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.
||Near zone - the region of a sound beam in which the beam diameter decreases as the distance from the transducer increases (Fresnel zone).|
||Focal zone - the region where the beam diameter is most concentrated giving the greatest degree of focus. |
||Far zone - the region where the beam diameter increases as the distance from the transducer increases (Fraunhofer zone).|
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.
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