The physics of diagnostic ultrasound

Sound waves progress through a medium at a velocity that is characteristic of that medium. In a particular medium, wavelength and frequency are inversely related, so that short wavelengths can only be obtained at higher frequencies. A beam of sound will not be directional unless the object that is sending (or reflecting) it is at least several times longer than the wavelength. In tissue directional beams of 1 cm or less in width can be produced at frequencies of 1 MHz or more. Other things being equal, the higher the frequency, the narrower the beam. The attenuation of beams of sound in tissue at ultrasonic frequencies is directly and almost linearly related to frequency in that the higher the frequency, the greater the attenuation and the weaker the return signals. In choosing an operating frequency for a particular purpose, one must therefore bear in mind that the advantages of the higher frequencies (i.e., better resolution, both in range and azimuth) are obtained at the bost of a shorter effective range. Accordingly, one makes the best compromise by selecting the highest frequency that permits a range adequate for the purpose. In scanning the abdomens of adults where the range may be fairly long, a 2 MHz transducer is used. For cardiac studies in pediatric patients where the range is shorter and the structures smaller, a 5 MHz probe can be used. In the globe of the eye the structures are minute, the medium one in which losses are quite low and the range is very short, permitting the use of frequencies in the 10 to 20 MHz range.