Breast ultrasound has become an indispensable tool for surgeons who wish to provide state-of-the-art care to patients with diseases of the breast. Surgeons depend daily on their interpretation of the images to make diagnoses and to guide interventional procedures. To understand the ultrasound images properly, the surgeon must have a thorough understanding of the physics involved in the generation of those images.
Sound is a form of mechanical energy that travels at varying speeds through different materials by causing temporary compression and rarefaction of the molecules of those materials. Consequently, unlike electromagnetic radiation, sound cannot travel through a vacuum. Sound travels in waves, and there is an inverse relationship between the wavelength (the distance from one point on the wave to the same point on the next wave) and the frequency of the waves. A human with perfect hearing might hear frequencies between 20 and 20,000 cycles per second or 20 Hz to 20 kHz. Sound with a higher frequency than audible sound is referred to as ultrasound. Diagnostic medical ultrasound is generally somewhere between 2 and 30 million cycles per second, or 2 to 30 MHz.
The fundamental operating principle of all ultrasound transducers is the piezoelectric effect, discovered by Pierre Curie and his less famous brother, Jacques. There are crystals that occur in nature and others that can be created that have the ability to transform electrical energy into sound and vice versa. A linear array transducer used in breast ultrasound has a series of tiny piezoelectric crystals extending from one end of the footplate to the other, each of which can be fired individually or in groups. When a voltage is applied to the piezoelectric crystal, the crystal is deformed and the electrical energy is converted into mechanical energy in the form of sound. A damping material adjacent to the crystal acts like a finger on a tuning fork to immediately stop the sound, resulting in a very short pulse. Operating according to the pulse-echo principle, the transducer is generating ultrasound less than 1% of the time. Greater than 99% of the time it is "listening" to returning echoes, which are in turn converted back into electrical energy and recorded.
When sound meets an interface between tissues with different acoustic impedances (an acoustic mismatch), part of the sound is transmitted into the second tissue, and part is reflected as an echo. The impedance of the tissue is simply the product of the density of the tissue and the rate at which sound travels in it. The greater the acoustic mismatch between adjacent tissues, the greater the echoes produced at that interface. The returning echoes are recorded on a scale in proportion to their strength or amplitude with a varying degree of brightness. Ultrasound using this scale is called B-mode scanning (the B stands for brightness). The highest strength echoes are displayed as ...