Novel methodologies have been recently developed to characterize the microgeometry of neural tissues and porous structures via diffusion MRI data. In line with these previous works, this article provides a detailed mathematical description of q-space in spherical coordinates that helps to highlight the differences and similarities between various related q-space methodologies proposed to date such as q-ball imaging (QBI), diffusion spectrum imaging (DSI), and diffusion orientation transform imaging (DOT). This formulation provides a direct relationship between the orientation distribution function (ODF) and the diffusion data without using any approximation. Under this relationship, the exact ODF can be computed by means of the Radon transform of the radial projection (in q-space) of the diffusion MRI signal. This new methodology, termed exact q-ball imaging (EQBI), was put into practice using an analytical ODF estimation in terms of spherical harmonics that allows obtaining model-free and model-based reconstructions. This work provides a new framework for combining information coming from diffusion data recorded on multiple spherical shells in q-space (hybrid diffusion imaging encoding scheme), which is capable of mapping ODF to a high accuracy. This represents a step toward a more efficient development of diffusion MRI experiments for obtaining better ODF estimates. Diffusion MRI (dMRI) data are routinely used for studying molecular diffusion in fluids (1). When the diffusion is constrained by the presence of obstacles, dMRI experiments yield information about the confining geometry (2). Recently, there has been a large amount of interest in using dMRI techniques to obtain information about the microgeometry of porous structures and biological cells, which is supported by the noninvasive nature of these measurements. Also, dMRI techniques are able to probe the pore geometry over a range of length scales not accessible to standard techniques like x-ray and small-angle neutron scattering.Diffusion tensor imaging (DTI) is the first dMRI technique that has been proposed to quantify in vivo the anisotropy of the water self-diffusion process in fibrous tissues (3). This approach allows estimating a second-order self-diffusion tensor D from a series of dMRI data without requiring exogenous contrast agents. DTI provides relevant information that is not available from other imaging modalities; however, this technique can only resolve a single fiber direction within each voxel.In the last few years a number of novel methodologies have been developed to characterize more accurately the microgeometry of neural tissues (for a partial list of such publications, see . Among these advanced methods and algorithms, q-ball imaging (QBI) (6) and diffusion spectrum imaging (DSI) (11) have generated considerable interest due to a number of benefits such as model independence, theoretical soundness, and the ability to resolve intravoxel orientational heterogeneity.In DSI the molecular displacement probability density function (also ...