Where does the heart beat now?
A new type of phase-contrast imaging that can take three-dimensional images of dynamic processes is revealing more about how hearts contract and may eventually help scientists discover the roots of heart disease.
In recent years, researchers have developed a flurry of phase-contrast imaging techniques that map changes in optical path length to reveal surface characteristics and mechanical activity. Until now, however, none has performed three-dimensional imaging.
Audrey K. Ellerbee and her colleagues at Duke University in Durham, N.C., have developed three-dimensional spectral domain phase microscopy, an outgrowth of optical coherence tomography (OCT). The technique exploits OCT’s high-resolution depth-sectioning capability and provides increased sensitivity to nanometer-scale changes. By employing lateral scanning optics or full-field swept-source topologies, the technique can collect phase information unique to each lateral position and longitudinal depth. The results are detailed surface contours for static structures and time-lapse imaging for dynamic processes.
Phase profiles (above) of three locations inside a chick embryo (left) show that the motion of the heart wall (M) and inside the heart (B) are similar, while the motion of the yolk membrane (T) is distinct. Reprinted with permission of SPIE.
The team examined contractions in heart cells of two-day-old chick embryos. Because diseased hearts often exhibit characteristics similar to those of immature ones, understanding cardiac development could be useful in finding cures for common heart ailments.
The noninvasive and noncontact technique allows it to sample repeatedly from many myocytes within the same preparation. In addition, it can be used on many cell shapes, such as the spherical developing heart cells or the irregular flattened cells in cultures.
The system includes a fiber-based common path interferometer fitted to the documentation port of a Zeiss inverted microscope with a second documentation port with a PixeLink CCD camera for video microscopy. The researchers used either a superluminescent LED coupled with a ThorLabs HeNe laser at 635 nm with a 90/10 splitter for visualizing the point of acquisition in real time, or a tunable mode-locked FemtoLasers Ti:sapphire laser for high-speed acquisition. A wideband 50/50 coupler delivers light to the sample arm, which is fitted with a pair of scanning galvanometer mirrors to enable spectral domain phase microscopy data acquisition in arbitrary scanning patterns on the lateral plane. A coverslip at the object plane is a reference reflector for cellular measurements.
At a 1.25-kHz line rate, the system had enough stability to capture phase changes induced by an isolated cell’s moving surface. However, because there was no perceptible resting period between heartbeats, there was no way to determine if the start of the beat occured prior to a crest or a valley in the phase waveform. Thus, the researchers presented data relative to the phase in the first frame of acquisition, not necessarily the beginning of the contraction. For the same reason, they could not say if the sampled cell points were moving up or down, but for a multicell preparation it was clear that two cells were moving together, while a third was moving in the opposite direction.
An analogous situation occurred in the investigation of the whole embryo. Two laterally displaced points within the embryo moved upward and returned to their original positions but showed an inter-peak delay of 30 ms relative to each other. The contractile event seems to begin on the inside and radiate outward toward the heart wall.
The team hopes to image younger embryos to determine the earliest time that heart cells develop the capacity to contract. Improvements in time resolution will enable researchers to identify the start location of the contractile wave and to conduct studies on the coordination of contractions in different parts of individual cells and in multiple cells.
The research was presented at SPIE’s Biomedical Optics Symposium in San Jose in January.
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