That being said, there's a type of radar system called "stepped frequency continuous wave" radar that uses a bunch of single frequency transmissions instead of an impulse transmission. The basic idea is that instead of using a high-bandwidth transceiver to send and receive the impulse, you can use a low-bandwidth transceiver to send and receive tones, and then hop this transceiver over the large bandwidth to get a high-resolution image (the tradeoff being that it takes longer to acquire an image). Since ultrasound is basically radar, I'd imagine this technique could be used for ultrasound too.
