Acoustic microscopy--principles and applications in the studies of biomaterial microstructure

The acoustic microscopy is based on totally different physical concepts than both the optical and electron microscopes. Therefore, the information obtained with an acoustic microscope is significantly different from those obtainable by the other two. By employing elastic waves, detailed microscopic information regarding biomaterial properties like density, elasticity, viscosity and viscoelasticity are obtained with an acoustic microscope. Live tissue can be examined without fixing or chemical staining. The Scanning Laser Acoustic Microscope (SLAM) and the Scanning Acoustic Microscope (SAM) are the only acoustic microscopes commercially available today. The SAM operates at as high as 4.2 GHz frequency providing at least five times better resolution than the optical limit. The SLAM, however, operates generally at 100 or 500 MHz and can therefore examine thicker samples than the SAM, but at lower resolutions. For optically translucent samples, the SLAM can also provide an optical image simultaneously with the acoustical one. A laser beam is employed in the scanning mechanism in the SLAM, compared to mechanically translating the sample in the SAM. Consequently, the scanning rate is much faster in the SLAM at about 30 frames per second, compared to several seconds per frame for the SAM. Both qualitative as well as quantitative information is obtainable about the material under examination. As a qualitative tool, the information is used to classify and sort materials and detect and localize flaws and defects in optically opaque samples. A microscopic map of the specimen's mechanical properties can be produced using information from the acoustic image. This information may then lead to better understanding of biomaterial microstructure and also can be a valuable aid in characterization of certain subtle morphological differences.