Sound is created when a surface vibrates, pushing the medium particles into a series of compressions and rarefactions. A human being would normally be able to hear sound with frequency between 20 Hz and 20000 Hz.
A piezo-electric crystal is able to expand or contract according to the applied voltage. If an alternating current is applied across this crystal, it will expand and contract according to the a.c. frequency. When the a.c. frequency is beyond 20000 Hz, ultrasound would be produced.
When a piezo-electric crystal changes shape, electric voltages are produced. When the ultrasound is incident onto a piezo-electric crystal, the crystal would expand and contract according to the ultrasound frequency. The series of expansions and contractions produce very small alternating voltages, which can be amplified with an operational amplifier. The signal would then be converted to images.