Micro-Ultrasound Molecular Imaging

Abstract

Perhaps using sound to identify objects, those which remained mysterious to sight and touch senses, is one of the very early diagnostic tools mankind has ever used. We all can distinguish the sound of a plastic cup from one made of glass when tapped by a finger nail no matter how similar they look. The sounds that we hear and those that we don’t, are mechanical vibrations (acoustic waves) traveling through a compressible and expandable (elastic) medium such as air or water. We can recognize the sound of a glass cup from a plastic one because of the fact that such mechanical vibrations of the particles of materials with dissimilar properties are unlike. Perhaps the most characteristic properties of an acoustic wave are its amplitude and its pitch (frequency). The amplitude of a sound determines how loud the sound is. The frequency of a sound is an indicator of how fast are the mechanical vibrations of the medium substances. The frequency of an acoustic wave is measured in cycles per second or Hertz (Hz) which is named after the German physicist Heinrich Hertz. The faster the vibration of the medium substances the higher the frequency or the pitch of the acoustic wave will be. For instance, imagine the strings of a guitar, the tighter the string the faster it will vibrate when picked and the higher the pitch of the sound it makes. Our devices for generating sound (e.g. string of a guitar) and detecting it (e.g. human ear) are limited to a certain frequency range. For instance, there is a limit to the pitch you can achieve by tightening a guitar string before breaking the string, and a normal healthy human ear can detect frequencies in the range of 20 to 20,000 Hz (20 kHz). Based on such frequency dependent limitation or better to say abilities, the sound can be categorized into three regimes: sonic, ultrasonic and infrasonic. Sonic (20 to 20 kHz) is the range where human ears operate. Ultrasound refers to sound with frequencies higher than 20 kHz and infrasound relates to those with frequencies below 20 Hz.