Which mode of ultrasound displays the amplitude of reflected signals as a function of time?

The mode of ultrasound that displays the amplitude of reflected signals as a function of time is A-mode, or amplitude mode. In this mode, the ultrasound machine sends out a pulse and receives the echoes that return from various tissues. The information obtained is then presented as a series of spikes on a graph, where the vertical axis represents the amplitude (strength) of the reflected sound waves, and the horizontal axis signifies time or the depth of the tissue being imaged. This graphical representation allows clinicians to assess the characteristics of the tissue based on the strength of the returning echoes.

In contrast, B-mode, or brightness mode, provides a two-dimensional image with varying brightness levels representing different tissue echogenicities but does not specifically show amplitude over time. M-mode, or motion mode, captures the motion of structures over time but does not focus solely on the amplitude of reflections. C-mode, or contact mode, is less commonly referred to in standard ultrasound contexts and does not fit the question's requirements. Therefore, A-mode is the appropriate choice for depicting amplitude as it relates to the timing and reflection of ultrasound signals.