The physical property that describes the ability of sound to move through an object is called acoustic impedance, which is a function of both density and speed of sound through a substance. When a sound wave reaches a boundary between two entities with different acoustic impedances, a portion of the wave is transmitted and the rest is reflected back to the source as an echo (Fig. 1.2). The degree to which a structure reflects sound pulses during an ultrasound examination is denoted as “echogenicity.” Highly echogenic, or hyperechoic, structures typically have relatively low water content, reflect a large proportion of incoming ultrasound, and are displayed as white on a B-mode image. Commonly seen hyperechoic tissues include cortical bone, tendon, organ and muscle sheaths, nerves, and gallstones. Extremely hyperechoic structures result in an artifact on the image known as acoustic shadowing; since most of the sound has been reflected to the probe at the structure’s surface, the area behind the echogenic material is displayed as a black streak (Fig. 1.3).

The simplest mode of ultrasound is known as A-mode (amplitude), in which a single piezoelectric transducer detects the distance to an echogenic structure by precisely measuring the time between emitting a pulse and receiving an echo, utilizing the equation distance=velocity * time. Thus, using the known speed of sound and the detected time to echo, the distance to an object can be calculated. A-mode itself is generally only of historic interest, but can be helpful to understand B-mode (brightness), which is the characteristic mode utilized in modern diagnostic ultrasound. In B-mode imaging, a row of transducers simultaneously act as just described, and the resultant echoes are compiled to produce a two-dimensional image. This is repeated at least 20 times per second to create a real-time effect. An important assumption in creating a B-mode image using the equation above is that the speed of sound through soft tissue is uniform, most frequently assumed to be a constant 1540 m/s, slightly higher than the speed of sound through pure water. However, in reality, the soft tissues encountered in medical ultrasound have a range of acoustic impedances and resultant velocities of sound, which can lead to imprecise localization of an object’s depth and is a limiting factor in the axial resolution of the modality in general.