Neuroimaging Flashcards
(37 cards)
1
Q
Structural imaging
A
- based on the fact that different types of tissue (e.g. skull, gray matter, white matter, cerebrospinal fluid) have different physical properties.
- These different properties can be used to construct detailed static maps of the physical structure of the brain.
- MRI and CT SCANS
2
Q
Functional imaging
A
- based on the assumption that neural activity produces local physiological changes in that region of the brain. This can be used to produce dynamic maps of the moment-to-moment activity of the brain when engaged in cognitive tasks
- FMRI and PET
3
Q
MRI
A
- Structure
- Magnetic Resonance Imaging
- used to create images of soft tissue of the body
- Anatomical
4
Q
What does the brain look like?
Is an example of what kind of imaging?
A
MRI
5
Q
FMRI
A
- Function
- designed to measure the moment-to-moment variable characteristics of the brain that may be associated with changes in cognitive processing
- Activity/function
- The more red- more active, mode blue- less active. Hot and cold=active and less active
6
Q
Where and When is there brain activity?
Which neuro image is this an example of?
A
FMRI
7
Q
MRI Over CT
A
- Safer
- Better special resolution
- Differentiates between grey and white matter
- Also has FMRI’s that can be used to detect function
8
Q
All tissues have
A
water
9
Q
Hydrogen nucleus
A
- (= 1 proton, + charge) spins around its own axis
10
Q
How is a magnetic field created?
A
spinning, and charged particles like to spin.
So, each hydrogen (millions) creates a small magnetic field
11
Q
Difference between outside and inside the MRI?
A
- Outside the MRI we have chaos
- Inside the MRI machine, the magnetic force bring an alignment to the hydrogen protons to a specific orientation. This is detected to create an image in MRI
12
Q
Radio Frequency Excitation
A
- Once all the protons are lined up in the magnetic field
- They spin and continue to spin and have a 90-degree angle as a result.
- We turn the radio off and it goes back to straight, turn it on and it goes back to 90
13
Q
Different body parts have different composition of water, therefore:
A
- that means that different tissues will have different relaxation time in the different magnetic axis
14
Q
What are we measuring in an MRI?
A
relaxation times into the magnetic field. This means we can differentiate between different tissues. We measure energy+time
15
Q
What happens when we turn off the RF energy?
A
the protons relax…
- Relaxation time (snapping back to main magnetic field) depends on tissue’s magnetic properties.
- Releases energy that we can measure!
- (Think of a zillion of these little magnets all snapping back at the same time…)
16
Q
Measurement of the MR signal
A
- A magnetic field produces an electric current in a loop of wire.
The current can be measured (and turned into an MR image)
17
Q
How do we get an MRI image?
A
- We apply a magnetic field to make protons spin in the same angle.
- We apply radio-frequency (RF) energy to tilt them a little.
- We let them relax – they give off energy
- Different tissues and fluids give off different patterns of energy
- We measure the energy from different angles, which tells us what tissue/fluid is in a given point in space.
- We assemble all the points to make a 3-D image.
18
Q
2 Radio-Frequency coils
A
- one for applying the RF pulse, and one for measuring the return energy from the brain after the pulse is turned off
19
Q
How do we know where we’re “looking” at?
A
- We want a 3D image so we do the process at different images
- Up-down: gradient in main magnetic field - excite only frequencies corresponding to slice plane
- Left-Right: slightly different radio frequency
- Top-bottom: slightly different phase of frequency
- The noises are switching from different angles/gradients
20
Q
Gradient coil-
A
applies RF waves
21
Q
- Different tissues → different relaxation time → different signal, i.e. different “color”
A
- CSF – Dark
- Bone – Dark
- White matter – white
- Gray matter – Gray
- Fat – Bright
- Infarct/Tumor – Dark
- MS plaque - Dark
- Blood - Bright
Good for structure
22
Q
Spatial resolution
A
magnetic energy created by magnetic fields.
23
Q
Temporal resolution
A
(important for FMRI)- how much time has passed from actual brain until I see it on the device/refers to the accuracy with which one can measure when a cognitive event is occurring
24
Q
Voxel
A
volumed up pixel
25
fMRI definition
* *Anatomical MRI image based on: different tissues behave differently under magnetic field).*
* A functional fMRI image is based on changes in the properties of the ***blood***.
* It ***does not*** measure neuronal activity directly.
* The assumption is that active neurons need more oxygen.
* Oxygen-rich blood reaches regions of active neurons, creating an fMRI signal.
* Active brain cell- needs blood supply with oxygen and glucose. So, if a brain region is active, we’ll have more blood supply and oxygen- this is what we measure in FMRI
26
when functional imaging researchers refer to a region being “active” they mean:
* that the physiological response in one task is greater relative to some other condition
27
Hemoglobin without oxygen
* creates a local magnetic field which disturbs the global magnetic field
* _hand_
28
Hemoglobin with oxygen
* much weaker local magnetic field, so effect of the global (MRI) magnetic field is stronger *= BOLD signal*
* **_Bold-_** “blood oxygen-level-dependent contrast”
* _Fist_
* Bolder here because eventually you’ll get more light. And doesn’t disturb the global magnetic field as much
29
Hemodynamic Response Function
* The way that the BOLD signal evolves over time in response to an increase in neural activity
30
Bold
blood oxygen-level-dependent contrast”
31
* Three phases of FMRI
1. **_Initial dip-_** As neurons consume oxygen there is a small rise in the amount of deoxyhemoglobin, which results in a reduction of the BOLD signal
2. **_Overcompensation-_** In response to the increased consumption of oxygen, the blood flow to the region increases. The increase in blood flow is greater than the increased consumption, which means that the BOLD signal increases significantly. This is the component that is normally measured in fMRI.
3. **_Undershoot-_** Finally, the blood flow and oxygen consumption dip before returning to their original levels. This may reflect a relaxation of the venous system, causing a temporary increase in deoxyhemoglobin again.
32
MRI Step by Step Detailed
1. First, a strong magnetic field is applied across the part of the body being scanned (e.g. the brain).
2. The single protons that are found in water molecules in the body (the hydrogen nuclei in H2O) have weak magnetic fields. (Other atoms and nuclei also have magnetic moments but in MRI it is the hydrogen nuclei in water that form the source of the signal.)
3. Initially, these fields will be oriented randomly, but when the strong external field is applied a small fraction of them will align themselves with this.
1. The external field is applied constantly during the scanning process.
4. When the protons are in the aligned state a brief radio frequency pulse is applied that knocks the orientation of the aligned protons by 90 degrees to their original orientation.
5. As the protons spin in this new state, they produce a detectable change in the magnetic field and this is what forms the basis of the MR signal.
6. The protons will eventually be pulled back into their original alignment with the magnetic field (they “relax”).
7. The scanner repeats this process serially by sending the radio wave to excite different slices of the brain in turn.
1. With the advent of acquisition methods such as echo planar imaging, a whole brain can typically be scanned in about two seconds with slices of around 3 mm
33
Positron Emission Tomography (PET)
* FMRI’s competition since it’s a function
* When a positron meets electrons, it cancels each other out and no longer exists. Emits 2 futons and they go in two separate directions- this is what is measures
* uses a radioactive tracer injected into the bloodstream
* The temporal and spatial resolution of PET is worse than for fMRI
34
CT Scanner
* Constructed according to the amount of X-ray absorption in different types of tissue. The amount of absorption is related to tissue density:
* **_bone_** absorbs the most (and so the skull appears white)
* **_cerebrospinal**_ _**fluid_** absorbs the least (so the ventricles appear black)
* **_brain matter_** is intermediate (and appears gray)
35
* Neurovascular coupling
36
Pet or FMRI?
* FMRI is more noisy
* FMRI has better special and temporal resolution
* FMRI doesn’t use radio activity
* FMRI small movements distort, as opposed to pet
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