科研之友
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摘要
The signal intensity in MRI depends on the proton density, T1, T2 and T2 relaxation processes of any ensemble of the spins within each imaging elements (voxel). Another important contrast mechanism in MRI is signal loss caused by proton dephasing in the presence of coherent and incoherent flow. Random diffusion of protons into areas of varying magnetic field strength leads to random phase shifts. The summation of signals from these protons with random phase shifts results in a net signal loss (hypointensity on DWI). In restricted diffusion (like acute infarction) the signal attenuation is decreased (hyperintensity on DWI). The observed proton diffusion rate and direction reflect the molecular and macromolecular barriers, or hindrances that proton experiences during its translational process. As water protons diffuse and thus interact with the lattice and spin microenvironment, their traveled path along any one direction can be described. The proton displacement is expressed by an apparent diffusion coefficient (ADC). The apparent diffusion in tissue is slowed if the protons are %26quot;hindered,%26quot; or slowed in their random motion by the presence of cell membranes, walls, and acromolecules but are not completely restricted. For example, myelin fiber and neurofibril orientation in white matter possesses a preferred direction for%26quot;nwater proton movement. This feature causes protons to diffuse faster along the path of least resistance, causing the ADC to be anisotropic, or directionally dependent. Measurements during proper times can fully reflect these hindrances to diffusion. DW images have become an essential part of clinical MR imaging, providing physiological information useful in identifying early ischemic infarction. They have also been used to differentiate cysts from solid tumors, to aid in the diagnosis of inflammatory conditions such as encephalitis and abscesses, and to evaluate various white matter abnormalities, such as multiple sclerosis.
