In vivo functional imaging of blood flow and wall strain rate in outflow tract of embryonic chick heart using ultrafast spectral domain optical coherence tomography

Abstract

During cardiac development, the cardiac wall and flowing blood are two important cardiac tissues that constantly interact with each other. This dynamic interaction defines appropriate biomechanical environment to which the embryonic heart is exposed. Quantitative assessment of the dynamic parameters of wall tissues and blood flow is required to further our understanding of cardiac development. We report the use of an ultrafast 1310-nm dual-camera spectral domain optical coherence tomography (SDOCT) system to characterize/image, in parallel, the dynamic radial strain rate of the myocardial wall and the Doppler velocity of the underlying flowing blood within an in vivo beating chick embryo. The OCT system operates at 184-kHz line scan rate, providing the flexibility of imaging the fast blood flow and the slow tissue deformation within one scan. The ability to simultaneously characterize tissue motion and blood flow provides a useful approach to better understand cardiac dynamics during early developmental stages.

Fig. 1

Schematic of ultrafast dual-camera SDOCT system. OC: optical coupler; CIR: circulator; PC: polarization control; CL: collimating lens; FL: focusing lens; M: mirror; GV: galvanometer; OL: objective lens; DG: diffraction grating. Trigger 1: to synchronize the OCT scanning; Trigger 2a and 2b: to synchronize the data acquisition of the dual camera.

Fig. 2

Extraction of myocardial wall boundary positions from the OCT images of embryonic chick heart OFT. (a) Typical OFT longitudinal section, and (b) OFT cross-section; (c) M-mode structural image along the vertical dashed line in (b) in which the boundary of myocardial wall is indicated by the solid curves; (d) relative depth positions of the myocardial wall boundary; (e) thickness variation of the myocardial wall over $\sim 1.7\sec$ period. M: myocardium; CJ: cardiac jelly; B: blood; V: ventricle; OC: outflow cushion (endocardial cushions). The $\text{vertical scale bar}=200\mathrm{\mu m}$; the $\text{horizontal bar}=0.1\mathrm{s}$.

Fig. 3

Schematic diagram describing the relationships between the OCT probe beam, the moving myocardial wall and the blood flow within lumen: (a) for the case when evaluating the myocardial wall motion; and (b) for the case when evaluating the blood flow. $L$ is the thickness of the myocardial wall; $V_{\mathrm{ri}}$ and $V_{\mathrm{ro}}$ are, respectively, the radial velocities of the inner and outer boundaries of the myocardial wall; $V_{\mathrm{li}}$ and $V_{\mathrm{lo}}$ are, respectively, the longitudinal velocities of the inner and outer boundaries of the myocardial wall; $V_o$ is the Doppler velocity of the outer boundary as measured by the OCT system; ${\alpha }_M$ is the angle between the radial velocity direction and the probe beam; $V_B$ is the blood flow velocity; $V_{\mathrm{BD}}$ is the Doppler velocity of blood flow as measured by the OCT system; ${\alpha }_B$ is the Doppler angle of blood flow.

Fig. 4

Depth-dependent Doppler velocity profiles (bottom row) along the central lines of the corresponding structural cross-sections shown in top row. Top and bottom are paired figures with the top showing the cross-sectional OCT images, and the bottom giving the corresponding velocity profiles across the central A-lines marked as horizontal dashed lines in the top row. Vertical dashed lines are defined by the boundaries of the cardiac wall, facilitating the localization of the velocity profiles that correspond to the wall ($z_o$, $z_i$ and corresponding $V_o$, $V_i$); (b) shows the case that the wall motion is within the detectable range of the system; (d) illustrates the wall motion is outside the detectable range, thus phase un-wrapping algorithm is used to correct the OCT so that the correct velocity profile can be obtained, the result of which is given in (f).

Fig. 5

Evaluation of both radial SR of myocardial wall and Doppler velocity of blood flow over cardiac cycles by the use of only one M-B scan afforded by the ultrafast imaging system used in this study: (a) M-mode structural image superimposed with radial SR of the myocardial wall and Doppler velocity of blood flow; (b) Doppler velocity along the dashed yellow line in Fig. 2(c); (c) Radial SR. The dashed vertical line indicates the three phases of the OFT activities, i.e., 1) the opening of outflow cushions (from $t_0$ to $t_1$); 2) the start of atrioventricular (AV) ejection or closure of the AV cushions (from $t_1$ to $t_2$); and 3) the closure of outflow cushions (from $t_2$ to $t_3$). TC: thickening of myocardial wall; TN: thinning of myocardial wall; F: forward flow; B: backward flow. The vertical bar in (a) represents 200 μm and the horizontal bar indicates 0.2 s.

Fig. 6

Averaged OFT longitudinal sections for Doppler angle correction vertical scale bar = 200 μm.