A zone is a focal region of the ultrasound beam. An ultrasound beam can be directed and focused at a transmit focal zone position. The axial length of the transmit focal zone is a function of the width of the transmit aperture.
The field to be imaged is deepened by focusing the transmit energy at progressively deeper points in the body, caused by the beam properties. Typically, multiple zones are used. The main reason for multiple zones is that the transmit energy needs to be greater for points that are deeper in the body, because of the signal's attenuation as it travels into the body.

Beam zones:
The tightest focus and the narrowest beam widths for most conventional transducers are in the mid-field within the zone where the acoustic lens is focused. The ultrasound beam is less well focused and, therefore, wider in the near and far fields which are superficial and deep to the elevation plane focal zone. The beam width is greater in the near and far fields, making lesions in these locations more subject to a partial volume artifact.

See also Derated Quantity.

Different sound velocities in tissue are causing refraction artifacts. With convex elastomer lens transducers, sound beam refraction at the skin interface can alter the transducer's focusing characteristics and beam profile, cause element to element nonuniformity, and cause phase changes in the acoustic wave. These cumulative refraction induced errors degrade the image quality through distortion and loss of resolution. Because the amount of refraction is proportional to the velocity mismatch, the greater the mismatch, the greater the refraction.