Depth
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The depth control determines the maximum depth of tissue displayed on the screen but not how far the ultrasound beam travels into the body (a common misconception)
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The depth scale is visible down one side of the image screen and is displayed in cm increments. It gives you an idea of the depth of a structure which is useful if you lose sight of it when trying to assess in different planes or when performing needle-guided procedures (also useful because it gives you an idea of the needle length you will need).
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Start off with a greater depth of tissue on the screen to help you with anatomical landmark location. Adjust the depth once you have the landmarks and know where you are
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Always adjust the depth to ensure that the region of interest (ROI) is in the centre of the screen – this is the area our eyes are automatically drawn to
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Remember increasing the depth makes the ROI smaller so you will have lots of ‘wasted space’ at the bottom of your image screen. Having too small an image will make thorough detailed assessment of a structure difficult and having too large an image on the screen will compromise your ability to navigate from one structure to the next as you scan
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Increasing the depth of tissue on the screen will compromise (reduce) the image resolution. This is because at greater depths, a divergent beam shape will have far field scan lines which are further apart from one another, thus the ability to differentiate between small, closely adjacent structures is compromised when compared to the nearer field
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Increasing the depth also reduces the frame rate of your real-time scan simply because it takes longer for a pulse to return to the transducer from deeper reflectors within the body. Consequently, the number of frames per second that can be processed and built into an image on the screen will be reduced. (When the frame rate is reduced below 20 frames per second, a lag or slight delay may be seen on the dynamic assessment)
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If you are looking at a superficial structure, you need to decrease the depth
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Decreasing the depth to include only the area of interest will increase your frame rate and the resolution
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Depth should always be optimised before zoom is used
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Magnification/zoom control is often combined with the depth control on ultrasound equipment
Magnification/Zoom
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Magnification is usually called zoom on ultrasound equipment. There are two types of zoom you can select – read and write
The use of the zoom function is important when the region of interest is small (eg GI wall layers, cystic duct) but you need to enlarge it on screen to assess it in more detail or measure it accurately
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Read zoom:
The read zoom magnifies the area of interest on the frozen image i.e., it is a ‘post processing’ function so it doesn’t improve the quality of the image. If the image is over magnified, the pixels become visible which will compromise the image quality. This function can be helpful if you have a mobile structure to assess or need to measure a structure on a stored/archived image
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Write zoom:
Write zoom is basically live zoom. Selecting the zoom control during the live scan generates a ‘box’, the size and position of which can be adjusted by the operator to assess the region of interest more accurately.
The image resolution in write zoom is better than the image obtained after using read zoom on a frozen image. This is because the ultrasound system recognises only the echoes returning from the zoom box so the line density and number of pixels producing the magnified image are increased.
Always remember the order, depth before zoom!