A: Axial in-phase T1-weighted magnetic resonance imaging; B: Axial opposed-phase T1-weighted magnetic resonance image; C: Axial fat-only magnetic resonance image from a two-point Dixon reconstruction which displays the difference between echos from A and B. The tumor (arrow) shows high signal on C due to the presence of microscopic fat. Incidentally, the liver also shows high signal on C due to hepatic steatosis.
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Diffusion weighted imaging (DWI) is designed to detect the random movements of water protons. Water molecules diffuse relatively freely in the extracellular space; their movement is significantly restricted in the intracellular space. Spontaneous movements, referred to as diffusion, rapidly become restricted in ischemic brain tissue. During ischemia, the sodium - potassium pump shuts down and sodium accumulates intracellularly. Water then shifts from the extracellular to the intracellular space due to the osmotic gradient. As water movement becomes restricted intracellularly, this results in an extremely bright signal on DWI. Thus, DWI is an extremely sensitive method for detecting acute stroke.