In which ultrasound mode is the amplitude of returning echoes visualized as a function of depth?

The correct answer is A-mode. This mode is the primary form of echograms in which the amplitude of returning echoes is displayed as a function of depth, typically along a single line. In A-mode ultrasound, the x-axis represents the depth of the reflectors, while the y-axis represents the amplitude of the echoes returned from the tissues, which is shown by spike-like structures.

This is particularly useful in certain diagnostic applications where precise measurements of depth and amplitude of echoes are required, such as in ophthalmology for measuring the depths of the eye structures. Each peak in the A-mode graph correlates with a sound reflection off a different tissue interface at varying depths.

In contrast, B-mode provides two-dimensional images and visualizes the amplitude of echoes as brightness on a display, offering a more comprehensive view of the structure being examined. Doppler mode focuses on measuring and visualizing blood flow and velocities rather than depth and amplitude alone. Lastly, C-mode is less commonly referenced and does not primarily deal with the representation of amplitude as a function of depth as A-mode does.