A mapping or imaging technique, used to obtain clinically useful information about the structure and functioning of tissues and organs, in which acoustic pulses are emitted from an acoustoelectric transducer, and echoes are received from acoustic impedance discontinuities along the assumed line-of-sight axial propagation path. A number of different modes of operation have emerged, each having areas of usefulness.
The A (amplitude) mode uses acoustic pulse emissions and echo reception along a single line-of-sight axial propagation path, and thus provides a one-dimensional mapping. This mode of operation cannot provide identification of structural features. It is, however, a most accurate method of measuring time delays and, therefore, distances between echo-producing structures or distances of structures from transducers, provided the speed of sound propagation in the medium is known.
The M mode of operation is used to display the movement of time-varying, echo-producing structures by intensity-modulating the trace as it is swept slowly across the oscilloscope screen in a direction at right angles to the fast time-base sweep. This mode of operation is used extensively in diagnosing disorders of the heart. See also Echocardiography; Heart disorders.
For a two-dimensional picture to be obtained, the line-of-sight propagation path must be scanned and the position and direction of the path monitored and used to form a two-dimensional picture. Typically, the B-mode display is formed by moving the transducer so that the line-of-sight path remains in a single plane. The time-base trace of the cathode-ray oscilloscope screen is moved to correspond, in position and direction, to the ultrasonic line-of-sight propagation path, and echoes are displayed as intensity modulations of the trace.
The C (constant-range) mode provides a two-dimensional image display at constant time delay, and presumed constant distance, from the ultrasonic transducer. The scanning is arranged so that the point at constant depth along the propagation path (beam axis) traverses a plane. See also Medical imaging; Ultrasonics.
