Contrast agents, such as bubbles, are used in ultrasound to enhance backscatter from blood. To increase contrast between these agents and tissue, nonlinear methods such as harmonic imaging can be used. Contrast is limited, however, by tissue second harmonic signals. We show that a major source of this signal is nonlinear propagation through tissue. In addition, we present methods to suppress this second harmonic generation. One simple approach is to decrease the fumber of the imaging system. Simulations show that doubling the size of the array, while keeping total power output constant, decreases propagating second harmonic generation. A second approach uses active noise cancellation to suppress second harmonic generation. A specific method, the harmonic cancellation system (HCS), is developed and presented as an example. In simulations, HCS decreased second harmonic generation by over 30 dB. Using such methods, contrast can be improved between tissue and bubbles in harmonic imaging.
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