DARPA had funded a research at Stanford to find a way of detecting underground explosives without touching the ground (which might trigger the explosion).
The team at Stanford worked out a way for doing that: they observed that pulses of microwave increase the temperature of an object, including the soil. This increase leads to an expansion of the object and to the compression of whatever the object is in contact with. This compression releases ultrasounds that can be detected and analysed. They are a sort of signature of the type of materials involved, since each substance when compressed will generate a very specific kind of ultrasound. Hence they have been able to create a device that can detect underground explosive.
That was the background. If one can detect underground explosive why not try to detect something hiding inside a human body, like a tumour? If one can prove that indeed a tumour has its own specific ultrasound signature one should be game!
Indeed, a tumours characteristics is to stimulate the growth of capillaries (blood vessels) and this makes the tumour look like a sponge, much more vascularised than the surrounding tissue. This physical difference can lead to generation of telltale ultrasounds if the tumour gets compressed.
They are reporting experiments where they placed a tumour like material (in terms of physical characteristics) in a piece of flesh and were able to detect it by sending a microwave pulse lasting a millisecond to hit the flesh, thus raising its temperature by 1/1000 of a degree, too little to create any suffering in the cells but sufficient to create a compression.
One of the problems faced is the dramatic reduction in the ultrasounds strength as they move from the solid body (that transmit them very well) to the surrounding air (this is why when you have an ecography the doctor place some liquid on your skin and press the ultrasound probe hard on your skin). Detecting the ultrasounds in the air required the development of a very sensitive detector, but that was what they did to detect underground explosive without touching the soil.
It might take 10 to 15 years more before we can see this sort of Star Trek tricorder in medial use, and who knows, in 25 years time it might end up in our door jamb: anytime we will walk through the door we will get scanned for tumour, you know, just in case!