signal measurement, only signal attenuation was measured. No resonant structure was observed. Synchronized growth colonies of mouse leukemia cells were investigated at both the plateau and log stages of cell growth. The attenuation of melanoma cells was measured with and without melanin. Also, various lines of tumor cells were investigated. For all of these cell suspensions, the attenuation in dB/mm increased linearly with the logrithm of frequency. Though the slope of the attenuation-frequency curves varied from cell line to cell line, the variation for the same cells under different biological conditions was not appreciable. For all of the above cell lines, no attenuation fell out of the range of 5 to 55 dB/mm.