3: imaging 2 Flashcards
white matter: how does water diffuse? (speed)
slower water diffusion perpendicular to tract. faster parallel.
fractional anisotropy: what does isotropic/anisotropic diffusion mean and FA values, and dark or bright?
isotropic diffusion - dark, FA = 0: when water is going the same speed in all directions. anisotropic, light, FA = 1 when water mostly goes just one direction
diffusion tensor imaging: what color is CSF and why?
CSF is dark = no directionality so isotropic, FA = 0, dark
biological interpretation of higher or lower FA?
higher FA = intact myelinated axon. lower FA = myelin loss and permeability, or axon collapse
what would cause increased directionality of water diffusion?
greater myelination, axon packing
direction of water movement = ?
direction of axons aka white matter connections
summary of what structural info you can get from MRI (3)
presence/location/extent of abnormalities and lesion in brain. quantitative volumetric analysis for total brain, white matter, grey matter, CSF, subregions. diffusion tensor imaging identifies white matter tracts, and a measure of microstructure (myelination + axon packing)
fMRI relies on what assumption?
BOLD: blood oxygenation level dependence
oxy vs. deoxyhemoglobin?
oxy = diamagnetics (doesn’t influence magnetic field). deoxy = paramaganetic. so would alter the H2O signal
what do you see with stimulus motor finger tapping and fMRI?
stimulus, on tap finger = increase in signal intensity. when not tapping = decrease in signal intensity
what does perfusion measure
blood flow at the capillary level
2 ways to tag the blood for perfusion studies
Gd: paramagnetic contrast agent: changes intensity as it goes through the brain (no blood flow = don’t see the signal). radio frequency label.
measuring absolute quantitative cerebral blood flow?
perfusion studies: see that blood flow is higher in cortex than white matter
perfusion studies in elderly?
not that good: brain looses the blood tagged in the neck due to sluggish flow
summary: info you get from functional MRI? (2) what is a key parameter of cerebral health?
insight into which regions of brain involved with specific tasks. can assess whether involvement of certain brain regions change with age or disorder. perfusion
MRS?
magnetic resonance spectroscopy (conventional MRI measures water in tissue, this measures other molecules like carbon, phosphorous, etc.)
7 examples of 1H visible brain metabolities
NAA, Cr/PCr, choline, myo-inositol, glutamate, lactate, lipids
summary: biochemical info from MRI (3)
non invasive quantitation of important metabolites in brain. can pick up abnormalities no visible with imaging (Ex. neuronal loss with NAA, excessive membranes with choline). more specific characterization of lesion.
white matter hyperintensities?
observed in healthy aging subjects: might be related to small vessel disease, some debate about its link with cognitive changes
volumetric changes with aging and gender
3D T1 weighted images: grey matter goes down, white matter stays the same, CSF goes up. males higher volume than females.
volumetric changes: problem?
so much variability on an individual basis. need to look at whole population group to see a pattern
white matter DTI and aging?
mean diffusivity shows opposite trend as FA. diffusivity small dip around 30 then up. FA peaks around 30 then goes down.
structural changes seen in AD?
hippocampal volume decreases. still LOTS of overlap between AD and control group though, lots of scatter
changes seen in ALS?
progressive loss of neuronal marker NAA in gray matter of motor cortex
fMRI and parkinson’s
greater activity in cerebellum, premotor area, parietal cortex, precuneus, PFC compared with normal subjects while doing autonomatic mvts like finger tapping
huntington’s and DTI?
DTI shows most changes are in deep dray matter: caudate, putamen, globus pallidus, thalamus. patients have a higher mean diffusivity value
cerebral perfusion and HIV?
blood flow is reduced in HIV patients: perfusion lower
MRI and prion disease?
reduced water diffusion in cortex/basal ganglia (see hyperintense areas)
MS and MRI?
MS plaques: with susceptiblity weighted iron see a ring around the lesions
concl: MRI provides detailed info on?
structure, metabolism, function of aging/diseased brain
MRi aids disease diagnosis in ___? detects ____ differences?
disease diagnosis in an individual. group differences in the brain linked to a disorder
