A, nodal line detection using ΔAPD. a, APD alternans in an LQT2 heart. b, phase of alternans at each pixel was represented as +1 when the following APD is longer, −1 when the following APD is shorter, and zero when ΔAPD is within small variation (<5%) (black line). c, after filtering alternans phase maps with a 15 × 15 Gaussian filter, contour lines were drawn along the zero phase (black) to divide two regions alternating out of phase (red and green). B, nodal line detection using cross-correlation. a, signal from reference location (red) at top trace, overlapped with a trace from a nodal line region (black) and out-of-phase region (green). b, cross-correlation will give the maximum correlation when a time delay between two signals is introduced such that their overlap is optimized. When discordant alternans occurs, two locations that are out of phase will have maximum correlation at the delay of one cycle length, signifying APD oscillation is out of phase (see green lines in the trace and cross-correlation plots). When a region with APD alternans is cross-correlated with a nodal line region, maximum correlation remains constant at delay the one cycle length (black). c, contour lines are drawn separating out of phase regions. Both methods showed compatible patterns of nodal lines.