fMRI/MRI introduction

Question 1

Isidor Isaac Rabi

Answer 1

First to show how the properties of certain atomic nuclei can resonate when placed inside a magnetic field (Won Nobel Prize in Physics 1944)

Question 2

Laterbur and Mansfield

Answer 2

Discovered that atomic nuclei of hydrogen is something we can image in humans

Question 3

What is the length of electromagnetic waves involved in MRI?

Answer 3

Long waves (up to 2m)

Question 4

Which atoms resonate in MRI?

Answer 4

Hydrogen-1 atoms

Question 5

Hydrogen in different tissues resonates…

Answer 5

in different ways.

Grey and white matter thus can be differentiated

Question 6

Atoms with even atomic mass (equal amount of protons and electrons)…

Atoms with an odd amount of protons…

Answer 6

• have no net spin (magnetisation)

- have magnetic properties (Hydrogen-1 atom has only one proton, has net magnetisation and can be detected in MRI).

Question 7

Our body is comprised of ~_% water

Answer 7

~80%

Question 8

What is the Larmor frequency?

Answer 8

The speed of precession

Question 9

What is M0?

Answer 9

Magnetic field in the Z axis

Question 10

Which way is the spin aligned with the magnetic field M0?

Answer 10

Spinning up = aligned with magnetic field

Spinning down = aligned against the magnetic field

Question 11

3T magnetic field is ____ times stronger than Earth’s magnetic field

Answer 11

90 000

Question 12

What is used to flip atoms (by 90 degrees) from the longitudinal axis (running from feet to head) into the transverse plane (running across the brain from left to right ear)?

Answer 12

Radiofrequency magnetic pulse (RF pulse)

Question 13

What happens to the hydrogen atoms following the RF pulse?

Answer 13

1. Protons flip 90 degrees into the transverse plane
2. The atoms precess in phase (go around in synch)

Thus, the new net magnetisation is in the transverse XY plane (not longitudinal M0)

Question 14

What are the two types of relaxation?

Answer 14

T1: relaxation/recovery back into longitudinal (M0) magnetisation (slower relaxation = less dense material: bone is so fast we cannot measure it!)

T2: relaxation/decay of transverse magnetisation (i.e. losing synchronisation of the precession and net magnetisation is lost)

Deoxy blood = paramagnetic (Faster T2 relaxation)

Oxy blood = ferromagnetic (Slower T2 relaxation)

Question 15

What are the differences in images between T1 and T2?

Answer 15

T1:
Fluid = dark
Grey matter = grey
White matter = white

T2:
Fluid = bright
Grey matter = light grey
White matter = dark grey

Question 16

What do the gradient coils do?

Answer 16

provide subtle shifts in phase and frequency of the magnetic waves to provide information on the location of the relaxation (creating a 3D image comprised of voxels to visualize)

Question 17

What is the spatial resolution of MRI?

Answer 17

1mm

Question 18

What is the spatial resolution of fMRI?

Answer 18

~3-5mm

Question 19

How does magnet strength influence spatial resolution in MRI/fMRI?

Answer 19

Higher magnetic strength = higher spatial resolution

Question 20

What does BOLD stand for?

Answer 20

Blood-Oxygenation Level Dependent

Question 21

Time for the BOLD signal to start rising following the initial dip

Answer 21

1-2s from stimulus onset

Question 22

Time for the BOLD signal to peak following stimulus onset

Answer 22

4-6s from stimulus onset

Question 23

fMRI resolution is affected by the initial…

Answer 23

overshoot in supply of oxygenated blood to active brain region. More blood than needed is initially supply thus greater chunk of the surrounding brain tissue becomes filled with oxygenated blood, thus, negatively impacting the spatial resolution of fMRI signal

!the initial dip has better spatial resolution but there is inconsistency with detecting it

Question 24

The brain uses nearly __% of glucose and oxygen in our bodies

Answer 24

20%

Grey matter consumes nearly twice as much as white matter

Question 25

What does blood flow in the brain depend on?

Answer 25

• Blood pressure
• Diameter of vessel
• Density of red blood cells
• amount of oxygen and CO2
• Age & health

Question 26

What is neurovascular coupling?

Answer 26

The changes in blood perfusion/flow as a result of neuronal activity

Question 27

Describe the journey from a stimulus-related neuronal activity to the fMRI BOLD response

Answer 27

1. Neuronal activity resulting in an action potential (which requires energy)
2. Neurovascular coupling/haemodynamic response (blood delivered to provide energy for neuronal activity)
3. The haemodynamic response is detected by the MRI scanner

Question 28

Describe the process of an action potential

Answer 28

1. Stimulus excites neuron > sodium channels open
2. influx of sodium ions into the cell
3. once a threshold is reached (-55mV) an action potential is fired
4. As potential travels across the axon, it triggers depolarization across the cell
5. Then the cell repolarizes by opening pottasium channels and letting out potassium
6. More potassium than necessary leaves the cell initially, causing brief hyper polarization before returning into baseline

Question 29

Energy requirement of depolarization

Answer 29

No energy required (natural flow in concentration gradient of sodium ions)

Question 30

Energy requirement or repolarization

Answer 30

Potassium pumps require oxygen and glucose (for ATP) to pump the potassium out of the cells leading to repolarization

Question 31

What is thermal noise caused by?

Answer 31

Electrons colliding with atoms

Question 32

What is scanner drift?

Answer 32

static field strength of the scanner changes over time

Question 33

What does distortion correction do?

Answer 33

Corrects for magnetic field inhomogeneities

Question 34

what does spatial and temporal filtering help with?

Answer 34

noise due to respiration, heart rate, uncorrected headmovement etc

Question 35

What is the T statistic?

Answer 35

Mean of all data points in each condition / shared standard error

Question 36

False positives are type _ error

Answer 36

type 1 error

Question 37

Bonferronig correction when used for controlling the Family-Wise Error Rate

Answer 37

• Decreases type 1 errors (false positives)

- increases type 2 errors (false negatives)

Question 38

False Discovery Rate (FDR) when used for controlling the Family-Wise Error Rate

Answer 38

Takes into account number of false positives (type 1 errors) you expect (usually around 5%) and adjusts the p value accordingly

!Best for fMRI

Question 39

What is the best fMRI approach when we don’t have apriori hypothesis?

Answer 39

Whole brain approach

Question 40

How can Type 1 /2 errors be reduced in fmri

Answer 40

ROI analysis (less t-tests performed)

Question 41

What does the process of normalisation involve?

Answer 41

a 3-dimensional coordinate system (known as an ‘atlas’) of the human brain, which is used to map the location of brain structures (all participant data normalized into standardised image space)