Week 1 - Brain Imaging Flashcards
how does X-ray computerized tomography work? (AKA CT or CAT Scan)
rotating X-ray beam images brain from several directions
- rate of attenuation varies by tissue, and detectors on opposite side of source pick up rate of weakness
- info reconstructs 3D image as patient is mvoed through scanner slowly
clinical applications of CT, advantages, and disadvantages
enhances visualization of bony anatomy, acute hemorrhage/stroke, and elements with high atomic numbers (bright), edema, infarction (dark)
- pro: faster/cheaper than MRI, and used as initial screening and assessment tool
- con: uses X-ray, less contrast differences between soft tissue, and lower spatial resolution (several mm in CT vs 1 mm in MRI)
what do infarcts look like in CT VS MRIs?
CT: only slight difference in gray matter, but hard to differentiate between gray and white
MRI: much easier to see change
what are CTs good at revealing?
hemorrhages/trauma (TBI)
generalized atrophy
how does anatomic MRI work?
based on principles of nuclear magnetic resonance
-produces high resolution images of brain and spine
clinical applications of MRI, advantages, and disadvantages
high resolution and detailed visualization of soft tissue
- visualizes anatomy (gray and white matter, CSF)
- identifies wide range of pathological processes
- pro: no radiation (uses radio waves)
- con: long study duration, no herromagnetic or electronic devices, and claustrophobic
what are MRIs good at revealing?
neoplasms, demyelination (MR spectroscopy), degenerative disorders (cortical atrophy), inflammatory disease (MS), epilepsy, cerebrovascular disease (stroke)
how do MRIs work?
spinning H+ placed in large, external magnetic field aligns with or against EMF, wobbling at a proportional frequency
- slightly more H+ will align with EMF, leading to net magnetization
- detect with radiofrequency pulse that tips H+ away from direction of magnetization
- when RF is turned off, H+ realign with EMF, and the energy absorbed from RF will decay, emitting an RF signal picked up with antennae and decoded into images with Fourier algorithms
how is the digital MRI image constructed?
spin of H+ decays, emits RF signals at different rates depending on composition of tissue
-each pixel is encoded with a numerical index that represents relative strength of signal, and assigned grayscale value (higher = darker)
what are pulse sequences in terms of MRIs?
clinician can vary timing of RF pulse which accentuates the tissue he/she is most interested in
-rapid repetitions of RF pulse enhances gray-white contrast
what are lesions best imaged with?
infrequent repetitions of RF pulse
-enhances signals from water, which is increased in pathological conditions
what are the necessary equipment for MRI?
magnet
gradient coil
RF coil
usually use 3 tesla, but increased strength will increase contrast
how is MRI imaging of a tumor manipulated?
- 5 T and 3T scanners at T1 weighted can’t show tumor (although increasing clarity)
- best with 3T scanner at T2 flair
what does magnetic resonance spectroscopy let us do?
study chemical structure of brain
- separates out components of chemical mixtures in brain (N-acetylaspartate, choline, creatine, lactate)
- derives concentrations and ratios of chemicals (metabolites)
how does MRS work?
an RF pulse is applied, after which each chemical component/metabolite emits specific frequency
- signals analyzed with Fourier transforms to generate NMR spectra of multiple peaks
- the higher the concentration, the larger the peak
what are the following MRS metabolites markers for?
- NAA (N-acetylaspartate)
- choline
- creatine
- lactate
- NAA: located in cell bodies and dendrites as neuronal marker
- choline: cell membrane synthesis and degradation; marker for demyelination
- creatine: glial marker
- lactate: found following ischemic events
but even if there’s no peak, it doesn’t discount disease
what does diffusion weighted/tensor imaging let us do?
visualize and measure integrity of white matter tracts in the brain
what gradients are applied in diffusion weighted images? what is the best estimate of rate of diffusion?
3 gradient directions are sufficient to estimate trace of diffusion tensor (average difusivity)
-intensity of each image element (voxel) reflects best estimate of rate of diffusion at that location
what is anisotropy and what is it used for?
measurement of water diffusion along different orientations within axons, used in diffusion tensor imaging
- can use up to 64 directions, but faster with just 3
- flow is restricted by myelin
what does functional MRI (fMRI) allow us to do?
acquire images of brain while patients are performing cognitive tasks in MRI scanner
-shows brain is more plastic than previously thought
what are advantages of fMRI?
- measure brain functioning in vivo, thus understanding neural systems that make performance of cognitive tasks possible
- understand changes in brain function associated with disorders and aging
- understand sites of neural reorganization following stroke or injury
how does fMRI work?
measure blood-oxygen-level dependent (BOLD) signal with MRI during baseline and experimental conditions, then compare
- neural activity increases when we process information, which causes increased in blood flow
- leads to changes in local cerebral blood volume, measurable via paramagnetic oxyhemoglobin
- able to see how much a task is stimulating neural activity in what regions of brain neural activity
how to analyze fMRI data (how do experimental and control conditions differ?)
experimental: task comprised of specific cognitive variable of interest
control: task comprised of all features of experimental task EXCEPT specific cognitive variable of interest
- subtract magnitude of neural activation during control from experimental
- generate “activation maps” superimposed on brain images
how does visual direction compare in controls VS velo-cardial facies syndrome?
much less stimulation in fMRI
what are limitations of fMRI?
limited temporal and spatial resolution
-relation between neuronal activity, blood flow, and fMRI signals has not been definitively established
what is PET (positron emission tomography)?
use of cyclotron to prepare radioactive isotope tracers
- tracer is incorporated into biologically active molecule (internal source)
- injection of tracers, which then binds to physiological sites
- scanner images positron-emitting tracer upon its decay
what disorders does PET measure?
psychiatric/addictive/degenerative disorders, epilepsy
what are PET advantages and disadvantages?
pro: functional imaging, physiological variables can be determined
con: ionizing radiation, costly and limited access, troublesome tracer production, poor spatial resolution
applications of PET
- blood flow and perfusion
- metabolism (resting and task dependent)
- -FDG traces glucose uptake (reflects synaptic activity), O traces O2 uptake
- ligands/neuroreceptor imaging
- -radiotracers bind to pre/postsynaptic neuroreceptors (dopamine synthesis and reuptake)
- -advanced pharmocological treatments
what are 3 applications of immaging modalities?
normal brain development
Alzheimer’s disease
image guided neurosurgery
how does gray and white matter differ between males and females
males have more of both types of matter
-gray matter decreases while white matter increases with age
what happens when fMRI maps emotions?
all 4 emotions (happy, sad, fearful, angry) caused amygdala activation
what happens when fMRI maps declarative memroy?
children, adolescents, and adults activated mesial temporal lobe structures involved in memories
- adolescents and adults activated prefrontal cortex (increased memory of scenes)
- indicates that PFC regions important for memory formation have prolonged maturational trajectory
what does PET show in terms of Alzheimer’s disease?
gray matter loss
-used in management, since hypometabolism in various brain regions has been associated with severity of clinical symptoms
what is the APOE-4 gene?
genetic risk factor for Alzheimer’s disease
- carriers have increased brain activation during memory tasks
- degree of baseline brain activity is correlated with memory decline
- shown via Bookheimer study
