Zoom Function: Magnifying an area of the image.
©Walter Rasmussen, R.D.C.S.
Most people are familiar with using a zoom lens for photography. Twisting the lens progressively isolates and magnifies a portion of the image while cropping-out everything else in the viewfinder. On an ultrasound machine the Zoom function is obtained in an entirely different manner.
There are two types of Zoom: Read Zoom and Write Zoom. Read Zoom isolates and magnifies a selected area of pixels from both a live or previously recorded image and although often of good quality, is not capable of rendering any more structural detail because it simply enlarges the pixels.
Unlike Write Zoom, Read Zoom cannot produce faster frame rates than the original background image. This becomes a problem when Read Zoom is used in combination with Color Doppler because the Color Doppler frame rate is based upon the entire sector image. Read Zoom can be panned to any part of the image while zoomed, whereas not all ultrasound machines allow Write Zoom to be re-positioned while zoomed. Read Zoom is usually initiated by twisting the Zoom or Magnify button.
Write Zoom can only be acquired live because it actually re-scans a selected smaller area of anatomy and has noticeably superior resolution to Read Zoom because the pixels are not just being magnified and the frame rate is maximized.
The Write Zoom function requires more steps to activate. The first press of the Zoom button displays an outline of the area of interest (identical to how the Color Doppler area of interest is selected), and after using the kidney button to adjust the size and position with a trackball, a second press of the Zoom button initiates its function. Faster frame rates are possible with Write Zoom because the width of the image is narrower than a full sector image.
Because Read Zoom is easier to use, many sonographers find it more attractive to choose over Write Zoom, even though it is inferior in quality.
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