The hearts of six adult mice (15 months) of either sex were perfused in Langendorff mode for the purposes of optical mapping of electrical impulse propagation (Olejnickova et al., 2021 (link)) followed by fixation for 24 h in 4% paraformaldehyde in PBS at 4 C. The hearts were then processed for micro‐computed tomographic (CT) examination essentially as described recently (Gregorovicova et al., 2022 (link)), keeping the specimens in iodine solution for 1 month. The specimens were scanned in a plastic tube immersed in phosphate‐buffered saline with the following scanning parameters: pixel size = 7.5 μm, source voltage = 90 kV, source current = 111 μA, filter: Al 0.5 mm + Cu 0.038 mm, rotation step = 0.2°, frame averaging = 2, specimen rotation of 180°, camera binning 2 × 2, scanning time = approx. 3 h per specimen. Flat‐field correction was updated prior to each scanning. Scans were acquired using SkyScan 1272 (Bruker micro‐CT, Belgium). Projection images were reconstructed with NRecon (Bruker micro‐CT, Belgium) with the adequate setting of correction parameters (misalignment, smoothing, ring‐artifact correction and beam hardening). 3D visualization was created by CT Vox (Bruker micro‐CT, Belgium). CTAn (Bruker micro‐CT, Belgium) was used to perform image processing.