Does Attenuation of Perfusion in Diffusion Weighted-MRI Behave as Exponential Decay or as Damped Oscillation ?

Introduction: Diffusion-weighted MR imaging has been used to characterize diffuse and focal liver disease (1,2). Disease processes such as fibrosis and malignancy have been shown to affect diffusion parameters estimated from two models of signal intensity vs. b-value: the mono-exponential model, representing apparent diffusion, and the bi-exponential model (below left), reflecting the separate attenuation of perfusing spins. For isotropic, plug-like blood flow, early diffusion theory predicted a third model in which signal decays not as an exponential, but instead as a sinc function (below right) of the velocity of perfusing spins (3,4). This damped oscillation arises from odd-echo dephasing to an average phase that is either parallel or opposite to the average phase of diffusing spins. Such behavior of the diffusion-weighted signal was not empirically demonstrated in phantoms or in vivo in these studies, but, if present, would be important to recognize for accurate quantification of diffusion parameters. In this study, we demonstrate the existence of damped oscillation of the diffusion-weighted signal in a flow phantom and investigate whether the same effect can be observed in the liver of human volunteers.