Measurement of Displacement in Isolated Heart Muscle Cells using Markerless Subpixel Image Registration

Cellular mechanics is an area that has been readily explored, yet there is still much that is unknown about the behaviour of heart muscle cells (myocytes) during contraction in both healthy and diseased states. Currently available systems for contractility measurements expect myocytes to be aligned within the imaging system and expect them to contract axially. However, myocyte contraction is not always uniform, and hence localised changes in the myocyte may not be captured in 1D measurements such as average sarcomere and cell length. With the availability of new and robust image correlation techniques, there is new potential to track displacements of cells without the need for external markers. Here, we present the use of a subpixel image registration (accurate to the millipixel) to track the internal movement of an isolated myocyte during unloaded contraction. Additionally, methods were developed to estimate the orientation of a contracting myocyte frame by frame, removing the need for myocytes to be perfectly aligned in the imaging system. This will allow measurement of sarcomere lengths and displacement regardless of what angle the myocyte is at. Furthermore, it offers the potential to find localised myocyte angle during contraction and therefore localised sarcomere lengths.