Since the discovery of piezoelectricity in 1880 by Pierre Curie, ultrasonic waves could be deliberately transmitted, and their echoes detected by the same element. In 1940 ultrasonic imaging (Reflectoscopy) was firstly introduced for industrial use, and afterward for medical use in 1941. Many expansions were developed for this modality like doppler-ultrasonography for speed-of-flow measurements, High-Intensity Focused Ultrasound (HIFU) for therapeutic lesioning, Elastography, etc.
Resolution improvement in ultrasound imaging
Ultrasound imaging resolution is limited by its mostly large wavelengths. Medium inhomogeneities also induce resolution degradation, as the ultrasonic image reconstruction (or focusing, as done in HIFU) is done under the assumption of a non-varying speed-of-sound in the medium, but speed-of-sound may vary relatively largely in many practical imaging scenarios (such as in biological tissue).
We are trying to implement super-resolution methods from optical imaging in ultrasound imaging modalities to overcome the system's acoustic diffraction limit.
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 Kyriakou, A., Neufeld, E., Werner, B., Paulides, M. M., Szekely, G., & Kuster, N. (2014). A review of numerical and experimental compensation techniques for skull-induced phase aberrations in transcranial focused ultrasound. International journal of hyperthermia, 30(1), 36-46.