| A new method is proposed to detect whether the sign of acoustic impedance change in space is positive or negative. Ultrasonic signed echoes were obtained by two times of transmission/reception at a center frequency ratio of N: N+1 (N: a natural number), calculating the (N+1)-th power of the received echo signal at a center frequency of Nf0, and the N-th power of the received echo signal at a center frequency of (N+1)f0, and then performing phase-sensitive detection between the resulting two signals at N(N+1)f0. The signed echo images of human carotid arteries were taken at 6.2 and 9.3 MHz (2:3). The layer structure of the arterial wall was clearly observed with a healthy volunteer while the sign of its echo was significantly less uniform with a volunteer with presumable plaques. |