Fine spatiotemporal activity in contracting myometrium revealed by motion-corrected calcium imaging

Key points: Calcium indicator fluorescence imaging is a key tool for investigating the physiological mechanisms that initiate and regulate myometrial contractions. However, inhomogeneous tissue motion severely limits the effective spatial resolution of measurable calcium activity. We present an image analysis method that locally tracks contractions to produce a complete description of tissue-wide motion. This renders calcium signals measurable at the micrometre scale. Additionally, the method simultaneously extracts kinematics of the motion that will inform biomechanical models of contracting myometrium. The method is demonstrated on calcium-imaging datasets of contracting myometrium. Using the method, we observed significant heterogeneity in calcium activity and identified a characteristic length scale of 40-50 μm below which tissue motion remains locally homogeneous. We provide freely available and modifiable code to process datasets affected by motion artefacts. The method has potential application to in vivo neuron imaging, as well as to calcium imaging of other smooth muscle tissue. Successful childbirth depends on the occurrence of precisely coordinated uterine contractions during labour. Calcium indicator fluorescence imaging is one of the main techniques for investigating the mechanisms governing this physiological process and its pathologies. The effective spatiotemporal resolution of calcium signals is, however, limited by the motion of contracting tissue: structures of interest in the order of microns can move over a hundred times their width during a contraction. The simultaneous changes in local intensity and tissue configuration make motion tracking a non-trivial problem in image analysis and confound many of the standard techniques. This paper presents a method that tracks local motion throughout the tissue and allows for the almost complete removal of motion artefacts. This provides a stabilized calcium signal down to a pixel resolution, which, for the data examined, is in the order of a few microns. As a byproduct of image stabilization, a complete kinematic description of the contraction-relaxation cycle is also obtained. This contains novel information about the mechanical response of the tissue, such as the identification of a characteristic length scale, in the order of 40-50 μm, below which tissue motion is homogeneous. Applied to our data, we illustrate that the method allows for analyses of calcium dynamics in contracting myometrium in unprecedented spatiotemporal detail. Additionally, we use the kinematics of tissue motion to compare calcium signals at the subcellular level and local contractile motion. The computer code used is provided in a freely modifiable form and has potential applicability to in vivo calcium imaging of neural tissue, as well as other smooth muscle tissue.