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When the acoustic wave propagates though the sample it may be scattered, absorbed or reflected at media interfaces. Thus, the technique registers the echo generated by the acoustic impedance (Z) contrast between two materials. Scanning acoustic microscopy works by directing focused sound from a transducer at a small point on a target object.
The notion of acoustic microscopy dates back to 1936 when S. Ya. Sokolov [1] proposed a device for producing magnified views of structure with 3-GHz sound waves. However, due to technological limitations at the time, no such instrument could be constructed, and it was not until 1959 that Dunn and Fry [2] performed the first acoustic microscopy experiments, though not at very high frequencies.
The scanning acoustic microscope, invented with a colleague in 1973, has resolution exceeding optical microscopes, revealing structure in opaque or even transparent materials not visible to optics. In 1981, Quate read about a new type of microscope able to examine electrically conductive materials.
AFAM. Atomic force acoustic microscopy (AFAM) is a type of scanning probe microscopy (SPM). It is a combination of acoustics and atomic force microscopy. The principal difference between AFAM and other forms of SPM is the addition of a transducer at the bottom of the sample which induces longitudinal out-of-plane vibrations in the specimen.
Scanning near-field ultrasound holography combines atomic force acoustic microscopy and ultrasonic force microscopy. Two transducers producing high frequencies are used. Usually frequency is higher than the resonant frequency of the cantilever. One transducer is placed below the sample and the other attached to the cantilever.
This involves scanning focused light on the tissue surface. The imaging depth (typically <1 mm) and quality of the resulting image are limited by optical diffraction and scattering, not by ultrasound diffraction. In other words, optoacoustic microscopy has the same limitations as conventional optical microscopy.