Ultrasound imaging uses the pulse-echo principle in which a short burst of sound waves are emitted from a transducer and directed to the underlying tissue. Diagnostic ultrasound uses sound waves of frequencies between 2 and 20 MHz. When a sound beam is directed toward the body, tissue reflects, absorbs, or scatters the beam. Each of these responses determines the nature of the received signal. The sum of acoustic energy losses from absorption, scattering, and reflection is called attenuation.
Reflection is a result of the passage of sound between tissues with different acoustic impedances, i.e., resistance. Absorption, meanwhile, is directly proportional to ultrasound frequency and determines resolution, or the ability to differentiate between adjacent structures. Resolution can be separated into two dimensions, parallel to and across the beam. Depth resolution is a function of length of the ultrasound pulse, and near-surface resolution implies distinguishing between superficial reflectors. The ability of ultrasound to detect reflectors at a certain depth is called sensitivity. A more sensitive system detects more intense reflection signals1, 2, 3, and 4 (Figure 2–1).
Schematic depicting the reflection, scattering, and absorption of ultrasound waves.
There are two types of ultrasound imaging commonly used today:
B-mode, which is a two-dimensional grayscale imaging. Images are dependent on amplitude information from sound beams passing through, or reflecting from, tissue.
M-mode in which the intensity of the backscattered beam is represented as pixel brightness. The image produced is the result of the compilation of different scan lines, each produced by a single pulse-echo. Thus the image will have high temporal resolution, which is essential in cardiac assessments.5, 6, 7, and 8
Anatomy seen on ultrasound imaging is described in terms of morphology, which consists of external contour (boundaries and shapes) and internal echo pattern (in practical terms, brightness). Contour of a structure also describes its margins. When an organ has a well-defined margin it is usually due to the presence of a capsule, showing a sharp margin around the less echogenic parenchyma.
Internal echo pattern is described by the echogenicity—or ability to reflect sound waves—of a structure relative to its surrounding structures. For example, a lesion that is more echogenic, or hyperechoic, will be brighter than the surrounding structures (Figures 2–2, 2–3, and 2–4). If structures contain lower echoes, they will be hypoechoic or echopenic and will appear darker (Figures 2–5 and 2–6). A lesion without echoes, such as a renal cyst containing simple fluid, is anechoic or sonolucent, and will appear completely black.
Angiomyolipoma (arrow) of right kidney, seen as a ...