Intravoxel incoherent motion

Intravoxel incoherent motion (IVIM) is a technique used in diffusion MRI to measure translational movements of water molecules within a voxel.

IVIM-based perfusion MRI does not require the injection of any tracer or contrast agent. IVIM is a concept and method used in diffusion MRI to assess microscopic translational motions that contribute to the acquired signal. It involves the quantification of both molecular diffusion of water in tissue and microcirculation of blood in the capillary network. IVIM refers to the distribution of speeds and amplitudes of translational movements within a voxel during the measurement time.

Physics

Intravoxel incoherent motion principles

• distinct environments: biological tissue consists of two distinct environments - molecular diffusion of water and microcirculation of blood in capillaries

• pseudo-diffusion: the flow of water in capillaries, known as "pseudo-diffusion", resembles a random walk as long as there is no net coherent flow in any direction

• signal attenuation: In diffusion MRI, the signal attenuation is affected by both molecular diffusion and pseudo-diffusion (perfusion) effects

• separation of effects: by utilizing very low b-values, the relative contribution of pseudo-diffusion to the diffusion-weighted signal becomes significant, enabling the separation of diffusion and perfusion effects

• signal attenuation equation: the signal attenuation in diffusion MRI can be expressed as a function of the volume fraction of incoherently flowing blood (f_IVIM), the signal attenuation from the IVIM effect (F_perf), and the signal attenuation from molecular diffusion (F_diff)

Models

• biexponential model

  • describes the presence of two water compartments with fast and slow diffusion, which can be used for IVIM analysis

  • combines diffusion and perfusion measurements to estimate parameters such as true diffusion coefficient (Dslow), and pseudo-diffusion coefficient (Dfast)

• kurtosis model: quantifies the deviation from Gaussian diffusion and can also be used for IVIM analysis

History and etymology

IVIM was introduced in 1986 as a way to account for the impact of blood flow on diffusion MRI measurements.