Real-Time Optical Mapping of Contracting Cardiac Tissues With GPU-Accelerated Numerical Motion Tracking

Abstract

Optical mapping of action potentials or calcium transients in contracting cardiac tissues are challenging because of the severe sensitivity of the measurements to motion. The measurements rely on the accurate numerical tracking and analysis of fluorescence changes emitted by the tissue as it moves, and inaccurate or no tracking can produce motion artifacts and lead to imprecise measurements that can prohibit the analysis of the data. Recently, it was demonstrated that numerical motion-tracking and -stabilization can effectively inhibit motion artifacts, allowing highly detailed simultaneous measurements of electrophysiological phenomena and tissue mechanics. However, the field of electromechanical optical mapping is still young and under development. To date, the technique is only used by a few laboratories, the processing of the video data is time-consuming and performed offline post-acquisition as it is associated with a considerable demand for computing power. In addition, a systematic review of numerical motion tracking algorithms applicable to optical mapping data is lacking. To address these issues, we evaluated 5 open-source numerical motion-tracking algorithms implemented on a graphics processing unit (GPU) and compared their performance when tracking and compensating motion and measuring optical traces in voltage- or calcium-sensitive optical mapping videos of contracting cardiac tissues. Using GPU-accelerated numerical motion tracking, the processing times necessary to analyze optical mapping videos become substantially reduced. We demonstrate that it is possible to track and stabilize motion and create motion-compensated optical maps in real-time with low-resolution (128 x 128 pixels) and high resolution (800 x 800 pixels) optical mapping videos acquired at 500 and 40 fps, respectively. We evaluated the tracking accuracies and motion-stabilization capabilities of the GPU-based algorithms on synthetic optical mapping videos, determined their sensitivity to fluorescence signals and noise, and demonstrate the efficacy of the Farnebäck algorithm with recordings of contracting human cardiac cell cultures and beating hearts from 3 different species (mouse, rabbit, pig) imaged with 4 different high-speed cameras. GPU-accelerated processing provides a substantial increase in processing speed, which could open the path for more widespread use of numerical motion tracking and stabilization algorithms during routine optical mapping studies.

Optical mapping of contracting hearts

“…3 Ab ) represent the direction of the contractile motion. Further details about motion tracking algorithms and their validation can be found in the work of Christoph & Luther ( 2018 ) as well as in Lebert et al ( 2022 ). Figure 3 B shows the application of 2D motion tracking where the motion amplitude (pixel displacement) of the heart during fixed frequency pacing and ventricular fibrillation is displayed as motion amplitude maps.…”

“…The motion tracking in these studies was performed on the optical mapping videos of whole heart (atria and ventricles) with a focus on ventricles during pacing and fibrillation. Recently, GPU‐accelerated motion tracking has been successfully applied to hiPSC‐CM and isolated hearts of mouse, rabbit and pig (Lebert et al., 2022 ). However, to our knowledge, no studies have performed motion correction specific to highly trabeculated endocardium or complex atrial structure.…”

“…2D motion tracking of the cardiac tissue requires motion tracking (Matlab, The MathWorks, Natick, MA, USA) and visualization (C++) algorithms that can be performed using a single CPU with a computation time of approximately 1 min per video image. However, the computation time can be reduced significantly by using parallel computing and it was recently demonstrated that optical mapping with numerical motion tracking can be performed in real‐time (Lebert et al., 2022 ).…”

Guidelines for assessment of cardiac electrophysiology and arrhythmias in small animals

“…As an alternative, motion artifacts can be minimized by gently constraining the heart for short intervals during a recording ( 334 , 338 ). Recent developments in binocular imaging, tracking algorithms, and computational hardware have enabled optical mapping studies of unrestrained contracting hearts ( 319 , 341 , 342 ).…”

Editorial: Cardiac optogenetics: Using light to observe and excite the heart

“…The review examines many of the novel techniques that optical physics have provided to extend the use of optical probes and actuators while also posing the next set of challenges to be addressed to extend further the applicability of these techniques. In this content, Jan Lebert and Jan Christoph present new algorithms for the analysis of voltage imaging with motion tracking stabilization to avoid the alterations of cardiac electrophysiology by contraction inhibitors with significant side effects ( Lebert et al, 2022 ). Furthermore, Wegener and colleagues took advantage of transgenic biosensor mouse models to analyze the cytosolic and mitochondrial glutathione redox potential in single cardiomyocytes and the intact heart.…”