An ultrasound is essentially the practice of propagating sound waves through the body and recording the echoes. When the sound waves bounce off internal structures such as bones, organs, an unborn baby, etc. an image is created that contains useful information for medical practioners. The physics of how this works is based on centuries of the slow accumulation of knowledge on how sound waves operate and then applied to how it operates in live tissue. What follows below is a brief discussion of how ultrasounds work, the physics behind it, complete with equations and their derivations, and then finally how ultrasounds have made an impact on the world.
As stated above medical ultrasounds work by generating soundwaves, sending them through organic tissue, and then receiving the echoes with a specialize machine that can interpret the different reflected sound waves. The most common method is known as Brightness Mode, or B Mode, which gives a two-dimensional image that is black and white. It can be used to image different slices of the tissue in slices that are less than one millimeter thick, depending on the anatomical site of the slice. The sound waves are created by piezoelectric crystals, which “are fabricated from material that changes electrical signals to mechanical vibrations and changes mechanical vibrations to electrical signals” (Abu-Zidan, Hefny, & Corr, 2011). As the wave goes through the body it hits different material, known as acoustic impedance, and reflect different levels of sound. Liquid transmits sound more easily so less is reflected, producing a blacker image, whereas denser materials such as bone reflect more, producing a white image.
Ultrasounds uses sound waves, the physics of which is well understood. A sound wave is best defined as a pressure wave, as it is a series of high pressure and low pressure areas moving through a material. Sound waves also move longitudinally and do not use transverse motion, as there is no up and down motion. A simple explanation of this is that beginning at the creator of the sound which vibrates, the material nearest it compresses, which in turn compresses the material next to it and so forth. For example, if a person claps his hands, then his hands compress the air between them, which in turn compresses the next air molecules and so forth. It then propagates three dimensionally with alternating areas of higher and lower pressure.
An important equation governing sound waves describes the speed of the wave with regard to the density and compressibility of the medium. It is (1/pk). Here, (C) is the speed of sound, (p) the density of the material, or the amount of mass per unit volume, and (k) is the compressibility of the material, or how easily it can be changed when a pressure is applied to it. This means that the speed changes depending on the material it is traveling through, which is important in an ultrasound to produce the image. That is, as the density goes up the faster the sound wave travels through it, and the less dense the material, the slower the sound wave is able to propagate through it. The human body is made up of various materials, some being denser than others. The generally accepted densities and speeds for a sound wave traveling through different mediums, especially relevant here are the parts of the human body, are 330 m/s for air, 1480 m/s for water, most human tissue is around 1500 m/s, and bone is about 4080 m/s. From this it is clear that sound travels much faster through bone than through other tissue in the body as bone is much more dense.
Another important equation in sound wave propagation is . Here, is the wavelength, or the length between two peaks in the wave, (c) is the velocity, or the speed of the wave through a medium, and (f) is the frequency, or the number of oscillations per unit of time. Using this it is clear to see how a reflected sound wave differs from one to the next depending on the material it is hitting. That is, when a wave changes from one medium to the next, the frequency remains constant but velocity changes and so then must wavelength. The different wavelengths are picked up by the ultrasound machine. This produces the different shades of black, grey, and white in the final ultrasound image. For example, if an ultrasound is done on a kidney to determine if there is a kidney stone, then as the wave travels through the kidney some of the energy of the wave is lost due to being transmitted through the soft tissue, while some is reflected back to the receiver. When the sound wave hits the stone, more of it will be reflected and less lost to the stone, producing more brightly colored images. The same is the case for bone and other dense areas of the body.
Another important equation is Snell’s Law which is . This states that as a wave hits a medium of a different density its angle of reflection (divided by its speed (c1) must equal the initial angle ( divided by its speed (also c1), which both must equal the angle of transmission through the medium (divided by the speed (c2). This shows the relationship between the velocity of the wave and the angle it approaches the medium. This affects the angle of reflection and the angle the wave propagates through the second medium, as well as their resultant velocities. That is, as the angle of the incident wave becomes greater so does the reflected wave and transmitted wave, as well as their velocities. For medical professionals, one thing to consider is the angle of the ultrasound so that they don’t create a total internal reflection of the wave. This can be seen using the above equation, as if the initial angle is too large then there is no transmitted wave, which means the there is no lost energy through transmission. In this case, the reflected wave appears stronger than it should, giving an improper reading. The image will appear bright white as if the wave is reflected something more dense like bone.
Ultrasounds are an important part in medicine and various other parts of society. The first practical application was during World War 1 when sound waves were used to detect enemy submarines. It was until the 1950s that it began to be used in the field of medicine. “First was introduced in the obstetrics, and after that in all the fields of the medicine (the general abdominal diagnostics, the diagnostics in the field of the pelvis, cardiology, ophthalmology and orthopedics and so on)” (Carovac, Smajlovic, & Junuzovic, 2011). Although most people associate ultrasounds with looking at a fetus in a pregnant woman, it can also be used to peer into all areas of the body with soft tissue to help medical professionals with their diagnosis. However, areas of the body that are high in bone density are not easily imaged using ultrasound and so it cannot be used effectively. For decades now, ultrasound has been one of the most useful tools in the field of medicine and it has undoubtedly saved countless lives since its introduction.