Imaging algorithm visualizes early heart abnormalities

Studying abnormalities that occur during early cardiac development is very important for understanding congenital heart defects. Optical coherence tomography (OCT) is a noninvasive imaging modality based on low-coherence interferometry that is capable of micrometer-scale resolution and 1–2mm of depth penetration in embryonic tissue. OCT can visualize and quantify real-time heart dynamics in 3D,making it a useful tool for assessing cardiac development. Real-time volume imaging performed with current systems, however, does not provide the necessary temporal resolution to analyze rapid cardiac dynamics in 3D. In the past, researchers used a physiological signal such as an electrocardiogram to trigger data acquisition, typically during multiple phases of the cardiac cycle.1–4 Alternatively, some recorded the physiological signal while imaging as fast as possible over several heartbeats at different spatial locations, and then used timing from the signal to create 3D volumes over time.1, 5–8 However, recording an additional signal is invasive and time-consuming. A possible remedy is retrospective cardiac gating,9–12 where one obtains data over multiple heartbeats without recording an external signal and rearranges the data after the fact to reconstruct 3D volumes over time. Our technique, called image-based retrospective gating, employs image similarity to rearrange out-of-order image data. The result is extremely high temporal resolution. Our image-based retrospective gating algorithm is influenced by the work of Liebling et al.10 However, due to the higher signal-to-noise ratio of our imaging setup, we work in the spatial rather than the wavelet transform domain. In addition, we created a novel quantitative validation technique to obtain a real-time reference scan of the same heart volume used for outof-order data acquisition. We then employ it to evaluate the accuracy of our algorithm. Figure 1. A 4D visualization of retrospective gated OCT data from a stage 14 embryonic quail heart. Movies of the data (90 volumes in a single cardiac cycle, or ∼270 volumes/s) are shown in (a) sagittal or en face (see video13), (b) transverse (see video14), and (c) coronal (see video15) views. In each case a cutaway view of the beating heart is shown with a moving orthogonal 2D slicer in the plane of interest.