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Methods for measuring the targeting registration error (TRE) in the water tank. (A) TRE in the lateral X–Y plane was measured by the distance between the planned target (black) and the actual measured target, which was the beam centroid (white). (B) TRE in the Z direction was measured by the difference between the time-of-arrival of signals acquired at the planned target (blue) and reference signal acquired at the actual target (red).
