17Bright = “hyper-attenuating” or “hyper-dense” Computed TomographyA CT image is a pixel-by-pixel map of X-ray beam attenuation (essentially density) in Hounsfield Units (HU)HUwater = 0Bright = “hyper-attenuating” or “hyper-dense”
18Computed Tomography Typical HU Values: Air –1000 Fat –100 to –40 Water 0Other fluids (e.g. CSF) 0–20White matter 20–35Gray matter 30–40Blood clot 55–75Calcification >150BoneMetallic foreign body >1000Brain
45Magnetic Resonance Imaging Diffusion DWI Highly sensitive to acute ischemia—+ within a few hours!No other imaging is more sensitive to acute ischemiaalthough perfusion imaging reveals hypoperfused tissue at risk for ischemiaAcute left MCA infarction
46Magnetic Resonance Angiography Axial “source” images……reformatted to “maximum intensity projections” (MIP)Multiple projections allow 3D-like displayNo need for IV contrast!
47Magnetic Resonance Angiography with Perfusion MR MRA Perfusion MR
48Magnetic Resonance Tissue contrast in MR may be based on: Proton densityWater/fat/protein contentMetabolic compounds (MR Spectroscopy)e.g. Choline, creatine, N-acetylaspartate, lactateMagnetic properties of specific moleculese.g. HemoglobinDiffusion of waterPerfusion (capillary blood flow)Bulk flow (large vessels, CSF)
49IV Contrast in Neuroimaging CT: Iodine-basedIodine is highly attenuating of X-ray beam (bright on CT)MRI: Gadolinium-basedGadolinium is a paramagnetic metal that hastens T1 relaxation of nearby water protons (bright on T1-weighted images)Tissue that gets brighter with IV contrast is said to “enhance” (Brightness, in and of itself, is not enhancement!)Enhancement reflects the vascularity of tissue, but…The blood-brain barrier keeps IV contrast out of the brain!Enhancement implies BBB is absent or dysfunctionalRemember: Some brain anatomy lives outside the BBB