fMRI/MRI introduction Flashcards
Isidor Isaac Rabi
First to show how the properties of certain atomic nuclei can resonate when placed inside a magnetic field (Won Nobel Prize in Physics 1944)
Laterbur and Mansfield
Discovered that atomic nuclei of hydrogen is something we can image in humans
What is the length of electromagnetic waves involved in MRI?
Long waves (up to 2m)
Which atoms resonate in MRI?
Hydrogen-1 atoms
Hydrogen in different tissues resonates…
in different ways.
Grey and white matter thus can be differentiated
Atoms with even atomic mass (equal amount of protons and electrons)…
Atoms with an odd amount of protons…
- have no net spin (magnetisation)
- have magnetic properties (Hydrogen-1 atom has only one proton, has net magnetisation and can be detected in MRI).
Our body is comprised of ~_% water
~80%
What is the Larmor frequency?
The speed of precession
What is M0?
Magnetic field in the Z axis
Which way is the spin aligned with the magnetic field M0?
Spinning up = aligned with magnetic field
Spinning down = aligned against the magnetic field
3T magnetic field is ____ times stronger than Earth’s magnetic field
90 000
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)?
Radiofrequency magnetic pulse (RF pulse)
What happens to the hydrogen atoms following the RF pulse?
- Protons flip 90 degrees into the transverse plane
- The atoms precess in phase (go around in synch)
Thus, the new net magnetisation is in the transverse XY plane (not longitudinal M0)
What are the two types of relaxation?
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)
What are the differences in images between T1 and T2?
T1:
Fluid = dark
Grey matter = grey
White matter = white
T2:
Fluid = bright
Grey matter = light grey
White matter = dark grey
What do the gradient coils do?
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)
What is the spatial resolution of MRI?
1mm
What is the spatial resolution of fMRI?
~3-5mm
How does magnet strength influence spatial resolution in MRI/fMRI?
Higher magnetic strength = higher spatial resolution
What does BOLD stand for?
Blood-Oxygenation Level Dependent
Time for the BOLD signal to start rising following the initial dip
1-2s from stimulus onset
Time for the BOLD signal to peak following stimulus onset
4-6s from stimulus onset
fMRI resolution is affected by the initial…
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
The brain uses nearly __% of glucose and oxygen in our bodies
20%
Grey matter consumes nearly twice as much as white matter
What does blood flow in the brain depend on?
- Blood pressure
- Diameter of vessel
- Density of red blood cells
- amount of oxygen and CO2
- Age & health
What is neurovascular coupling?
The changes in blood perfusion/flow as a result of neuronal activity
Describe the journey from a stimulus-related neuronal activity to the fMRI BOLD response
- Neuronal activity resulting in an action potential (which requires energy)
- Neurovascular coupling/haemodynamic response (blood delivered to provide energy for neuronal activity)
- The haemodynamic response is detected by the MRI scanner
Describe the process of an action potential
- Stimulus excites neuron > sodium channels open
- influx of sodium ions into the cell
- once a threshold is reached (-55mV) an action potential is fired
- As potential travels across the axon, it triggers depolarization across the cell
- Then the cell repolarizes by opening pottasium channels and letting out potassium
- More potassium than necessary leaves the cell initially, causing brief hyper polarization before returning into baseline
Energy requirement of depolarization
No energy required (natural flow in concentration gradient of sodium ions)
Energy requirement or repolarization
Potassium pumps require oxygen and glucose (for ATP) to pump the potassium out of the cells leading to repolarization
What is thermal noise caused by?
Electrons colliding with atoms
What is scanner drift?
static field strength of the scanner changes over time
What does distortion correction do?
Corrects for magnetic field inhomogeneities
what does spatial and temporal filtering help with?
noise due to respiration, heart rate, uncorrected headmovement etc
What is the T statistic?
Mean of all data points in each condition / shared standard error
False positives are type _ error
type 1 error
Bonferronig correction when used for controlling the Family-Wise Error Rate
- Decreases type 1 errors (false positives)
- increases type 2 errors (false negatives)
False Discovery Rate (FDR) when used for controlling the Family-Wise Error Rate
Takes into account number of false positives (type 1 errors) you expect (usually around 5%) and adjusts the p value accordingly
!Best for fMRI
What is the best fMRI approach when we don’t have apriori hypothesis?
Whole brain approach
How can Type 1 /2 errors be reduced in fmri
ROI analysis (less t-tests performed)
What does the process of normalisation involve?
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)
