Task 5 Flashcards
What does an MRI meassure ?
Structural imaging (no activation) -> just pyhsical properties (grey vs white matter)
How does an MRI work ?
Look at sheet
What does MRI Relie on ?
Water molecules
How is a Water Molecule constructed ?
- > Hydrogen atom and Oxygen atom
- > Hydrogen atom have a magentic propertie
What does a Hydrogen atom consist of ?
-> Proton
What is a proton ?
- > Unsynchronized Spinning charged particle
- > Oriented randomly
What happens if you apply a stong magentic field ?
- > High energy protons point downwards (antiparallel)
- > Low energy protons point upwards (parallel)
What happens if you apply a brief readio frquence impulse?
- > Turns Low energy protons to high energy protons (90 degree switch)
- > Protons move now synchronized
What are the effects of synchronized protons?
-> Produce a detectable magenetic field
What happens when the redio frequency stops ?
-> Protons will be pulled back into original allingment
What are the two types of meassurment in an MRI scan ?
- > T1 / T2
- > T2 *
What does T1 and T2 meassure ?
T1 = meassure the anatopical properties
-> how much time is needed for the previously low energy protons which changed to high energy protons due to the RF pulse to change back to low energy protons
-> Grey matter and white matter vary in timing on how much they spin back
T 2 = how much time till the presscsion spins randomly again
What does T2 * meassure ?
-> meassure functional property
What does functional imaging mean?
- > Always a hemodynamic response
- > Indirect measurement (look at the consequences of neurons firing)
- > How good can methods measure function
What does Hemodynamic mean?
- > When the activity of neurons increases, the blood supply to that region increases relatively to others
- > providing it with more glucose & oxygen
What are the limitations of functional imagaing ?
- > Poor temporal resolution
- > To relate function and structure, must map data of fMRI & PET onto corresponding MRI scans
How does Pet work ?
- > inject radioactive tracer into bloodstream
- > Tracer measures local variation in blood flow
How does the tracer meassure local variation in blood flow?
- > Tracer are most often isotope consiting of a positron and a oxygen molecule
- > The positron correlates with an electron
- > The collison releases two gamma rays
Where is Pet most oftenly used ?
- > It can measure the concentration of creatin molecules or pathways
- > Alzheimer molecule
What is the assumption of PET scan ?
-> More active region receive more tracer since they need paired molecule oxgen !
How does an fMRI work?
- > Active regions need oygenated hemoglobin
- > When neurons consume oxygen it get converted ito deoxyhemoglobin
- > High lvl of Oxygen are detected with fMRI via sending in multiple electrical fields
- > oxygen and deoxygen have different magentic properties
Explain the Hemodynamic Response function:
- Intial dip = Higher deoxy hemoglbion
- Overcompenatsion = increased in blood flow is greater then consumption (6 - 10)
- Undershoot -> Bold signal decrease higher doexgynated
What is meant by bold ?
the ratio of oxygenated to deoxygenated hemoglobin
What is important for an fMRI design:
-> Be aware of when you present a stimuli since oxygen lvl must be at baseline
What is something you need to look out for in an fMRI scan ? Before you scan !
- > Correction for head movement
- > If the head moved the postion of active region also moves
What do you need to be aware of after the fMRI Scan ?
- > Steretatctic Normalization
- > Smoothing
- > Statistical Comparison
What is meant by Steretatctic Normalization ?
- > It is the process of mapping regions on each individual brain onto a standard brain via stretching or squashing
- > Brain gets divided into thousands of voxels which are given Talairach coordinates (x,y,z)
What does the X coordinate refer to ?
- > Left and right
- > Left is negative right is positive
What does the Y coordinate refer to ?
- > referes to front and back
- > Front is positive and back is negative
What does the Z coordinate refer to ?
- > refers to top and bottom
- > Top = is positive and bottom is negative
What is menat by smoothing ?
- Spread some of the raw activation level of a given voxel to neighbouring voxels
- > The closer the neighbour the more activation it gets
What are the benefits of Smoothing ?
- > Enhances SNR (signal to noise ratio)
1. isolated voxel = (noise) gets turned off
2. Signal = Cluster activity
What is meant by statistical power ?
- > “is the mean activity at a particular voxel greater in the experimental condition than in the baseline condition?”
- > Usually treshold must be lower then 0.05
What are data interpretation ?
-> Activation in image is the difference between two conditions (Task A – Task B)
What is the problem of fMRI activation?
- > Just because one region is active, it doesn’t mean it’s essential for the task
- > Activation can either mean inhibition or excitation
How to increase the spatial resolution of fMRI ?
- > higher tesla
- > Increasing voxel size = lowering spatial rsolution
- > reducing voxel size = increasing spatial resoultion
- > Higher spatial rsolution comes of the coast of less temporal resolution
- > Spatial resolution = dependent on voxel
How to increase temporal resolution ?
- > Time to repetition (TR)
- > Time betweeen to excitation pulses
- > Shorter time between impulses = higher temporal resolution
How to increase both spatial / temporal resolution in an fMRI study ?
- > Via jittering
- > Parallel imaging
What is meant by jittering ?
-> Different delays between the start of the sampling of the brain volume images relative to the start of the stimulus presentation
Name some factors about fMRI:
- > Based on blood oxygen conectartion
- > No radioactivity
- > Participants can be scanned multiple times
- > Less expensive
- > Better spatial resolution
Name soe factors about PET:
- > Based on blood volume
- > Involves radioactivity
- > Partcipants can only be scanned once
Which type of protons do we use in an fMRI scan ?
-> Protons which alling (cause they add up) due to magnetic field of scanner
But what do we actually meassure in an FMRI ?
- > The precission has the speed of mega herz which is in the radio frequency range
- > increases via higher tesla (lamour equation)
- > the radio frquency pulse makes the precession equally fast plus it chnages the position of protons
Explain how we see the different clouors (grey vs white):
- > More protons = more spinning = white
- > less protons = les spining = grey
Oxygenated blood lead to ?
- > high fmri signal
- > does not have a magnetic field
- > slow decrease in spinning T2
Deoxygenated blood lead to ?
- > low fmri signal since they have their own magnetive field
- > fast decrease in spinning T2
What are drifts ?
-> really low frequencies which we want to filer out
What does temporal smoothing remove ?
-> high frequency signals
