The far field (also called Fraunhofer zone) is the distal part of an ultrasound beam characterized by a diverging shape and continuous loss of ultrasound intensity with distance from the transducer. The angle of divergence increases with lower transducer frequency and with smaller transducer diameter.

See also Sonographic Features.

The acoustic lens is placed at the time the transducer is manufactured and cannot be changed. The acoustic lens is generally focused in the mid field rather than the near or far fields. The exact focal length varies with transducer frequency, but is generally in the range of 4-6 cm for a 5 MHz curved linear probe and 7-9 cm for a 3.5 MHz curved transducer.
Placing the elevation plane (z-plane) focal zone of the acoustic lens in the very near or far field would improve the beam width at precisely those depths. However, this would degrade the beam width to a much greater and unacceptable degree at all other depths.
There are some chemicals in ultrasound couplants that can degrade the acoustic lens, destroy bonding, or change the acoustic properties of the lens. Problematic chemicals include mineral oil, silicone oil, alcohol, surfactants, and fragrances. Fragrance can affect the transducer's acoustic lens or face material by absorption over time into elastomer and plastic materials, thus changing the material's weight, size, density, and acoustic impedance. Surfactants can degrade the bond between the lens and the piezoelectric elements and contribute to the accelerated degeneration of the lens.

See also Retrolenticular Afterglow.

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 divergence is an ultrasound beam characteristic of the far field. The beam divergence angle q, depends on the transducer frequency and diameter according to the following approximation:
sin q 1.22 ld
where l is the wavelength of the ultrasound in the medium of propagation and d is the diameter of the transducer element.

Usually, multiple probes are used because most transducers are only able to emit one frequency because the piezoelectric ceramic or crystals within it have a certain inherent frequency.
Multi-frequency probes have multiple crystals with different frequencies and the desired specific frequency can be selected. Advanced probes can emit sound waves at different frequencies for the near and far fields. The disadvantage is that multi-frequency (multifrequency) probes have slower frame rates and therefore they are only useful for imaging of static structures.

See also Dual Frequency Phased Array Transducer and Tri-Frequency Probe.