Brain Imaging Flashcards
1
Q
intro
A
modern imaging modalities provide information about the living brain
- essential to understanding effect of both development and disease on:
- structure of the brain
- how structures are integrated as coherent networks
- which networks subserve diverse cognitive linguisitc and emotional systems
- how they are altered by disease
2
Q
CT
A
- computed axial tomography
- rotating X ray beam
- images the brain from several directions
- rate of attenuation varies by tissue
- radiodensity information is detected
- used to reconstruct a 3D image
3
Q
clinical applications of CT
A
- enhances visualization of
- bony anatomy
- acute hemorrhage or stroke
- elements with high atomic numbers show up better-Calcium, iron, iodine, barium, lead
- advantages-faster and less expensive then MRI, can be used as an initial screening and assesment tool
- disadvantages-use of Xray, less contrast differences between soft tissues, lower spatial resolution, several mm in CT vs 1 in MRI
- used for infarction, tumors, calcifications, hemorrhage, bone trauma
- hypodense-edema/infarction
- tumors from anatomical distortion or surrounding edema
4
Q
TBI
A
- heme released, causes sub-arachnoid hemorrhages, intraparenchymal contusions, hematomas
- intrinsic cellular injury
5
Q
anatomic MRI
A
- based on principles of nuclear magnetic resonance
- produces high resolution images of the brain and spine
- no radiation, radio frequency waves used
6
Q
clinical applications of MRI
A
- high resolution and detailed visualization of soft tissue
- visualizes anatomy (gray and white matter, CSF)
- identifies a wide range of pathological processes
7
Q
physics of MRI
A
- body comprised of 63% hydrogen atoms
- protons in hydrogen atoms spin like a top
- spin produces small magnetic field
- spinning proton placed within a large external magnetic field will align with or against the external field
- it will also precess (wobble) at a frequency proportional to the magnetic field
- slightly more protons will eventually align with the external field
- net magnetization of the tissue
- to detect magnetization, apply radiofrequency pulse which tips protons away from the direction of magnetization
- when the pulse is off, they realign
- the energy that each spinning proton absorbed from the RF decays
- signal emitted
- decoded into images with Fourier transform algorithms
8
Q
digital image construction
A
- the spin of the proton decays, and emits RF signals, at different rates depending on the composition of the tissue in which they are located
- each pixel of the image is encoded with a numerical index that represents the relative strength of the RF signal in the area of the brain to which that pixel corresponds
- each numerical index is assigned a gray scale value
9
Q
pulse sequences
A
- the clinician can vary the timing of the RF pulse which accentuates the tissue they are most interested in
- rapid repetitions of the RF enhance gray/white contrast
- lesions-best imaged with infrequent repetitions of RF pulse-enhanced signal from water, which is usually increased in pathological conditions
10
Q
equipment for MRI
A
- magnet
- gradient coil
- RF coil
11
Q
diagnostic applications of MRI
A
- inflammatory disease-MS
- neoplasms
- epilepsy
- cerebrovascular disease- stroke
12
Q
magnetic resonance spectroscopy
A
- permits us to study chemical structure of the brain
- separates out components of chemical mixtures in brain
- N-acetylaspartate (NAA)
- choline
- creatine (Cr)
- lactate
- can derive concentrations and ratios of chemicals
13
Q
how does MRS work?
A
- an RF pulse is applied, after which each chemical component emits a specific frequency
- the signals are analyzed with Fournier transforms to generate NMR spectra
- the concentration of each chemical component of interest is represented by the size of the peak that is produced
14
Q
NAA
A
-located in cell bodies of dendrites, considered a neuronal marker
15
Q
choline
A
- cell membrane synthesis and degradation
- marker for demyelination
16
Q
creatine
A
glial marker
17
Q
lactate
A
found following ischemic events
18
Q
diffusion weighted imaging and diffusion tensor imaging
A
-allows us to visualize and measure the integrity of white matter tracts in the brain
19
Q
diffusion weighted images
A
- intensity of each image element (voxel) reflects the best estimate of the rate of water diffusion at that location
- three gradient directions are applied, sufficient to estimate the trace of the diffusion tensor or average diffusivity
- putative measure of edema
20
Q
diffusion tensor imaging
A
- measures water diffusion along different orientations within axons
- PM and myelin restrict water flow
- water flows relatively organized longitudinally and perpendicular
- if not doing this, pathology
- axial and radial diffusivity
- small coefficient, small amt of water along axis, its elsewhere
- ellipsoid indicates water is diffusing along longit axis and represents integrity of myelin
- RD represents integrity of myelin
21
Q
anisotropy
A
measurement of water diffusion along different orientations within axons
- based on AD and RD
- high means more water is diffusing along long vs perpendicular-ellipsoid
- sphere is low anistropy and no organization
- vectors define orientation of axons
- transverse red, long green, horizontal blue
- x,y,z
22
Q
fMRI
A
-allows us to acquire images of the brain while patients are performing cognitive tasks in the MRI scanner
23
Q
advantages of fMRI
A
- brain functioning in vivo
- previously relied on single cell recordings, animal studies, and lesion studies
- functional neuroimaging has shown that the brain is much more plastic than we thought
- understand neural systems that makes performance of cognitive tasks possible
- understand changes in brain function associated with disorders and with aging
- understand sites of neural reorganization following stroke or injury
24
Q
how fMRI works
A
- measure blood oxygen level dependent (BOLD) signal with MRI during baseline and experimental conditions
- compare BOLD MRIs between two conditions
25
Q
analyzing fMRI data
A
- experimental condition-the task is comprised of the specific cognitive variable of interest
- control-task is comprised of all features of exp except the specific cognitive variable of interest
- subtract the magnitude of neural activation during the control from exp
- generate activation maps that are superimposed on brain images
26
Q
limitations of fMRI
A
- limitations of temporal/spatial resolution
- relation between neuronal activity, blood flow, and fMRI signals has not been definitively established
- new phrenology?
27
Q
PET
A
- use of cyclotron to prepare radioactive isotope tracers
- tracer is incorporated into a biologically active molecule
- glucose, oxygen, dopamine transporters/receptors
- injection of tracers, which then bind to physiological sites
- scanner images the positron emitting tracer upon it’s decay
28
Q
applications of PET
A
- blood flow and perfusion
- metabolism
- ligands/neuroreceptor imaging-dopamine syn and reuptake
- can detect radiation necrosis from tumor recurrence
29
Q
limitations of PET
A
- need cyclotron
- injection of radioactive tracer
- poor spatial resolution
30
Q
applications of imaging modalities
A
- normal brain development
- AD
- image guided neurosurgery
31
Q
mapping emotion
A
- normal adolescents
- happy, sad, fearful and neutral faces presented during fMRI scan
- amygdala activation during presentation of all emotions
32
Q
mapping declarative memory
A
- children, adolescents, adults activated mesial temporal lobe structures involved in memory
- adolescents and adults activated pre-frontal cortex
- indicates the PFC regions important for memory formulation have prolonged maturational trajectory
33
Q
AD
A
- gray matter loss
- PET used since hypometabolism is associated
- APOE-4 allele- risk
- fMRI comparison of effect of allelic variation during memory task
- APOE-4 allele carriers increased brain activation during memory tasks
- after 2 years, degree of baseline brain activity correlated with memory decline
34
Q
image guided neurosurgery
A
- pre-op planning
- 3D volume data set- CT images overlayed with FLAIR weighted MRI, DTI, fMRI to define speech areas in a patient with low grade astrocytoma
35
Q
intraoperative MRI
A
-used during surgery
36
Q
conclusion
A
- exponential leaps in our understanding of neural mechanisms in learning, aging, disease
- development of effective, well targeted pharmacological agents
- identification of functionally important brain areas prior to neurosurgery
- understanding of neural reorganization and development of remediation strategies
- recognition of plasticity of human brain well into adulthood
