Fluctuation analysis for the T-tubular distance in a myocyte in an in vivo mouse heart at rest. (A) Confocal image showing a CellMask-treated myocyte in the left ventricle of the heart at rest in a mouse (with greater than ∼5% isoflurane). The T-tubular distance in the yellow-outlined rectangle (indicated by the arrowhead) was analyzed. Scale bar, 10 µm. See Video 5. (B) Time course of changes in the T-tubular distance. Imaging performed at 197 fps. (C) Histogram showing the variance of the T-tubular distance. Average T-tubular distance was 2.11 ± 0.11 µm, indicating that the SD (i.e., index of single T-tubular distance measurement resolution) was 110 nm.