Choosing a sensor for my Ultrasound Scanner project

I need help choosing a sensor for my ultrasound/sound sensor project. I need a sensor which is cheap, because this is a personal project, and I want one which could work in a B-mode ultrasound scanner. Those goals are probably mutually exclusive, but I think I can find something better than my current sensor, Digikey's "SPM0404UD5", the only US sensor I found for sale in small quantities. Any suggestions would be appreciated.

Reading material suggestions for someone hoping to build a B-mode ultrasound scanner would also be great. I doubt there's a good book addressing this exact task, but I'd be happy to be proven wrong.

Below is a description of my project goal, my background, and my very early design of my project.



I think B-mode medical ultrasound scanners are really, really neat. I saw one in real life once, while the transducer was being inserted in my esophagus  but for various reasons I was too distracted to enjoy the experience. Later I learned more about the concept of ultrasound imaging in a medical image processing course, but that didn't cover the mechanics of those beautiful machines. I think a hands on project is just what I need. so I'm going to try to build one.

If you choose to give me advice on this problem, you should probably know a bit about what I know and what I've got planned. I know a little bit about electronics and digital signal processing. I attended and enjoyed courses on digital signal processing and the audio and medical imaging applications thereof, as well as a few courses on electronic system design. I have designed and built a microprocessor-based sensor system, but have not worked with FPGAs or audio sensors.

With that in mind, I'm planning to build an ultrasound scanner using an FPGA dev board (Saxo), a sensor acquisition device (FlashyD High-Speed ADC), an analog multiplexer (perhaps a CD4051), and a batch of 8 or more ultrasound sensors (like the SPM0404UD5, if needs be).

I like looking at diagrams, as long as they're not my own, so I hope someone else enjoys this:

"Schematic" of My Ultrasound Scanner, Version 1

• Where I've written "MEMORY" and crossed it out, that's because I haven't bought an FPGA daughter board which connects a stick of RAM or an SDCard-writer, and I don't intend to buy or build one until I get other stuff working. 
• For now, the FlashyD/Saxo combination can (according to the manufacter's documentation) record 512kb of data, stop recording, transmit that data to the PC, and begin recording again. I hope that is sufficient for a prototype. 
• Where I've drawn a "MULTIPLEXER", the first version of the device won't use the multiplexer, because it will involve connecting 2 sensors directly to the FlashyD. I'm not really sure that I've thought through my multiplexing ultrasound-sensor-sweep-sampling idea, after all. 
• I am hoping to cycle between my sensors very quickly, so that I can record from all 8 or 16 of them with a single FlashyD. 
• Where I've drawn a row of "SENSOR"s, I will actually need to make a wee little sensor amplifier circuit which outputs a signal in the (-100,+1000)mA range required by the FlashyD. Building such a sensor amplifier is really pretty easy. The hard part will be building one which works with the particular input and output characteristics I require. I sure hope I was paying attention in all those circuit design courses. 
• Regarding the EMITTER: A medical ultrasound scanner emits a loud noise (well, a high-amplitude noise at a frequency you can't hear, so technically it's silent), and records the resulting echos. I'm not sure what sort of emitter I should use, so for now I'm going to use a hammer and a wall. This is crude, so crude that it is probably a bad idea, but it's very cheap, and I hope I will be a wiser person by the time I give up this silly idea and start using electronics to make my chirp.