  Current tomographic imaging systems need major improvements, especially when
multi-dimensional, multi-scale, multi-temporal and multi-parametric phenomena
are under investigation. Both preclinical and clinical imaging now depend on in
vivo tomography, often requiring separate evaluations by different imaging
modalities to define morphologic details, delineate interval changes due to
disease or interventions, and study physiological functions that have
interconnected aspects. Over the past decade, fusion of multimodality images
has emerged with two different approaches: post-hoc image registration and
combined acquisition on PET-CT, PET-MRI and other hybrid scanners. There are
intrinsic limitations for both the post-hoc image analysis and dual/triple
modality approaches defined by registration errors and physical constraints in
the acquisition chain. We envision that tomography will evolve beyond current
modality fusion and towards grand fusion, a large scale fusion of all or many
imaging modalities, which may be referred to as omni-tomography or
multi-tomography. Unlike modality fusion, grand fusion is here proposed for
truly simultaneous but often localized reconstruction in terms of all or many
relevant imaging mechanisms such as CT, MRI, PET, SPECT, US, optical, and
possibly more. In this paper, the technical basis for omni-tomography is
introduced and illustrated with a top-level design of a next generation
scanner, interior tomographic reconstructions of representative modalities, and
anticipated applications of omni-tomography.
