  Ventricular coordinates are widely used as a versatile tool for various
applications that benefit from a description of local position within the
heart. However, the practical usefulness of ventricular coordinates is
determined by their ability to meet application-specific requirements. For
regression-based estimation of biventricular position, for example, a symmetric
definition of coordinate directions in both ventricles is important. For the
transfer of data between different hearts as another use case, the consistency
of coordinate values across different geometries is particularly relevant. To
meet these requirements, we compare different approaches to compute coordinates
and present Cobiveco, a symmetric, consistent and intuitive biventricular
coordinate system that builds upon existing coordinate systems, but overcomes
some of their limitations. A novel one-way transfer error is introduced to
assess the consistency of the coordinates. Normalized distances along bijective
trajectories between two boundaries were found to be superior to solutions of
Laplace's equation for defining coordinate values, as they show better
linearity in space. Evaluation of transfer and linearity errors on 36 patient
geometries revealed a more than 4-fold improvement compared to a
state-of-the-art method. Finally, we show two application examples underlining
the relevance for cardiac data processing. Cobiveco MATLAB code is available
under a permissive open-source license.
