Contrast, image patient motion

Several factors affect the contrast of an image, namely count density, scattered radiations, type of film, size of the lesion, and patient motion. Each contributes to the contrast to a varying degree. These factors are briefly discussed here. 

D time of flight (3D TOF) Noninvasive High spatial resolution Shows complex vascular flow Less susceptible to intravoxel dephasing Can be repeated if suboptimal Can be obtained after contrast Only small volumes due to marked saturation effects Cannot image slow flow because of saturation effects Time-consuming, susceptible to patient motion Artifact from T1 hyperintense lesions Routine to evaluate circle of Wfllis for large vessel stenoses/ occlusions Can estimate carotid bifurcation stenoses... 

Recent studies have demonstrated the usefulness of MRI in patients with suspected TIA, particularly diffusion-weighted imaging (DWI) (Figs. 10.3-10.9) (Schulz et al. 2003 Schulz et al. 2004). This modality relies on changes in the Brownian motion of water molecules to generate contrast. During early ischemia, there is decreased water proton movement caused by cytotoxic edema as water moves from the less-restricted extracellular environment into the more-restricted intracellular environment. Reduced proton diffusion leads to... 

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