Neuroimaging

Question 1

Structural imaging

Answer 1

• 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

Question 2

Functional imaging

Answer 2

• 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

Question 3

MRI

Answer 3

• Structure
• Magnetic Resonance Imaging
• used to create images of soft tissue of the body
• Anatomical

Question 4

What does the brain look like?

Is an example of what kind of imaging?

Answer 4

MRI

Question 5

FMRI

Answer 5

• 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

Question 6

Where and When is there brain activity?

Which neuro image is this an example of?

Answer 6

FMRI

Question 7

MRI Over CT

Answer 7

• Safer
• Better special resolution
• Differentiates between grey and white matter
• Also has FMRI’s that can be used to detect function

Question 8

All tissues have

Answer 8

water

Question 9

Hydrogen nucleus

Answer 9

• (= 1 proton, + charge) spins around its own axis

Question 10

How is a magnetic field created?

Answer 10

spinning, and charged particles like to spin.

So, each hydrogen (millions) creates a small magnetic field

Question 11

Difference between outside and inside the MRI?

Answer 11

• 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

Question 12

Radio Frequency Excitation

Answer 12

• 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

Question 13

Different body parts have different composition of water, therefore:

Answer 13

• that means that different tissues will have different relaxation time in the different magnetic axis

Question 14

What are we measuring in an MRI?

Answer 14

relaxation times into the magnetic field. This means we can differentiate between different tissues. We measure energy+time

Question 15

What happens when we turn off the RF energy?

Answer 15

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…)

Question 16

Measurement of the MR signal

Answer 16

• A magnetic field produces an electric current in a loop of wire.
  The current can be measured (and turned into an MR image)

Question 17

How do we get an MRI image?

Answer 17

1. We apply a magnetic field to make protons spin in the same angle.
2. We apply radio-frequency (RF) energy to tilt them a little.
3. We let them relax – they give off energy
4. Different tissues and fluids give off different patterns of energy
5. We measure the energy from different angles, which tells us what tissue/fluid is in a given point in space.
6. We assemble all the points to make a 3-D image.

Question 18

2 Radio-Frequency coils

Answer 18

• one for applying the RF pulse, and one for measuring the return energy from the brain after the pulse is turned off

Question 19

How do we know where we’re “looking” at?

Answer 19

• 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

Question 20

Gradient coil-

Answer 20

applies RF waves

Question 21

• Different tissues → different relaxation time → different signal, i.e. different “color”

Answer 21

• CSF – Dark
• Bone – Dark
• White matter – white
• Gray matter – Gray
• Fat – Bright
• Infarct/Tumor – Dark
• MS plaque - Dark
• Blood - Bright

Good for structure

Question 22

Spatial resolution

Answer 22

magnetic energy created by magnetic fields.

Question 23

Temporal resolution

Answer 23

(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

Question 24

Voxel

Answer 24

volumed up pixel

Question 25

fMRI definition

Answer 25

• 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

Question 26

when functional imaging researchers refer to a region being “active” they mean:

Answer 26

• that the physiological response in one task is greater relative to some other condition

Question 27

Hemoglobin without oxygen

Answer 27

• creates a local magnetic field which disturbs the global magnetic field
  
  • hand

Question 28

Hemoglobin with oxygen

Answer 28

• 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

Question 29

Hemodynamic Response Function

Answer 29

• The way that the BOLD signal evolves over time in response to an increase in neural activity

Question 30

Bold

Answer 30

blood oxygen-level-dependent contrast”

Question 31

• Three phases of FMRI

Answer 31

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.

Question 32

MRI Step by Step Detailed

Answer 32

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

Question 33

Positron Emission Tomography (PET)

Answer 33

• 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

Question 34

CT Scanner

Answer 34

• 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)

Question 35

• Neurovascular coupling

Answer 35

Question 36

Pet or FMRI?

Answer 36

• 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