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Diffusion-Weighted Imaging (DWI) "Diffusion-weighted (sensitized) MRI was first achieved in l985 and has rapidly advanced into the clinical realm over the past few years.9, 10 Diffusion is the random, incoherent motion of molecules driven by thermal energy. DWI typically employs strong Stejskal-Tanner (S-T) pulsed gradients. These paired gradients are balanced about a 180-degree excitation pulse and reduce the signal of diffusing protons relative to more stationary spins. In the presence of the S-T gradients, random motion leads to uncompensated spin dephasing and hence signal attenuation. Stationary protons on the other hand acquire no net phase shift as they experience identical magnetic field perturbations before and after the inversion pulse." 16 "Signal attenuation related to random molecular motion involves an exponential with argument (-b D), where Dis the diffusion coefficient and b is a gradient factor dependent on pulse sequencing. For the S-T arrangement, b=g 2 G2 d2 ( D- d/ 3 ) where g is the gyromagnetic ratio, G is the gradient strength, and dand Dare time intervals" 12, 13, 16 as illustrated below.
Figure 3.Stejskal-Tanner diffusion-weighted pulse sequence. "Diffusion-weighting can be used as a contrast mechanism. Additionally, by varying b in a series of DWI images, it is possible to calculate a "diffusion coefficient" of water in the direction of the diffusion sensitizing gradients. This is analogous to creating a T2 relaxation map by varying TE or a T1 relaxation map by varying TR. Higher b values denote greater diffusion weighting. Unfortunately large diffusion gradients induce strong eddy currents and sensitize images to patient motion, blood flow, system RF instability and gradient instability. These effects degrade images and may significantly influence the experimentally measured value of D, which is thus more appropriately termed the Apparent Diffusion Coefficient (ADC)." 16 DWI is the most sensitive imaging technique to evaluate hyperacute stroke. Restricted diffusion has been demonstrated in less than one hour after the ictus. "DWI is helpful in distinguishing acute from chronic infarcts, often a vexing clinical problem. In the setting of suspected stroke, trace ( orientationally averaged) diffusion images are typically obtained. Trace diffusion images demonstrate contrast associated with the magnitude of the ADC in each voxel, but offer no information concerning the direction of diffusion. Trace images remove confounding white matter anisotropy and improve image quality through signal averaging." 16 |
