A 5 MHz ultrasound pulse images a tumor 2 cm deep in soft tissue. Its round-trip echo time is t. If frequency changes to 2.5 MHz, what is the echo time from the same tumor?
For a fixed path in the stated nondispersive soft-tissue model, echo time is 2d/c and does not depend on carrier frequency. Halving frequency changes wavelength, not the propagation time to the same depth and back. Therefore it remains t, B.
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