Zero offset is the measurement recorded by an ultrasound machine under conditions of zero input, caused e.g. by occlusion of the vessel. A two point calibration can be performed by the combination of zero offset determination with timed collection.

Anatomic structures respond with characteristic features on ultrasound scanning.
There are some ultrasound terms, referring to the echo appearance, that describes tissue appearance in a uniform manner:

Tendons characteristically are hyperechoic on ultrasound because of the fibrillar pattern. Ligaments appear hyperechoic when the beam is perpendicular to the tissue. Peripheral nerves are hyperechoic relative to muscle.
Muscle appears relatively hypoechoic to tendon fibers. Close observation reveals hypoechoic muscle fibers separated by hyperechoic septae that converge on a hyperechoic aponeurosis. Articular hyaline cartilage appears hypoechoic. The presence of fluid within the joint outlining the cartilage produces a thin bright echo at this interface.
Sound beams do not penetrate the bone cortex. The very bright echo produced at the interface allows both recognition of the bone cortex but also can demonstrate fracture, spurring and bone callus bridging. Abnormal soft tissue calcification and ossification also produces bright reflective echoes.
Cysts or fluid filled areas are without internal echoes and are called echo free or anechoic and may demonstrate enhanced soft tissue echoes posterior to the fluid collection. Inflamed metatarsal bursae and calcaneal bursae clearly depict fluid swelling.

See also Beam Pattern and Zero Offset.

Peripheral veins are easily tested using a 5 to 10 MHz transducer. The venous walls are smooth, thin, and compressible. Venous ultrasound imaging requires the compression of the veins in the transverse view. If compression is performed in the longitudinal view, the vein may roll away from the transducer possibly creating a false-negative examination.
The lumen of the normal vein is echo free. Increasing the gain will display low level echoes representing venous blood moving towards the heart. When performing Doppler spectral analysis or color Doppler the gate should be placed in the center of the vessel. In case of a non-obstructing or recanalized thrombosis, the Doppler gate should be placed within the remaining vessel lumen for flow detection.

See also Maximum Venous Outflow and Zero Offset.