intravoxel

Intravoxel is a descriptive term in medical imaging referring to processes or measurements that occur within a single voxel. A voxel is a three-dimensional element of an image representing tissue, with dimensions defined by imaging resolution.

In practice, many biological tissues are heterogeneous at the voxel scale. Intravoxel heterogeneity arises when a voxel contains multiple tissue types or microstructures, leading to signal or parameter estimates that are averages of different components. This can produce partial-volume effects and complicate interpretation.

In diffusion magnetic resonance imaging, intravoxel incoherent motion is a notable intravoxel phenomenon. IVIM attributes part of the signal decay to microcirculation of blood in the capillary network within the voxel, in addition to molecular diffusion. The IVIM model separates these contributions with parameters D, D*, and f, often via diffusion-weighted imaging across multiple b-values. The expression S(b)/S0 = (1 - f) e^{-bD} + f e^{-bD*} summarizes the idea, where D represents true diffusion, D* reflects pseudo-diffusion from perfusion, and f is the perfusion fraction.

Applications of IVIM and related intravoxel concepts include characterization of liver disease, tumors, kidneys, and other organs, with the appeal of non-contrast perfusion information. Limitations include sensitivity to noise, fitting instability, and the need for carefully designed acquisition protocols that cover a range of b-values. More generally, intravoxel interpretation requires consideration of voxel size, partial-volume effects, and the potential coexistence of multiple tissue components within a single voxel.