There is provided a computer-implemented method for generating virtual chimera populations of multi-part organ shapes for use in an in-silico trial, wherein organ shapes are represented as surface or volumetric meshes, the method comprising using a part-aware generative model to learn a latent representation of each part of a multi-part organ shape for inclusion in a virtual population and output synthesised parts of the multi-part organ shape, using a spatial composition model to align the outputted synthesised parts of the multi-part organ shape and output an anatomically meaningful example of an overall multi-part organ shape as a virtual chimera and storing the virtual chimera in the virtual population, for use in the in-silico study. There is also provided an apparatus and computer readable medium embodying the method.