- > Unsynchronized Spinning charged particle - > Oriented randomly

- > inject radioactive tracer into bloodstream - > Tracer measures local variation in blood flow

Task 5 Flashcards by Pine Tree

What does an MRI meassure ?

Structural imaging (no activation) 
-> just pyhsical properties (grey vs white matter)

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How does an MRI work ?

Look at sheet

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What does MRI Relie on ?

Water molecules

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How is a Water Molecule constructed ?

> Hydrogen atom and Oxygen atom

- > Hydrogen atom have a magentic propertie

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What does a Hydrogen atom consist of ?

-> Proton

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What is a proton ?

> Unsynchronized Spinning charged particle

- > Oriented randomly

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What happens if you apply a stong magentic field ?

> High energy protons point downwards (antiparallel)

- > Low energy protons point upwards (parallel)

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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

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What are the effects of synchronized protons?

-> Produce a detectable magenetic field

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What happens when the redio frequency stops ?

-> Protons will be pulled back into original allingment

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What are the two types of meassurment in an MRI scan ?

> T1 / T2

- > T2 *

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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

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What does T2 * meassure ?

-> meassure functional property

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What does functional imaging mean?

> Always a hemodynamic response
> Indirect measurement (look at the consequences of neurons firing)
> How good can methods measure function

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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

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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

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How does Pet work ?

> inject radioactive tracer into bloodstream

- > Tracer measures local variation in blood flow

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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

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Where is Pet most oftenly used ?

> It can measure the concentration of creatin molecules or pathways
> Alzheimer molecule

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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