Ultrasound physics explores the principles behind using high-frequency sound waves for various applications. It involves understanding how these waves are generated, travel through materials, and how their interactions provide information beyond human hearing.
The Nature of Ultrasound Waves
Sound travels as a wave, a mechanical vibration that requires a medium, such as air, water, or tissue, to propagate. The characteristics of these waves, including their frequency, wavelength, and amplitude, determine how they behave.
Frequency refers to the number of wave cycles passing a point per second, measured in Hertz (Hz). Ultrasound utilizes frequencies above the human hearing range, typically 20 kilohertz (kHz) to megahertz (MHz). Wavelength is the distance between two consecutive peaks or troughs of a wave.
The speed at which a sound wave travels varies depending on the medium’s density and stiffness. For instance, sound travels much faster through denser materials like bone compared to air. Amplitude describes the intensity or strength of the wave, which correlates with the energy it carries.
Interaction of Ultrasound with Tissues
When ultrasound waves encounter tissues, they undergo several physical phenomena. Reflection occurs where a portion of the sound wave bounces back when it encounters a boundary between two materials with different acoustic properties. The amount of reflection depends on the difference in acoustic impedance, a property related to the density and speed of sound within a material. Greater differences in acoustic impedance lead to stronger reflections, such as those occurring at the interface between soft tissue and bone.
Absorption occurs where the energy of the ultrasound wave is converted into heat as it travels through tissue. Tissues with higher protein content or viscosity tend to absorb more ultrasound energy.
Scattering occurs when ultrasound waves encounter small, irregular structures that are roughly the size of the wavelength or smaller. Instead of reflecting in a single direction, the waves are dispersed in multiple directions. Red blood cells, for example, are common scatterers of ultrasound waves. Refraction is the bending or change in direction of an ultrasound wave as it passes from one medium into another at an angle, caused by a change in the wave’s speed.
From Sound Waves to Images
Creating an image from ultrasound waves begins with a transducer, a device that generates ultrasound pulses and receives the returning echoes. The transducer contains piezoelectric crystals that vibrate when an electrical current is applied, producing sound waves. After emitting a pulse, the transducer switches to a listening mode.
When ultrasound waves encounter tissue boundaries, echoes reflect back. These echoes strike the piezoelectric crystals in the transducer, causing them to vibrate and generate electrical signals. The strength of these electrical signals corresponds to the intensity of the returning echoes.
A computer then processes these electrical signals, interpreting the time it took for an echo to return and its intensity. Shorter travel times indicate structures closer to the transducer, while longer times suggest deeper structures. The computer uses this information to construct a real-time, two-dimensional image, often referred to as B-mode (Brightness mode) imaging, where the brightness of each pixel corresponds to the strength of the echo from that point.
Detecting Motion: The Doppler Effect
The Doppler effect is used in ultrasound to detect and measure motion, particularly the flow of blood. This effect describes the change in frequency of a wave relative to an observer moving either toward or away from the source of the wave. In ultrasound, the frequency of sound waves reflected from moving structures, such as red blood cells, will be different from the frequency of the waves originally sent out by the transducer.
If the red blood cells are moving towards the transducer, the reflected waves will have a higher frequency. Conversely, if they are moving away, the reflected waves will have a lower frequency. The computer analyzes this frequency shift to determine the speed and direction of blood flow. This application of ultrasound provides dynamic information about physiological processes, complementing the static images provided by B-mode.