As the depth of interest changes from one structure to another, the depth to which the ultrasound beam is focused needs to be altered. This will improve lateral resolution, making it easier to delineate small structures. This is particularly important when imaging structures such as nerves or foreign bodies that are small, and that may be similar in brightness level to surrounding tissues.

The homogeneous speckle pattern characteristic of many solid organs is, in part, a product of the effective beam width. As beam width is reduced, contrast resolution will improve and structures, such as nerves, will be better visualised. Focus may need to be adjusted throughout the examination.

On some hand held systems, focus is optimised for the entire depth of view and is altered automatically, as the depth setting is changed. On these systems focus cannot be altered independently.

In this image, the depth of focal zone is indicated by the on-screen marker. Click the image to see a larger version.