Neuroimaging Flashcards
Structural Methods
CT
MRI
DTI
Post-Mortem
Functional Methods
fMRI PET EEG EMG Optical
fMRI
PET
High spatial resolution (1-3 mm)
Low temporal resolution (6+ seconds)
EEG
EMG
Optical
Low spatial resolution (1-3 cm)
High temporal resolution (msec)
CAT
Computerized Axial Tomography
Structural Method
X-ray from various degrees calculate density
Uses Hounsfield Units of Density
MRI
Magnetic Resonance Imaging
Structural Method
Detect pulse and relaxation of molecules in magnetic field
1 Tesla
T1; T2
20,000 times earth magnetic field
Pulsing time: T1 = 0.5 sec, T2 = 2.5 sec
H2O Relaxation
relaxes at 2.5 seconds; T1 scan no water signal (dark), T2 scan prominent water signal (white)
Right-hand rule
In a coil of current, right-hand rule can determine the direction of magnetic field by curling fingers in direction of current. Thumb will point in direction of magnetic field.
B1
Every molecule/proton has own magnetic resonance pointing in B1 direction
B0
Larger, environmental magnetic field; will affect small B1s
resonance frequency
specific frequency at which a molecule resonates (ex. plucking a G string on violin will cause G string on harp to resonate)
Basic MRI Procedure
1) radio frequency pules at resonance frequency with B0
2) B1 conforms to B0
3) Stop pulse
4) B1 slowly returns to original state
5) change in B1 emits rf signal
6) image generated
DTI/DSI
Diffusion Tensor/Spin Imaging
Water flow in white matter tracts
Structural method
fMRI
detects increase in amount of oxygenated hemoglobin in area after increased firing. Takes 7 seconds for blood rush to occur
Functional method
PET
Positron Emission Tomography
Functional Method
Produces 3D image
1) radioactive tracer isotope injected into living subject
2) tracer chemically incorporated into molecule
3) isotope goes through radioactive decay (becomes positron) and emits photons to short distance
4) gamma photons emitted in opposite directions
5) photons detected by detector arrays, many collisions increases concentration
FDG vs PIB
Both detect Alzheimer’s dementia. PIB more efficient at detecting non-Alzheimer’s dementia
EEG
Electroencephalography
Function method
Source: Dendritic post-synaptic potentials from pyramidal cells
Cellular Basis of EEG
Apical dendrite receives EPSP, rush of positive ions inside creates temporary dipole moment between positive charge near soma and negative charge near dendrite (vice versa for extracellular space). Positive extracellular dipole is detected by electrode
Evoked Potential (EP)
early latency sensory responses such as the BAEP (5msec) or Visual Evoked Potential (100msec)
Event-Related Potential (ERP)
refers to time locked perceptual, cogntive, or response potentail (200-800msec)
Signal Averaging
Sum repetitive events time locked to a stimulus or response
The background EEG can be assumed to be random
Averaging random activity sums to zero
The EP or ERP emerges from ongoing EEG
Number of trials needed is dependent on amplitude of EP or ERP of interest
Signal to noise is proportional to the square root of number of sums
Geometry Constraints on EEG
Open field: pyramidal cells line up parallel, dipoles sum up for activity
Closed field: Not parallel or oppositely oriented, dipoles cancel and decrease signal
Near field: signal originates from near surface of cortex (ex. SEP)
Far field: signal originates from deep inside brain stem (ex. BAEP)
Fourier Analysis
any complex time series can be broken down into a series of superimposed sinusoids with different frequencies
Frequency bands
Outline segments of different frequencies indicating different mental states. Over years, technology has improved frequency observable, now up to high gamma
MEG
magnetoencephalography
Functional method
More expensive than EEG
Basis: all electric currents also generate magnetic field
More reliable since signal does not deteriorate
SQUID
Superconducting Quantum Interference Device
Sensitive magnometer used to measure subtle magnetic fields
Josephson Tunneling effect
Phenomenon of supercurrent where electrons move freely for an indefinitely long time without voltage applied
ECoG
Electrocorticography
Functional Method
Many electrodes spaced closely together placed directly onto cortex to record activity.
Used to determine eloquent tissue via stimulation mapping.
Extract high gamma and theta waves and couple them, mapped out activity of verb generation, and has great single-trial reliability
Coherence and Cognition
Increase coherence increases communication between areas
Optical Imaging
Using light to image brain function
Types of measures: slow (NIRS) and fast (EROS)
Types of information: intensity and phase
non-invasive
Optical imaging information: intensity and phase
Intensity: Number of photons to reach detector
Phase: time it takes photons to reach detector
Optical imaging: slow signal (NIRS)
measure of light absorption
light intensity at detector compared to source
High spatial resolution, medium temporal
Optical imaging: fast signal (EROS)
measure of light scattering
light phase at detector compared with source
High spatial and temporal resolution
Optical Imaging (slow) advantages/disadvantages
adv: noninvasive, inexpensive, portable, allows separate measure of deoxy/oxy hemoglobin concentrations
dis: cannot reach deep structures, measurement of blood oxygenation, not neuronal activity (indirect)
Optical Imaging (fast) advantages/disadvantages
adv: noninvasive, inexpensive, portable, direct measure
dis: can’t measure deep structures, low signal to noise ratio (need many trials)
TMS
Transcranial Magnetic Stimulation
Function method
Can causally and reversibly test necessity of different brain areas for neural function and behavior
TMS Basic Procedure
Magnetic field from coil induces current inside brain
Pulse of around 2 Tesla, returns to baseline in 1ms
Induced electric field changes action/membrane potentials
TMS Resolution
Spatial: 1-2cm
Temporal: brief pulse, functional effects long
TMS Application
1) enhance neural function (stroke recovery enhanced by 4 Hz TMS)
2) excite motor cortex pathways for spinal surgery
3) disrupt local neural function to test causality for imaging techniques
4) treat depression (30% response rate)
TMS
Transcranial Magnetic Stimulation
Function method
Can causally and reversibly test necessity of different brain areas for neural function and behavior
TMS Basic Procedure
Magnetic field from coil induces current inside brain
Pulse of around 2 Tesla, returns to baseline in 1ms
Induced electric field changes action/membrane potentials
TMS Resolution
Spatial: 1-2cm
Temporal: brief pulse, functional effects long
TMS Application
1) enhance neural function (stroke recovery enhanced by 4 Hz TMS)
2) excite motor cortex pathways for spinal surgery
3) disrupt local neural function to test causality for imaging techniques
4) treat depression (30% response rate)
Neural Prosthetics
Concept: Brain activity activates control for machine prosthetic
BMI assistive technology
Cochlear implant: microphone, speech processor, transmitter, receiver/stimulator, electrode array lined on cochlea
Neurostimulator and DBS for Parkinson’s
P300 Speller
Detects recognition brain wave P300 when user attenuates wanted letter
Center Out Task
Monkeys learn to use neural signal to move small circle into larger circle
