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.

The dead or ring down zone is the distance from the front face of the transducer to the first echo that is identifiable. The signals from this region are unsuitable. The dead zone is the result of transducer ringing and reverberations from the interface between the transducer and the scanned object. Impedance matching between the transducer and the receiver is important to avoid electrical ringing.
With an increase of the frequency, the pulse length and the depth of the dead zone decrease, if all other parameters remain constant. The acoustic power also affects the depth of the dead zone.

The focal zone is the region within the transmitted sound beam in which the beam narrows to its minimum size. The lateral resolution is best within the focal zone of the beam.

The near field (also called Fresnel zone) is the proximal part of an ultrasound beam. The Fresnel zone is adjacent to the transducer surface and has a converging sound beam profile. A narrow beam shape is maintained in the near field owing to constructive and destructive interference patterns of sound wavelets emitted from the transducer crystal.
The length of the near field is equal to
r²/l = d²/4l
where r is the radius, l is the ultrasound wavelength in the medium of propagation and d the diameter of the piezoelectric crystal.

See also Beam Pattern, and Sonographic Features.

Ultrasound waves propagate poorly through a gaseous medium that result usually in inadequate imaging.
The effect of propagation through dense zones is that nearly all of the ultrasound waves are reflected. Structures below dense zones, for example bone, calcium, metal are also poorly imaged.

See also Signal Coupling.