Joie P. Jones - jpjones@uci.edu
Department of Radiological Sciences, University of California Irvine
Irvine, CA 92697-5000
Popular version of paper 1pBB1
Presented Monday afternoon, December 2, 2002
First Pan-American/Iberian Meeting on Acoustics, Cancun, Mexico
For hundreds of years, simple optical microscopes have enabled students as well as scientists to see into a world which our own eyes are incapable of viewing. Such instruments have origins going back thousands of years, when our ancestors in both Assyria and Mexico independently discovered that small magnifying glass spheres from meteorites could be used to observe nature at a near-microscopic level. We now know that these sections of meteorites, as well as contemporary microscopes, use glass lenses to focus the light so that we can view structures that the lenses in our own eyes are not capable of resolving.
Limitations on our ability to magnify are determined by the wavelength or frequency of the light; that is, the higher the frequency of the light, the shorter the wavelength, and the greater the potential resolution or ability to magnify. With visible light the resolution limitations are about 0.5 micron (0.5 x 10-9 meters), which corresponds to a magnification of about 2000 times that normally visible with our eyes.
Instead of light (or electromagnetic radiation), we can use other forms of energy to produce microscopic images. For example, if we replace light with sound waves and replace the optical lens with an acoustical lens we can produce an acoustical microscope. Rather than "listening" to the sounds produced by an acoustical microscope we record them electronically and view the acoustical image on a TV monitor.
Now, our ears are capable of hearing sounds only in a limited range of frequencies, between 20 and 20,000 cycles (or 20 Hz to 20 kHz). Since such frequencies of sound have very long wavelengths (typically a meter or so), this means that if we are to build an acoustical microscope with resolution we could call "microscopic", we must use sound waves with frequencies far above that which our human ears can detect. It turns out that if we want our acoustical microscope to have a resolution similar to a light microscope we must use sound waves (or, more appropriately, ultrasound) with frequencies in the range of a GHz (a billion Hz or 1000 MHz). Ultrasonic waves with a frequency of 1 GHz have a resolution of about 1 micron.
The concept of acoustical microscopy was first proposed by Sokolov, in the Soviet Union, in the 1930's. However, developments in computer and ultrasound technologies were required before the first experimental systems could be constructed. Two very different systems were developed in the 1970's: one at Zenith Labs in Chicago by Korpel and Kessler and another at Stanford University by Quate and Lemons. Since that time, numerous research and commercial systems have been developed, but all derived from the original instrumentation design. Why then would we want to develop and utilize an acoustical microscope when the far simpler optical microscope is available and provides images with similar resolution? Optical microscopy provides an "image" of the optical (or the electrical) properties of the target material. Since the optical properties of many materials are very similar (for example, the optical properties of soft tissue vary by only 0.5%), the only way to extract useful information from an optical microscope image of such materials is to prepare the specimens with appropriate stains designed to bring out specific features. For example, appropriate stains applied to soft tissue will identify specific pathologies or biochemical processes.
Acoustical microscopy, on the other hand, provides an "image" of the acoustical (or the elastic) properties of the target material. Since, for many materials, the acoustical properties have a far wider range of values than the optical properties, acoustical microscopy provides a far more sensitive tool to study those materials. For example, since the acoustical properties of soft tissue vary by some two orders of magnitude (as compared to the optical properties which vary by only 0.5%), acoustical microscopy provides an extremely sensitive tool with which to image soft tissue structures, without staining or elaborate sample preparation.
Although acoustical microscopy now plays a major role in materials science, non-destructive testing, and a broad range of industrial application, it has, to date, played only a limited role in biology and diagnostic medicine, limited to a number of research groups. However, the biomedical applications of acoustical microscopy would seem to be a major growth area for this technology, given the fact that it provides an extremely sensitive tool for evaluating tissue, without elaborate preparation, and for examining living organisms. Acoustical microscopy could provide an immediate assessment of pathology, long before conventional methods could be carried out. A specialized ultrasound scanner could be applied directly to the skin providing real-time microscopy on an in-vivo basis. Implemented on a catheter-based system, pathological assessments could be made in a minimally invasive manner without a biopsy. Finally, acoustical microscopy of cells or tissue in culture permits the examination of living structures, which would be impossible with optical methods.