How to obtain the best ultrasound images?

The frequency of ultrasound refers to the number of wave repetitions per second. The frequency for ultrasound diagnosis ranges from 2 to 15 MHz. Higher frequency equals better image resolution.

The higher the frequency, the clearer the image closer to the transducer. However, higher frequency has weaker penetration, so the image becomes blurry further away from the transducer. Conversely, lower frequency has strong penetration but low image resolution, resulting in blurry images.

Adjust the frequency to balance penetration depth and resolution. Choose the highest frequency that still allows for a clear image at the desired depth.

Focus

The ultrasound transducer emits many ultrasound wave beams, and interference exists between different waves. The focal zone is where the ultrasound wave beams converge (narrow), which is the narrower section in the pink beam on the left; the image at this depth is clearer.

The two most frequently used controls are focus and depth; they complement each other. The ultrasound beam (main beam) is narrowest in the focal zone and then widens or diverges at greater depths. The focal zone should be positioned at or just below the area or organ to be examined. It is usually displayed as a triangle or arrow-like icon and can be moved between the near field and far field using a trackball.

Below are three ultrasound images with different focus settings: The focal zone is placed in the near field, making the anechoic circle in the near field clearer, but the echoic circle in the far field is unclear (A); the focal zone is placed at the hyperechoic circle in the mid-zone, achieving optimal imaging (B); the focal zone is placed in the far field, causing reduced resolution of structures in the near and mid-zones (C). In all three images, the depth adjustment is appropriate.

The number of focal zones can also be adjusted on the ultrasound machine, but it is not recommended to use too many focal zones, as this will affect the frame rate, making the image jerky and slow, ultimately affecting the examination. In practice, it is recommended to not use more than two focal zones, generally using only one.

Depth

Adjust the overall depth of the image so that the area to be examined occupies most of the screen, facilitating a more comprehensive and thorough regional diagnosis.

The depth is determined by the organ to be examined. For example, kidney assessment may only require 3 cm depth, while whole liver assessment may require 6 to 7 cm depth.

Gain

Gain is an image processing method that adjusts the overall brightness of the image; it does not affect the propagation and generation of the ultrasound beam.

Overall gain adjusts the overall brightness of the image to an appropriate level. Time gain compensation (TGC) adjusts the brightness of different layers to be consistent vertically.

These are three ultrasound images with different gain settings. A's overall gain setting is normal (usually around 60%). B's overall gain is set too low, resulting in an overall hypoechoic image (gain set at 50%). C's overall gain is set too high, resulting in an overall hyperechoic image (gain set at 78%). These gain values are relevant to ultrasound use; therefore, ensure that the machine's gain settings are appropriate.

When using time gain compensation or depth gain compensation, use the mid-section as a starting point; then adjust the slider from a vertical straight line position to a slight angle, setting the top slider further to the left (reducing the overall gain of the near field) and the bottom slider further to the right (increasing the overall gain of the far field). This should compensate for the attenuation of the ultrasound beam in deeper tissues.

Below are images of time gain compensation or depth gain compensation. A's near-field time gain compensation slider is moved too far to the right, causing the near field to be too "white." B's near-field time gain compensation slider is retracted too much, so the overall echo is uneven, with areas closer to the transducer appearing too dark. C's sliders for distance markers of 1, 3, and 4 cm are set to the far left, resulting in a complete absence of image.