Ultrasonic attenuation and propagation speed in normal human brain

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Attenuation and propagation speed in the 1- to 5-MHz frequency range were measured in vitro in 22 tissue samples from six anatomic sites in five normal human brains obtained unfixed at autopsy. Two methods were used to measure attenuation: (1) received amplitude reduction resulting from insertion of tissue sample between a pair of transducers, and (2) radiation force reduction resulting from insertion of tissue sample between a transducer and a reflector. The piezoelectric transducer method gave attenuation values −10% to +90% higher than the radiation force method. Attenuation (by transducer method) was a function of frequency to the 1.08 power with a value of 0.87 dB ⋅ cm−1 at 1 MHz. Propagation speed was measured by observing signal arrival time change resulting from insertion of the tissue sample between the transducer pair. Propagation speed was 1562 m ⋅ s−1 at 1 MHz with a dispersion of 1.2 m ⋅ s−1 ⋅ MHz−1 over the 1- to 5-MHz frequency range. Ages at death were 3 days, 20, 21, 42, and 72 years. Results were similar in the 20- to 72-year range but attenuation for the 3-day case was about one-third that for adult. Formalin fixing decreased propagation speed by 10 m ⋅ s−1. One-day aging of unfixed tissue resulted in up to a 21% decrease in attenuation and up to a 10 m ⋅ s−1 decrease in propagation speed. White matter (corna radiata, medulla, pons) had attenuation 1.5 times that for gray (Island of Riel, cerebellum) with mixed (basal ganglia) having an intermediate value. Attenuation decreased with increasing temperature. Propagation speed as a function of temperature exhibited a minimum at about 15 °C.