Longitudinal describes a position parallel to the long axis of the body and along the path of the ultrasound beam.

A longitudinal wave is a waveform in which the particle motion is parallel to the direction of the wave travel. Sound propagates as longitudinal waves. A series of longitudinal waves make up the ultrasound beam.
A longitudinal wave is produced when a piezoelectric ceramic in an ultrasound transducer, transmits it's back and forth oscillation into a continuous, elastic medium. The particles of the medium are made to oscillate in the direction of the wave propagation, but are otherwise stationary.

Biplane endorectal and endovaginal probes are commonly used in obstetrics, gynecology and urology. They are generally composed of 2 arrays one linear for imaging of the longitudinal plane and a highly curved one to image the transverse plane. These two planes allow a perpendicular visualization of the region of interest.
Ultrasound imaging in both transverse and longitudinal planes enables e.g., precise multiple biopsies by simultaneous visualization of both planes in real-time.

(TIB) The bone thermal index is an exposure model for the case that the ultrasound beam passes through soft tissue and a focal region is in the immediate vicinity of bone.
The longitudinal waves of ultrasound are reflected and transformed into transverse waves, creating a heating effect. Muscle and bone absorb more energy at interfaces with other heterogeneous tissues.

See also Sheer Wave.

From Hitachi Medical Systems America Inc.;
The EUB-2000 B/W is a compact system capable of B-mode, M-mode, and optional with grayscale Doppler (EUB-2000 B/W Dop). This unit features intuitive operation, high mobility, and exceptional image quality. When color Doppler is not required, this system is an excellent choice for general purpose scanning.