Optical signal interrogation depth in Tyrode and blood-perfused myocardial tissue

Abstract

Photon attenuation length (d) in biological tissues determines interrogation depth, spatial resolution, and amplitude of fluorescence signal in various types of optical imaging, including imaging of cardiac excitation using voltage-sensitive dyes. We assessed d in human and pig myocardium at excitation/emission wavelengths of commonly used and recently developed near-infrared voltage-sensitive dyes (NIR). We also compare d in Tyrode vs blood-perfused tissues and simulate respective fluorescent signals in the context of potential clinical applications. Experiments were conducted in isolated slabs of ventricular myocardium. Light decay inside the tissue was measured at 520, 650, and 715nm. d in blood-perfused porcine and human myocardium were similar (at 520nm 0.46±0.01 mm and 0.48±0.02 mm; at 650nm 1.83 ± 0.02 mm and 1.98 ± 0.07 mm, at 715nm 2.31 ± 0.05 mm and 2.74 ± 0.13 mm in porcine and human respectively), which makes porcine myocardium a good model for development of clinical imaging applications. Blood- vs Tyrode perfusion significanctly reduces d, particularly at 520nm. However, our simulations show that, the resulting reduction of optical action potential is\16% (for NIR dyes), which would not be a major impediment for in vivo imaging of cardiac excitation.