The effects of microscopic tissue parameters on the diffusion weighted magnetic resonance imaging experiment

Abstract

This review examines the way in which microscopic tissue parameters can affect MR experiments which are sensitive to diffusion. The interaction between the intra-and extravascular as well as that between the intraand extracellular spaces is examined. Susceptibility gradients due to the presence of deoxyhemoglobin can cause diffusion-induced signal losses which are significant in functional magnetic resonance experiments, particularly at higher main magnetic field strengths. This is also true of the fast response that manifests itself as an early negative signal change in functional magnetic resonance experiments. The fields surrounding paramagnetic vessels are described and the way in which diffusion in these fields contributes to functional signal changes is examined. Flow in the capillary bed can be a confounding factor in experiments which aim to examine the diffusion characteristics of extravascular water. It is potentially also a method for assessing capillary perfusion. The intravoxel incoherent motion experiment is described in terms of how significantly this effect can influence diffusion attenuation curves from water. The major models for describing water diffusion in tissue are presented, as are the main experimental results that have contributed to an understanding of the mechanisms of diffusion contrast. The widely accepted view that changes in the diffusion characteristics are caused by a shift of water to the intracellular space and a concomitant change in extracellular tortuosity is examined critically. More recent experiments that indicate that a reduction in the intracellular diffusion may occur simultaneously with the cell swelling are described and their compatibility with existing models discussed.

Neuroimaging Weighs In: Humans Meet Macaques in “Primate” Visual Cortex

“…Basically, axons constrain the flow of fluids around and inside them, producing freer diffusion parallel with long axis of the axon. MR imaging can resolve such diffusion anisotropies, but the underlying biophysics is complicated (Norris, 2001).…”

Utility of diffusion-weighted magnetic resonance imaging for liver fibrosis in chronic active hepatitis

“…In a study conducted with cirrhotic live dogs, the ADC was lower in the cirrhotic lobes than in normal lobes; however, lobal differences in the ADC values disappeared after portal vein clamping (43). Thus, the study concluded that the decrease of ADC in the liver was caused by decreases in blood perfusion effects as a result of cirrhosis (22,35,41). In another rat study comparing conventional morphological imaging and DW-MRI, diffuse hepatic disease could be detected earlier by DW-MRI, resulting in the suggestion of using ADC values as a marker for early diagnosis (44).…”

“…The ADC difference among dead and living rats was related to perfusion effects. Decreased perfusion due to liver fibrosis has been seen in a number of studies (32,35,41,42). In a study conducted with cirrhotic live dogs, the ADC was lower in the cirrhotic lobes than in normal lobes; however, lobal differences in the ADC values disappeared after portal vein clamping (43).…”

MRI perfusion measurements calculated using advanced deconvolution techniques predict survival in recurrent glioblastoma treated with bevacizumab

“…As generation of the AIF is an imperfect process and can vary depending on the software used, mathematical models that remove the timing dependence of the AIF can produce more robust perfusion measurements. The concentrationtime curves in perfusion imaging are also influenced by the permeability and tortuosity of the blood vessels; while permeability is modeled and accounted for during data analysis by the leakage correction [15], perfusion processing software does not often account for vessel tortuosity, although models for tortuosity do exist [26].…”