Reflection of the sound beam occurs when it hits a boundary between materials having different acoustic impedance. The reflection (echo) is the portion of a sound that is returned from the boundary. The reflection time (the time taken for the wave to return to the probe) can be used to determine the depth of the object.
The reflection within the body produces the ultrasound image, but should be minimized at an ultrasound couplant to skin boundary where the couplant acts as an acoustic window through which the image is seen. The amount of sound waves, which are reflected back at the interface between two tissues is depend on the angle of incidence and the difference between the acoustic impedance values of the two tissues.
If the difference is great, a large part of the sound waves will be reflected back. If too much sound is reflected back and not enough waves are remaining to be able to penetrate the tissue, the imaging will be poor.
If the difference is small, a small amount will be reflected back. Enough sound signal remains to continue with ultrasound imaging.
If the ultrasound beam meets a rough surface or small object, the beam is scattered in all directions and only a small amount will be received by the probe.

See also False Distance Artifact, Target Strength, and Snells Law.

From Siemens Medical Systems;
'The SONOLINE Omnia™ ultrasound system offers mobility, high performance, and ease of use. This digital imaging system delivers excellent 2D, color flow, and Doppler image quality for a variety of general imaging exam types. In addition, the SONOLINE Omnia with cardiac option includes special features that enable use for adult cardiac imaging.'

The skinline is the ultrasonic penetration depth corresponding to the skin / transducer interface on the display.

The word 'sonar' is derived from 'sound navigation and ranging'. Sonar describes a device that transmits sound waves in water and registers the vibrations reflected back from an object. Sonar is used in detecting objects such as submarines, locating schools of fish, or determining water depth.

Sonography [aka: ultrasonography] is a term that encompasses the entire process of performing ultrasound examinations and interpreting the obtained images.
Sonography involves the skilled application of ultrasound technology by trained professionals known as sonographers or ultrasound technologists. These specialists operate the ultrasound equipment, manipulate the transducer, and acquire the necessary pictures for diagnostic imaging purposes. Sonography requires in-depth knowledge of anatomy, physiology, and pathology to accurately interpret the ultrasound images and provide valuable information to the treating physician.
Sonography uses equipment that generates high frequency sound waves to produce images from muscles, soft tissues, fluid collections, and vascular structures of the human body. Obstetric sonography is commonly used during pregnancy. Sonography visualizes anatomy, function, and pathology of for example gallbladder, kidneys, pancreas, spleen, liver, uterus, ovaries, urinary bladder, eye, thyroid, breast, aorta, veins and arteries in the extremities, carotid arteries in the neck, as well as the heart.
A typical medical ultrasound machine, usually a real-time scanner, operates in the frequency range of 2 to 13 megahertz.

See also Musculoskeletal and Joint Ultrasound, Pediatric Ultrasound, Cerebrovascular Ultrasonography and Contrast Enhanced Ultrasound.