Dynamic range in ultrasound

Dynamic Range of Ultrasound Imaging Systems

By definition, the dynamic range of an ultrasound imaging system is similar to that of other systems: it is simply the ratio of the maximum detectable ultrasound signal to the minimum detectable ultrasound signal. For a fully digital ultrasound imaging system, each receiving channel has an analog-to-digital converter (ADC) that converts the analog signal into a digital signal. The bit width of the ADC is the most important factor affecting the dynamic range of the ultrasound system. Obviously, the wider the bit width of the ADC (or the larger the number that can be represented), the greater the ratio of the maximum signal to the minimum signal that can be converted, and the greater the dynamic range. In addition, the number of receiving channels and the background noise of the ultrasound system itself also have a significant impact on the dynamic range of the ultrasound system.

Assuming that the ADC bit width is N, the number of receiving channels is K, and the background noise can be ignored in certain circumstances, then the system dynamic range in the most ideal case should be 6.02*N+1.76+6*sqrt(K); where 6.02*N+1.76 is the dynamic range contributed by the ADC in the ideal case, and 6*sqrt(K) is the dynamic range provided by the channels. Currently, the most commonly used ADC bit width in ultrasound is 10-14 bits, and the number of channels is 64-256. According to the most ideal situation, the dynamic range is approximately as follows when various combinations of channel numbers and ADC bit widths are used:

Conclusion:

• For ultrasound waves returning from the superficial part of the human body, such as the skin, the signal strength is high, and the ultrasound system does not need to amplify the signal. At this time, the dominant noise in the system is the ADC quantization noise, which has a small amplitude, so the system dynamic range can be close to the ideal value.

• For ultrasound waves returning from deeper parts of the human body, the signal strength is very weak, and the ultrasound system must use something similar to a "loudspeaker" to amplify the signal. At the same time, the system's background noise is also greatly amplified, which has a great impact on the system's dynamic range. Taking our system as an example, the loss compared to the most ideal situation is approximately 20-25dB.

Dynamic Range of Ultrasound Images

The vast majority of ultrasound imaging systems use grayscale images to display two-dimensional B-mode images. Grayscale images are similar to black and white photos, using 256 grayscale levels from black to white (0-255) to display the image. Regardless of the actual dynamic range of the ultrasound image, the number of grayscale levels that can be displayed is 256. The grayscale level 255 (white) corresponds to the maximum signal in the image, and the grayscale level 0 (black) corresponds to the minimum signal in the image.

Due to differences in the imaging site and patient's physical condition, the actual dynamic range of the obtained ultrasound images often varies greatly. In order to fully utilize the grayscale levels, it is best to let the actual maximum signal display a grayscale of 255, and the actual minimum signal display a grayscale of 0. Therefore, most ultrasound systems allow ultrasound physicians to select the dynamic range based on the actual imaging results.

For ultrasound physicians, being adept at adjusting the dynamic range is quite important:

The following three images show the results obtained with dynamic ranges set to 30dB, 50dB, and 90dB respectively, while keeping other conditions unchanged.

In the first image, the dynamic range is too small relative to the actual signal range: some large signals become bright white, while areas in the muscle layer that should have signals have become completely black.

In the third image, the dynamic range is too large relative to the actual signal range; the entire image appears somewhat hazy and lacks contrast.

Relatively speaking, the 50dB setting in the second image is more appropriate, with no obvious signal loss and good contrast.

Finally, here are two tips for adjusting the dynamic range:

• 1. If the dynamic range is chosen too small, some of the actual signals will inevitably not be displayed: some strong signals will be displayed as bright white, and some weak signals will be suppressed to black. If this part of the signal contains diagnostic information, it will be quite problematic.

  2. If the dynamic range is chosen too large, only part of the 256 grayscale levels will be used, and although no signal will be lost, it is not the optimal display effect, and is equally disadvantageous for diagnosis.

  1. If the patient's physical condition is poor and there is a lot of noise, the dynamic range can be appropriately reduced, or even moderately reduced, especially in cardiovascular examinations. Although some signals may be lost, noise can be effectively suppressed. For example, in the three images above, I believe some doctors would find the first image good, because some noise in the vascular lumen has been suppressed.

  2. The dynamic range values displayed by different manufacturers vary greatly. The best approach is to ignore the displayed value and adjust it larger or smaller as needed.

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