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Figure 4 (A) Spectral computation of T-wave alternans (TWA). In the original aligned ECG beats, alternans at each time point within the T-wave (vertical arrows) results in down-up-down oscillations. Fast Fourier transformation (FFT) yields a spectrum, in which the alternating component of these oscillations causes the 0.5 cycles/beat peak (T). In the final spectrum (summated for all time points), T is related to spectral noise to compute V_alt and k-score (see panel B).