ULTRASONIC-ABSORPTION ANOMALY OF BRAIN-TISSUE

The internal friction of bovine brain tissue has been studied by the m easurements of longitudinal ultrasonic propagation in the megahertz re gion and Young's modulus in the frequency region from 10 to 1000 Hz. T he ultrasonic absorption of bovine brain tissue increased monotonously as the frequency was decreased. The imaginary part of Young's modulus appeared to have a finite value at the lower frequency limit. The res ults of the two kinds of mechanical measurements led to the conclusion that apparent viscosity of brain tissue increased anomalously as the frequency decreased from 5 MHz to 10 Hz. There are two possible mechan isms for the ultrasonic behaviors of brain tissue: anomalous absorptio n due to the existence of lamellar structures and very broad distribut ion of relaxation phenomena. Experimental methods to determine the con tribution of the two mechanisms were discussed based upon the essentia l difference in the expected behaviors of the real part of moduli.