Statistical description of microcirculatory flow as measured with an MR method.

Quantification of microcirculatory flow is important for the functional assessment of biologic systems. The authors describe a method of analyzing the dependence of the magnetic resonance (MR) signal intensity on microcirculatory flow. A gel bead phantom was used to simulate the randomly oriented flow capillaries, and the MR signal intensity of the phantom was studied at different flow velocities by using velocity-sensitized and -compensated spin-echo pulse sequences. A theoretical model based on the spin-phase phenomenon is proposed to elucidate the effect of flow on signal intensity. The velocity phase of a spin depends on its path and the corresponding velocity-encoding gradients. A Monte Carlo simulation was used to generate the path of a spin on the basis of a statistical model for flow through a random capillary network. From the velocity-phase distribution of a group of spins within a voxel, the signal attenuation due to flow can be calculated. The results of the statistical model and experimental measurements agreed well. Also, T1 and T2 effects in MR flow measurements were investigated. The current study provides a theoretical framework for understanding MR measurements of microcirculatory flow.