What happens in shadowing during ultrasound imaging?

In ultrasound imaging, shadowing occurs when the ultrasound beam encounters a solid structure that has a higher acoustic impedance compared to the surrounding tissues. This difference in acoustic impedance results in a decrease in the ultrasound beam's intensity as it penetrates the solid structure. The solid structure reflects most of the ultrasound waves rather than allowing them to pass through, leading to a region of decreased signal or "shadow" behind the structure.

When shadowing happens, the area beneath or behind the highly reflective solid structure appears darker or shows a loss of detail on the ultrasound image. This phenomenon is often useful as it can help identify the locations of calcifications, stones, or other dense materials. In contrast, the ultrasound beam's strength does not increase as it passes through solid structures; rather, it becomes attenuated. Hence, the correct understanding of shadowing is rooted in its association with differences in acoustic impedance and the resulting attenuation of the ultrasound beam.