C. The phase is bounded to certain values

Yes, this is the correct answer! There is nothing wrong about the signal phase generated by the tissues. However, when we sampled the signal, the phase values are constrained to the range -\pi \leq phase < \pi. When the true phase is outside of this range, a \pi will be added or subtracted to the true phase such that the apparent phase we observed from the data will still be inside the range.

../../_images/phase_wrap.png

Figure 1: An illustration of phase wrapping. The actual phase developed outside the range -\pi \leq phase < \pi over time but our measurement will only show the apparent phase.

Note

Think of an analogue clock with one hand only representing the hour. Every 12 hours the hand starts a new cycle again. If you look at the clock 13 hours after the first watch, it apparently advanced 1 hour only but the actual time difference between the two observations is 13 hours.

Back to Exercise 1.