NORMAL CYTOTOXIC EDEMA (Acute Ischemia) Diffusion MR Signal Diffusion MR Imaging
Magnetic Resonance Imaging Diffusion Highly sensitive to acute ischemia— + within a few hours! No other imaging is more sensitive to acute ischemia although perfusion imaging reveals hypoperfused tissue at risk for ischemia Acute left MCA infarction DWI
Magnetic Resonance Angiography Axial “source” images… …reformatted to “maximum intensity projections” (MIP) Multiple projections allow 3D-like display No need for IV contrast!
MRA Perfusion MR MRA Perfusion MR Magnetic Resonance Angiography with Perfusion MR
Magnetic Resonance Tissue contrast in MR may be based on: Proton density Proton density Water/fat/protein content Water/fat/protein content Metabolic compounds (MR Spectroscopy) Metabolic compounds (MR Spectroscopy) e.g. Choline, creatine, N-acetylaspartate, lactate Magnetic properties of specific molecules Magnetic properties of specific molecules e.g. Hemoglobin Diffusion of water Diffusion of water Perfusion (capillary blood flow) Perfusion (capillary blood flow) Bulk flow (large vessels, CSF) Bulk flow (large vessels, CSF)
1.CT: Iodine-based Iodine is highly attenuating of X-ray beam (bright on CT) MRI: Gadolinium-based Gadolinium is a paramagnetic metal that hastens T1 relaxation of nearby water protons (bright on T1-weighted images) 2.Tissue that gets brighter with IV contrast is said to “enhance” (Brightness, in and of itself, is not enhancement!) 3.Enhancement reflects the vascularity of tissue, but… The blood-brain barrier keeps IV contrast out of the brain! Enhancement implies BBB is absent or dysfunctional Remember: Some brain anatomy lives outside the BBB IV Contrast in Neuroimaging