3-DIMENSIONAL STRAIN-RATE IMAGING

Strain-rate imaging uses large velocity encoding gradients to obtain m easurements of velocity that are extremely insensitive to the effects of random noise. The spatial differential of velocity yields the veloc ity gradient from which the strain-rate and twist-rate tensors can be determined. These tensors represent the distortion of the material and are of interest in the analysis of the dynamic behavior of living tis sue (e.g., that of the myocardium). This work presents a new technique that uses the magnitude of the signal in the velocity encoded data to measure through-plane velocity variations at the resolution of the vo xel size. The magnitude of the MR signal contains information about th e range of phases present within a voxel. When the phase is dependent on the velocity (as in phase velocity imaging), the magnitude contains information about the range of velocities within a voxel. The method presented in this work uses unbalanced slice-refocusing gradients to s ample the magnitude variation introduced by the interaction of velocit y encoding gradients with spatially dependent velocities. The previous ly developed in-plane velocity gradient methods can be easily integrat ed with this new through-plane measurement to characterize the deforma tion of the myocardium in three spatial dimensions with high accuracy. The applicability of these methods is demonstrated theoretically, in phantoms and in vivo.