The laminar myocardial sheet architecture and its dynamic change play a key role in myocardial wall thickening.
Histology, confocal optical microscopy (COM), and diffusion tensor MRI (DTI) have been used to unveil the structures
and functions of the myocardial sheets. However, histology and COM require fixation, sectioning, and staining
processes, which dehydrate and deform the sheet architecture. Although DTI can delineate sheet architecture
nondestructively in viable hearts, it cannot provide cellular-level resolution. Here we show that photoacoustic
microscopy (PAM), with high resolution (~1 μm) and label-free detection, is appropriate for imaging 3D myocardial
architecture. Perfused half-split mouse hearts were also imaged by PAM in vitro without fixation, dehydration, nor
staining. The laminar myocardial sheet architecture was clearly visualized within a 0.15 mm depth range. Two
populations of oppositely signed sheet angles were observed. Therefore, PAM promises to access dynamic changes of
myocardial architectures in ex vivo perfused-viable hearts.