fig3

Figure 3. Pulse inversion and amplitude modulation. A: pulse-inversion technique is used in second harmonic imaging. Pulse 1 excites microbubbles, generating a linear fundamental response along with higher harmonic components. The inverted pulse 2 generates the same frequency components, however with different phases. The linear fundamental response from tissue experiences a 180˚ phase shift relative to the pulse 1 components, whereas the second harmonic response from microbubble experiences a 360˚ (= 0˚) phase shift. As a result, the fundamental responses are canceled out and the second harmonic responses are constructively added together; B: an amplitude modulation technique also plays a role in ultrasonic nonlinear imaging. An amplitude pulse is transmitted to eliminate the linear response and to elicit a nonlinear response. Upon reception, the pulse 2 components are rescaled and subtracted. Then, the fundamental response from tissue is canceled, and the second harmonic response from microbubble is leaked out