Signed echo imaging with high axial resolution

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)f ₀. An ultrasonic signed echo image of human carotid arterial walls was taken at 5 and 10 MHz with a healthy volunteer. The high axial resolution of the echo image was maintained so that the layer structure of the arterial walls can be observed in the signed echo image as well as in the corresponding conventional B-mode image.