Understanding the focus
The word focus has a variety of meanings, some examples are ‘the centre of interest or activity’, ‘clearly defined’ or ‘sharp’ and these meanings also describe perfectly the role of the focus control in ultrasound
The overall aim of the focus control is to produce a high-resolution image of the region of interest (and this should be in the centre of the screen)
The ultrasound beam has a finite width, and the function of the focus is to effectively narrow the beam width. This takes place using electrical compression of the scan lines (line density), bringing them closer together thereby improving resolution and so the ability to perceive detail in the direction across the screen (known as lateral resolution)
The part of the beam which is narrowed is known as the focal zone, the area beyond the focal zone i.e., where the beam becomes more divergent (widens) is known as the far zone and the ability to perceive detail is reduced in the far zone. It is important to adjust your focal zone to be positioned at (or just behind) the region of interest. Your equipment manual/applications specialist will advise you where you should position the focus on your machine but the best way to appreciate it is to use it in practice
It is possible to increase the focal zone over a greater depth by using multiple focal zones, and this can be useful if examining a large ‘static’ structure such as the liver or spleen
Introducing more focal zones to create a deeper focal zone has an impact on frame rate (FR) (the number of frames or images displayed per second). The more focal zones added, the slower the frame rate. This is because the scan memory needs to apply focal narrowing for each focal zone in turn, which takes longer to process and build each frame and the final image. If the frame rate drops to less than 20 (you can see the FR you are working with on the periphery of the image screen when scanning), there is a perceivable time ‘lag’ so technically, the scanning is no longer ‘real time’.
The focus is usually displayed as a small triangle or arrow on the depth scale at the side of the image screen display. The arrow/triangle relates to the depth in the tissue at which the ultrasound beam is at its narrowest.
Remember; set your focus, at or just below (on screen) the level of the region of interest.